VIEWS: 3 PAGES: 60 POSTED ON: 6/27/2011
1 Chapter III - Terrestrial Laser Scanners By Gordon Petrie & Charles Toth General Introduction As discussed in the introduction to Chapter One, there has been a widespread use of lasers in land and engineering surveying for the last thirty years. This can be seen in the incorporation of lasers into standard surveying instruments such as total stations and the laser rangefinders, profilers, levels and alignment devices that are in daily use within these fields of activity. Given this background, it would appear to be a natural development for scanning mechanisms to be added to total stations that were already equipped with laser rangefinders and angular encoders and for a terrestrial or ground-based laser scanner to be developed for use in field surveying. In this way, instead of individual points being measured on very specific ground features to a high degree of accuracy by a field surveyor using a total station - as required for large-scale cadastral, engineering and topographic maps - the laser scanner would allow the automated measurement and location of tens or hundreds of thousands of non-specific points in the area surrounding the position where the instrument had been set up, all within a very short time-frame. Besides this favourable technological background against which terrestrial laser scanner development could have been expected to take place, there was no requirement for the supporting geo-referencing technology of integrated GPS/IMU systems that were essential for the development of airborne profiling and scanning. Indeed the locations of the ground stations where a terrestrial laser scanner had been set up on a tripod could easily be carried out using the traditional traversing and resection methods that are used by land surveyors. 2 However, for whatever reasons, the development of terrestrial or ground-based laser scanners has tended to lag behind that of airborne laser scanners. As Pfeifer and Briese (2007) have pointed out, terrestrial laser scanning has matured and been accepted by the overall geoinformatics industry rather later than its airborne equivalents. It may be that this is partly the result of the fact that a large segment of the professional surveying community comprises individual practitioners or small partnerships who operate purely on a local basis. Many of these do not possess the financial resources needed to acquire laser scanners costing much more than the total stations, GPS receivers, theodolites and levels that are the traditional forms of instrumentation being used in surveying. In fact, in many cases, the first users came from the surveying departments of large airborne mapping companies who had already been using laser scanners and thus had a good understanding of the technology. In this particular context, it is also interesting to note that, when it did take place, much of the early development of terrestrial laser scanners, especially over short ranges, was carried out by companies that were active in the manufacture and supply of measuring instruments and equipment for use in industrial metrology rather than the mainstream suppliers of surveying instruments – though the latter have been quick to adopt the technology once it had been fully developed. Another interesting consequence of the developments that have come from the metrology side has been the introduction and widespread adoption of phase measuring techniques for the laser rangefinders that are used in short-range terrestrial laser scanners. This is in complete contrast to the situation in airborne laser scanning where the pulse-ranging technique is totally dominant. Now that terrestrial laser scanners have become sufficiently developed and have been accepted by the surveying profession, they have proliferated quite quickly and in large numbers, even though the 3 investment is by surveying standards considerable - although it is only around one-tenth of that required for the purchase of an airborne laser scanner. Turning next to the vehicle-mounted laser scanners that form part of certain mobile mapping systems, the technology has, in many respects, more similarities to that of airborne laser scanning systems than that used in terrestrial laser scanners. In particular, the requirements for the continuous geo-referencing of the moving vehicle have resulted in the use of integrated GPS/IMU systems as essential components of the overall mobile mapping system. However a special problem with these systems concerns their application to the mapping of built-up urban areas where the interruptions in the GPS signals caused by high buildings result in poor satellite configurations or even a complete loss of signal (Bohm & Haala 2005). Besides which, the reflections of signals from nearby buildings can give rise to multi-path effects which can reduce the positional accuracy of the GPS measurements. These deficiencies in the GPS measurements have resulted, in some cases, in their supplementation by distance measuring devices attached to the wheels of the vehicles on which the laser scanners have been mounted. In this context, it should also be noted that many terrestrial laser scanners are permanently installed on vehicles to improve their production efficiency - basically, to allow for their faster deployment in the field and, in general, to give more flexibility. In most cases, the laser scanner is attached to a mast that can be erected to higher positions, providing a better observation potential as compared with tripod- mounted installations. Obviously, these “stop-and-go” systems do not fall into the category of mobile mapping systems. However terrestrial laser scanners, whether tripod- or vehicle-mounted, do have the great 4 advantage of providing the details of building facades as required for the production of realistic 3D city models. In this respect, the data derived from terrestrial laser scanners complements the data acquired using airborne stereo-photogrammetric and laser scanning methods. The latter can produce the overall 3D city model at roof or ground level, but lack detailed information on the facades. Thus a combination of the two methods (airborne and terrestrial) is becoming increasingly common for the production of these 3D city models (Bohm & Haala 2005). As noted above, the terrestrial laser scanners that are being utilized for topographic mapping and modelling operations can be operated either from a static (or stationary) position - for example, being mounted on a tripod over a ground mark - or they can be operated from a dynamic (or moving) platform such as a van, truck or railcar. Quite different instrumental or system designs will result, depending on which of these two main operational modes has been adopted for the acquisition of the terrestrial laser scan data for mapping operations (Ingensand 2006). Thus the coverage of terrestrial laser scanner technology that will be given in this chapter has been organized to fall into these two distinct categories – those covering static and dynamic laser scanners respectively. III.1 - Static Terrestrial Laser Scanners The types of laser scanner that will be considered in this section of the chapter are those that measure the topographic features that are present on the ground in the area around the fixed (static) position that has been occupied by the instrument. They do this through the simultaneous measurement of slant range by a laser rangefinder and the two associated angles by angular encoders in the horizontal and vertical planes passing through the centre of the instrument (Fig. 1). 5 In most cases, prior to the actual scanning process, the angular increments in both directions, comprising the azimuth and vertical rotations, can be set by the user. Typically the angular step sizes are set to identical values; thus the scanner provides an equal spatial sampling in an instrument-centered polar coordinate system. Fig. 1 – The range and the horizontal and vertical angular rotations that are measured towards objects in the terrain using a terrestrial laser scanner. (Drawn by M. Shand) These simultaneous measurements of distance and angle are carried out in a highly automated manner using a pre-determined scan pattern often at a measuring rate of 1,000 Hz or more. As discussed in Chapter One, the distance measurements that are made by the laser rangefinder will utilize either the pulse ranging or the phase difference measuring technique. However, in this introductory discussion, it should be mentioned that there also exist a number of very short-range laser scanners, which are based on other measuring principles and which operate over ranges of a few metres, often to accuracy values of a fraction of a millimetre. A comprehensive review of the principles and technologies utilized in these ultra-high accuracy, very short-range scanners is given 6 in Blais (2004). These instruments are much used in metrology, industrial applications and reverse engineering; in body-scanning and medical research; and in the recording of objects by museum staff and archaeologists. A representative example of such an instrument – one among very many - is the Konica Minolta VIVID laser scanner. This operates on the basis of optical triangulation with the target or object being scanned with laser stripes whose reflected images are being recorded simultaneously by a digital camera. The maximum measuring range of such an instrument is only 2.5 m. These very short range laser scanners will not be considered here where the emphasis is exclusively on topographic applications. III.1.1 - Overall Classification The primary classification that has been adopted widely in the published literature on static terrestrial or ground-based laser scanners differentiates between those instruments that utilize the pulse ranging or time-of-flight (TOF) measuring principle and those that employ the phase measuring technique - as set out in Chapter One. Within the specific context of static terrestrial laser scanners, it can be said that the phase difference method produces a series of successive range measurements at a very high rate and to a high degree of accuracy, but only over distances of some tens of metres. Whereas the pulse-based time-of-flight method allows much longer distances of hundreds of metres to be measured, albeit at a much reduced rate and a somewhat lower (though quite acceptable) accuracy as compared to those of the phase measurement method. The lower rate of the time-of-flight method will be increased in the future using the multi-pulse technique, However this simple classification based on the measuring technique that is used to measure distance does not take account of the angular scanning action that is used to ensure the required 7 coverage of the ground and of the objects that are present on it. Therefore it is essential to have a secondary classification which will account for both the scanning mechanism and the pattern or coverage that this produces over the ground. The classification that has been adopted here is that introduced by Staiger (2003) - which differentiates between three types of static terrestrial or ground-based laser scanners - (i) panoramic-type scanners; (iii) hybrid scanners; and (iii) camera-type scanners; (Fig. 2). Fig. 2 – The classification of terrestrial laser scanners based on their respective scanning mechanisms and coverages. (Source: Staiger (2003); re-drawn by M. Shand). (i) Within the first of these three categories, panoramic-type scanners carry out distance and angular measurements in a systematic pattern that gives a full 360 angular coverage within the horizontal plane passing through the instrument's centre and typically a minimum 180 coverage in the vertical plane lying at right angles to the horizontal plane – thus giving hemispheric coverage. However a still greater vertical field of view of 270 or more is not uncommon - which means that a substantial coverage of the ground lying below the instrument's horizontal plane can be achieved. Indeed the only gap or void in the coverage of a full sphere on a number of instruments is that 8 produced by the base of the scanner instrument and its supporting tripod. While this panoramic scanning pattern is very useful in the context of topographic mapping, it is even more desirable, indeed often obligatory, in the measurement of complex industrial facilities, large quarries and open-cast mines and the facades of buildings within urban areas - or even indoors in large halls, churches, rooms, etc. (ii) The instruments falling within the second category of hybrid scanners are those where the scanning action is unrestricted around one rotation axis - usually the horizontal scanning movement in the azimuth direction produced by a rotation of the instrument around its vertical axis. However the vertical angular scan movement in elevation around the horizontal axis of the instrument is restricted or limited - typically to 50 to 60. This reflects the situation that is commonly encountered in medium- and long-range laser scanning carried out for topographic mapping purposes where there is no requirement to measure objects overhead or at steep angles, as will be needed within buildings (iii) The camera-type scanners that make up the third category carry out their distance and angular measurements over a much more limited angular range and within a quite specific field-of-view (FOV). Typical might be the systematic scanning of the surrounding area over an angular field of 40 x 40 in much the same manner as a photogrammetric camera - at least in terms of its angular coverage, though obviously not in terms of the actual measurements that are being made and recorded. As with airborne laser scanners, a third or tertiary classification can be envisaged based on the 9 range or distance over which the static terrestrial or ground-based laser scanners can be used. The first group that can be distinguished comprises those laser scanners that are limited to short ranges up to 100 m - indeed some are limited to distances of 50 to 60 m. They mostly comprise those instruments that employ the phase measuring principle for distance measurement using the laser rangefinder – although, as will be seen later, there are instruments that employ pulse (time-of- flight) ranging over these short ranges. Usually the limitations in range of these instruments are offset by the very high accuracies that they achieve in distance measurement - often to a few millimetres. A second group, almost entirely based on pulse ranging using time-of-flight measurements of distance, can measure over medium ranges with maximum values from 150 to 350 metres at a somewhat reduced accuracy. While a third long-range group, again using the pulse ranging technique, can measure still longer distances - up to 1 km or more in the case of the Optech ILRIS-3D instrument. However the gain in range is normally accompanied by a reduction in the accuracy of the measured distances and in the pulse repetition rate - though this is still appropriate to the applications and is very acceptable to the users of these instruments. III.1.2 - Short-Range Laser Scanners Both the main measuring principles outlined above - pulse ranging and phase measurement - are in use in those ground-based laser scanners that measure over short ranges - typically with maximum ranges from 50 m up to 100 m. It will be seen that all of the instruments that have been included in this group fall into the category of panoramic scanners. This enables them to be used indoors within buildings as well as outdoors where their operational characteristics make them well suited for use in the surveys and mapping of urban areas where the measurement of short distances and high vertical angles is required. 10 III.1.2.1 - Short-Range Scanners Using Phase Measurement Two of the most prominent manufacturers of this type of scanner are Zoller+Fröhlich (Z+F) and Faro - both of which are based in Germany. A third supplier is Basis Software Inc., which is based in the U.S.A. Zoller+Fröhlich The current Z+F ground-based laser scanner is called the Imager 5006; a previous model from this manufacturer is the very similar Imager 5003. Both of these instruments have a servo motor and angular encoder that implements the angular scan movement in the horizontal plane (in azimuth) around the instrument's vertical axis through a rotation of the upper part of the instrument against its fixed base - which is normally mounted on a tripod so that it can be set accurately over a ground mark. The scan movement in the vertical direction is implemented using a lightweight, fast- rotating mirror placed on the horizontal trunnion axis of the instrument which is supported on two vertical standards or pillars (Fig. 3). The horizontal rotation in azimuth covers the full circle of 360, while the rotational movement of the mirror in these Imager scanners allows a scan angle of 310 within the vertical plane. The manufacturer's claimed accuracy in both horizontal and vertical angular measurement is ±0.007 - which is equivalent to ±6 mm accuracy at the measured points in both directions in the plane that is perpendicular to the laser direction at 50 m object distance. The maximum scan rate in the vertical (elevation) plane is 50 Hz; however a more typical rate in actual operational use is 25 Hz. The LARA laser rangefinder that is used in the Imager scanner instruments employs a Class 3R continuous wave (CW) laser that operates in the near infra-red part of the spectrum at λ = 780 nm. In the earlier Imager 5003 model, the rangefinder was 11 available in two alternative versions giving maximum ranges of 25.2 m and 53.5 m respectively (Mettenleiter et al 2000). The current Imager 5006 model has a maximum range of 79 m. The accuracy in distance quoted by the manufacturer is ±6.5 mm over a range of 25 m. A detailed investigation into the accuracies of the distance and angular measurements of the Imager 5003 has been made by Schulz & Ingensand (2004) of the ETH, Zurich. The processing of the measured range and angular data from the Imager scanners is carried out using the Z+F Light Form Modeller software. Besides measuring the distance and angles to the ground objects, the Z+F Imager scanners also measure the intensities of the signals that are reflected from these objects. This allows grey-scale reflectance images of the object to be formed. Data is stored either on the internal hard disk drive of the instrument or it can be transferred via an Ethernet interface to a laptop computer. An integrated control panel for the operation of the instrument is located on the side of one of the vertical standards of the instrument. The instrument can also be controlled remotely from a PDA using a wireless interface and connection. The Imager scanners can deliver in excess of 500,000 measured points per second, though in normal operation, the rate is likely to be somewhat lower. 12 Fig. 3 – The Z+F Imager 5003 phase-based high-speed laser scanner. (Source: Zoller+Fröhlich) Fig. 4 – The Leica Geosystems HDS6000 laser scanner which has the same specification as the Z+F Imager 5006. (Source: Leica Geosystems) The Z+F Imager 5003 instrument has also been marketed and sold very widely for mapping applications by Leica Geosystems as its HDS4500 laser scanner, where HDS is an acronym for 'High Definition Surveying'. This instrument has been available in the same two versions - with ranges of 25.2 m and 53.5 m respectively - as the Imager 5003 and with the same angular coverages of 360 in azimuth and 310 in the vertical plane. Similarly the newer Z+F Imager 5006 instrument is now being offered by Leica as its HDS6000 scanner with the same product specification as the Z+F instrument (Fig. 4). Faro The Faro company, which is mainly based in North America, is involved in the manufacture of a wide range of portable computer-based measuring instruments, including laser trackers, gauges and measuring arms, for use in industrial plants and factories. Its range of terrestrial or ground-based laser scanners was developed originally by the IQvolution company, based in Stuttgart, Germany which Faro purchased in 2005. The IQvolution company's laser scanner was called the IQsun 880. The instrument was renamed LS 880 after the take-over by Faro. Since then, two new shorter- range models - the LS 420 and LS 840 - have been introduced to the market. The basic design, construction and operation of all three models of this panoramic-type scanner is 13 very similar. Furthermore they all use a Class 3R continuous wave (CW) semi-conductor laser operating at λ = 785 nm in the near infra-red part of the spectrum as the basis of their rangefinders. The output power of the laser is 20 mW in the case of the shortest-range LS 420 model (Fig. 5) which has a maximum range of 20 m. The output powers of the lasers used in the two longer-range LS 840 and LS 880 models are 10 mW and 20 mW respectively, allowing maximum ranges of 40 m and 76 m to be measured respectively. With regard to the phase measuring technique used to measure distances, taking the LS 880 model as an example, the output laser beam is split and amplitude modulated to operate at three different wavelengths - 76 m, 9.6 m and 1.2 m - as shown in Fig. 6. This allows the measured range to be determined to 0.6 mm in terms of its resolution value. The claimed accuracy of the measured ranges is ±3 mm at a distance of 10 m. The obvious advantage of using the phase measurement technique over the time-of-flight or pulse ranging method is its speed of measurement. In the case of the LS 880 model, the laser rangefinder can measure distances at rates up to 120,000 points per second. Fig. 5 – The Faro LS 420 laser scanner that measures its ranges employing phase differences. (Source: Faro) 14 Fig. 6 – The measurement scheme used in the Faro LS 880 laser scanners showing the three different frequencies corresponding to distances of 76 m, 9.6 m and 1.2 m respectively at which the phase differences are measured. The example is shown for a measured distance of 13 m. (Source: Faro; re-drawn by M. Shand) In all three models, the laser rangefinder is mounted in the horizontal plane and aligned with the horizontal (trunnion) axis of the instrument. The output beam from the laser passes into the centre of a continuously rotating (motor-driven) mirror that deflects it through a fixed angle of 90 to produce a vertical profile scan of the laser beam giving an angular coverage of 320 in the vertical plane. This places it in the category of panoramic scanners. The reflected signal that is returned from each point along the profile that is being scanned in the object field is then compared with the reference output signal to measure the phase differences, so determining the measured range. Besides which, the intensity of the return signal from each pulse that hits the object can also be measured to a 9-bit level. The 360 azimuth scan is implemented using a motor whose power is derived from a 24 volt DC battery pack. The measured range, intensity and angular data can be recorded either directly on the instrument's internal hard disk or remotely via an Ethernet interface to an external laptop computer. The processing of the measured data is carried out using the Faro 15 Scene software. Improved versions of the different models of the Faro scanners with upgraded electronics and better positional accuracy were introduced in March 2008 at the SPAR 2008 Conference under the name Photon and are now called the Photon 20 and Photon 80. Basis Software Inc. This American company is based in Redmond, Washington. Its main laser scanner product that can be used in topographic applications is the Surphaser (Fig. 7), which again is a panoramic-type scanner. The company also manufactures short range laser scanners for industrial use that will not be discussed here. The Surphaser is available in two models - the 25HS (Hemispheric Scanner), which has a maximum range of 22.5 m, and the 25HSX which has an extended range of 38.5 m. The basic design and construction of the Surphaser instruments is somewhat similar to that of the Faro scanners described above in that they both employ a laser diode rangefinder that emits its continuous beam of laser radiation along the instrument's horizontal axis on to a continuously rotating mirror located at the end of a motor-driven shaft (Lichti et al 2007). This turns the beam through a right angle to produce the required scanning motion in the vertical plane giving an angular coverage of 270 (Fig. 8). The receiving lens focuses the reflected radiation from the object on to the photo diode that acts as the receiver allowing the continuous measurement of the phase differences and the intensity values. The maximum rate of range measurement of the laser rangefinder is 190,000 points per second, though often, in practice, a lower rate will be used. The semi-conductor laser diode that is used in this instrument emits its radiation at the wavelength of 690 nm on the red edge of the visible part of the spectrum with a power of 15 mW. The 360 angular scan in the horizontal plane is implemented through the rotation of the upper part of the instrument which is driven in azimuth by a stepping motor and gearbox located in the lower (fixed) 16 part of the instrument which can be fitted into a standard Wild/Leica tribrach. The instrument's power requirements are supplied by a standard 18/24 volt DC battery. The measured data is transferred via a FireWire interface to be recorded on a laptop computer. Fig. 7 – The Surphaser 25HS laser scanner produced by Basis Software. (Source: Basis Software) Fig. 8 – Diagram showing the design and the main features of the Surphaser laser scanner. (Source: Basis Software; re-drawn by M. Shand) In November 2007, a version of the Surphaser was introduced by Trimble as the FX Scanner, the instrument being supplied by Basis Software to Trimble under an OEM agreement. The FX scanner is used in conjunction with Trimble‟s FX controller software, while Trimble‟s Scene Manager software is used to locate the positions where the scanner has been set up for its scanning and measuring operations. The measured data can then be used in Trimble‟s LASERGen suite of application software. III.1.2.2 - Short-Range Scanners Using Pulse Ranging Only a single company - Callidus from Saxony in Germany - has adopted pulse ranging based on 17 time-of-flight (TOF) measurement technology for its short-range laser scanners. Callidus Callidus has manufactured a short-range laser scanner based on pulse ranging for over eleven years - since 1996. Originally its scanner instrument was simply called Callidus 1.1. Later it was re- christened as the Callidus 3D Laser Scanner and, as such, it was also sold by the Trimble company. Now it is known as the Callidus CP 3200 scanner (Fig. 9a) . In this context, it should be noted that the Callidus company also builds other very short-range laser scanners designed for the measurement of comparatively small objects over very short distances indoors - e.g. in factories or museums. These are the CT 900 and CT 180 instruments, both of which utilize laser triangulation for the measurement of object up to 1.5 m and 30 cm respectively. As such, they will not be considered any further in this account which is concerned with the scanners that are used for topographic applications. 18 Fig. 9 (a) – The Callidus CP 3200 scanner that uses pulsed (time-of-flight) based laser ranging technology. (Source: Callidus); (b) the vertical angular coverage, and (c) the horizontal angular coverage of the CP 3200 of the instrument. (Source: Callidus; re-drawn by M. Shand) The CP 3200 instrument is a panoramic-type scanner providing a full rotation of 360 in azimuth (Fig. 9c) in the horizontal plane driven by a servo motor that provides a range of angular step sizes between 0.0625 and 1.0 that are selectable by the operator. A rotatable mirror attached to an angular encoder is used to establish each successive step position over an arc of 280 in the vertical plane (Fig. 9b). The angular step interval over this vertical scan is also selectable between 0.25 and 1.0 with a scan rate of up to 77 Hz. For each step position, the laser rangefinder measures the appropriate range in the vertical plane. The laser rangefinder that is used in the CP 3200 scanner utilizes a semi-conductor diode laser with quite a large (0.25) beam width operating at λ = 905 nm in the near infra-red part of the spectrum. Different measuring ranges can be selected - up to 8 m; 32 m and 80 m respectively - according to the reflectivity of the surfaces and objects that are being measured. The quoted accuracies that can be achieved with the instrument are ±5 mm in distance at a range of 50 m and ±0.005 and ±0.009 in terms of its horizontal and vertical angles respectively. A typical measuring rate that is used with the instrument is 1,750 points per second – although this rate is reduced if multiple measurements of each point are made. The resulting data is processed using Callidus's own 3D Extractor software. The CP 3200 also incorporates a software-controlled small-format (768 x 576 pixels) CCD camera with variable focal settings that can be used to generate digital frame images to supplement the range data being provided by the laser rangefinder. 19 Fig. 10 – The Callidus CPW 8800 scanner utilizes a combination of pulsed (time-of-flight) and phase-based laser ranging technologies. (Source: Callidus) Callidus has introduced its new CPW 8000 laser scanner at the Intergeo trade fair held in Leipzig in September 2007 (Fig. 10). The instrument combines the two main mensuration methods - those of time-of-flight pulse ranging and continuous wave phase measurement. The technique involves the basic measurement of the distance using the pulse ranging method. However the pulses are also modulated with a high frequency which allows the phase difference between the emitted and received pulses to be measured. With this elegant solution, no attempt needs to be made to resolve the ambiguity that is inherent in a single phase measurement. Instead the fine measurement of the distance given by the single phase measurement is combined with the overall distance value given by the pulse ranging measurement to provide the final range value to a high precision - ±2 mm at a range of 30 m. The maximum range of the instrument is quoted as being 80 m. A Class 3R laser emitting its pulses at λ = 658 nm in the red part of the spectrum is used as the basis of the instrument‟s rangefinder. A panoramic-type configuration ensures a full horizontal angular coverage of 360, while the coverage of the scan in the vertical plane is 300 with the smallest 20 angular resolution of 0.002 in both directions. Using this novel dual-measuring technique, a measuring rate of 50,000 measurements per second can be achieved - e.g., resulting in 54 minutes of scan time at an angular resolution of 0.02 in both directions. III.1.3 - Medium-Range Laser Scanners As defined above, this group of scanners can measure distances over medium ranges with maximum values lying between 150 m and 350 m. While most of the manufacturers of short-range laser scanners, such as Faro, Callidus and Basic Software, have strong interests in metrology - and indeed manufacture various other mensuration products that fall into that area - the situation is quite different with the manufacturers of medium-range laser scanners. In this area, several of the principal system suppliers of these scanners, such as Leica, Trimble and Topcon, are major manufacturers of surveying instrumentation such as GPS receivers, total stations and laser levels. The medium-range ground-based laser scanners that are manufactured and supplied by each of these companies are all based on the pulse ranging technique. Leica Geosystems Leica's entry into the field of terrestrial or ground-based laser scanners was made through its acquisition of the Cyra Technologies Inc. company in January 2001. This purchase was made at the same time as Leica's acquisition of another quite separate and independent American company, Azimuth, which ensured its entry into the field of airborne laser scanning - as already discussed in Chapter Two. The Cyra company had been set up originally in California in 1993. It developed its Cyrax terrestrial laser scanning instrument that finally entered the market in 1998. Leica first invested in the Cyra company as a minority shareholder in March 2000 before purchasing the rest 21 of the shares in the company a year later. At first, the Cyra company kept its name, operating as a division of Leica Geosystems. However, in April 2004, its name was formally changed to Leica Geosystems HDS Inc. - HDS being an acronym for 'High Definition Surveying'. The original model that was built and sold by the Cyra company was the Cyrax 2400. Its rangefinder used a Class 2 semi-conductor diode laser operating at λ = 532 nm in the green part of the spectrum. This allowed a maximum speed of measurement of 800 points per second with a maximum range of 100 m - though 50 m was more realistic with objects having a moderate reflectivity. The stated accuracy in range was ±4 mm over a distance of 50 m. The Cyrax 2400 was a camera-type scanner that could scan a 40 x 40 field of view or window using a twin mirror optical scanning system. The scanner main body sat in a simple non-motorized pan and tilt mount that allowed it to be pointed manually in steps over an angular range of 360 in azimuth and 195 in the vertical plane (Fig. 11). It was followed by the improved Cyrax 2500 model (later called the Leica HDS2500) with a similar specification (Sternberg et al 2004). The Cyrax instruments were supplied with the accompanying Cyclone software. 22 Fig. 11 – The Cyrax 2500 “camera-type” laser scanner with its manually operated tilt mount allowing an angular rotation around instrument‟s horizontal axis. Fig. 12 – The Leica Geosystems ScanStation laser scanner with its scan mirror providing “panoramic” coverage of the surrounding area. (Source: Leica Geosystems) In 2004, the popular Cyrax models were replaced by the Leica HDS3000 which had a very different design and specification. In particular, the camera-type layout of the previous Cyrax instruments was replaced by a dual-window design that gave a fully panoramic coverage of 360 in azimuth and 270 in the vertical plane. However the two windows are not in use simultaneously; thus two separate horizontal scans may be required to complete the required coverage. The HDS3000 scanner used servo motors to rotate the scan mirror in the vertical plane and for the azimuth drive, resulting in a much higher scan rate than its Cyrax predecessors. The rangefinder used in the HDS3000 was based on a Class 3R laser, again operating in the green part of the spectrum (at λ = 532 nm) and with a maximum operating range of 100 m - though the normal operational range was from 1 to 50m. The accuracy of the measured distance was stated to be ±4 mm at a range of 50 m, while the maximum measuring rate was 1,800 points per second. The HDS3000 scanner also incorporated a calibrated high-resolution digital video camera that was located internally within the instrument. This generated digital image data that could be overlaid on the scanned range and angular data using the Cyclone software. In 2006, the HDS3000 was superseded by the ScanStation (Fig. 12). This retained the overall design and construction of the HDS3000 panoramic-type instrument. However it had a much 23 greater maximum range of 300 m; a maximum measuring rate of 4,000 points per second; and the incorporation of a number of additional features such as a dual-axis compensator to allow conventional surveying operations such as resection and traversing to be carried out using the instrument. Most recently, in July 2007, the latest model in the series, called ScanStation 2, was introduced. This has the same overall design and appearance of the HDS3000 and the original ScanStation. However the ScanStation 2 instrument has a new laser rangefinder and timing electronics that allows it to carry out its range measurements at a very much higher speed - with a maximum rate of 50,000 points per second - while still retaining the same maximum range of 300 metres with a reflectivity of 90% that had been achieved in the previous ScanStation model. Recently (in 2007) the production of the Scan Station 2 instruments has been moved from the factory in San Ramon, California to the main Leica manufacturing plant in Heerbrugg in Switzerland. Trimble Trimble is another of the major suppliers of surveying instrumentation that has entered the field of terrestrial or ground-based laser scanning through the acquisition of a much smaller company that specialized in this area. It did so in September 2003 through its purchase of the Mensi company based in Fontenay-sous-Bois in France - which had been one of the pioneering developers of laser measurement technology. This company had been founded in 1986 and had designed and built a number of short range laser scanners - in particular, the S-series (including the S10 and S25 models) based on laser triangulation - that were used principally in metrology applications carried out within industrial facilities. 24 In 2001, the Mensi company introduced a longer-range laser scanner in the form of its GS 100 instrument, based on pulse ranging (Fig. 13). This was an early example of a hybrid type of scanner with a motorized 360 rotation in azimuth and a 60 angular rotation in the vertical plane using a lightweight scanning mirror. The angular resolution in the horizontal direction was 0.0018, while that in the vertical direction was 0.0009. The GS 100 instrument also used a rangefinder based on the use of a Class 2 laser operating at λ = 532 nm in the green part of the spectrum. The user could control the laser focus to produce a very small spot of 3 mm at 50 m to allow very precise measurements to be made. Alternatively an autofocus mode could be set which allowed the laser spot to be re-focused automatically according to the measured distance. The instrument could measure up to 5,000 points per second with a measuring precision of ±5 mm. The data processing was carried out using Mensi's RealWorks Survey and 3Dipsos software packages that had been developed in-house. A second model in the series, called the GS 200, followed soon after (Kersten et al 2005). This had a similar design and angular coverage to the GS 100, but utilized more sophisticated receiver circuitry that allowed the maximum range to be doubled to 200 m and a higher precision of ±2 mm to be achieved. It also featured an overscan technique that allowed data to be captured at ranges up to 350 m. Furthermore the GS 200, like the GS100, incorporated a calibrated video camera with a zoom capability that was fitted internally within the instrument. This produced colour video images with a format size of 768 x 576 pixels. The video images could be mosaiced and overlaid over the range and angular data that was being produced by the laser scanner. The GS 200 instrument could also be integrated together with a GPS receiver or a total station prism, these items being mounted on top of the laser scanner using a standard adapter. 25 Fig, 13 - The original Mensi GS100 laser scanner. Fig. 14 – The latest Trimble GX 3D laser scanner. (Source: Trimble) The latest models in the series are the Trimble GX 3D scanners (Fig. 14). These have a quite similar specification to that of the GS 200 model - with a hybrid-type angular coverage of 360 x 60 and a measuring rate of 5,000 points per second. However the instrument has been re-designed with a multi-shot capability to give an improved accuracy - with an absolute accuracy of ±3 to 8 mm depending on the range and the reflectivity of the objects being measured. It also incorporates automatic calibration of the zero index error together with a dual-axis compensator to correct any dislevelment of the instrument. The instrument also has the capability of polygonal framing - which allows the operator of the instrument to define an irregular shaped box, within which the instrument will carry out its scanning and measurements. The instrument also has a hand-held control unit which allows it to be operated remotely. In October 2007, Trimble introduced the GX Advanced model featuring its SureScan technology. This uses real-time data analysis to “regularize” the point density over the surface being measured. The user can set the desired grid interval and the algorithm changes the horizontal and vertical angles to provide an even density of 26 points over the measured surface. Since March 2007, the production of the GX instruments has been shifted from France to the former Spectra Precision factory in Danderyd, a suburb of Stockholm, the Swedish capital city. Topcon Another of the major manufacturers of surveying instrumentation, Topcon, has announced that it will enter the field of terrestrial laser scanners early in 2008 with the introduction of its GLS1000 model (Fig. 15). The preliminary information that has been released about this instrument is that it will be a hybrid-type instrument with a vertical angular coverage of 70, besides the normal horizontal rotation in azimuth of 360. The instrument‟s pulse-based rangefinder will feature a Class 1 laser that has a maximum range of 330 m to objects having a high (90%) reflectance and 150 m to those objects having a low reflectance (of 18%). The maximum rate of measurement is quoted as being 3,000 points per second. The GLS1000 scanner will have an integral camera and will feature an integrated control panel located on the side of the instrument. The data measured by the scanner will be recorded on a removable storage card. 27 Fig.15 – The newly announced Topcon GLS1000 terrestrial laser scanner. (Source: Topcon) III.1.5 Long-Range Laser Scanners Within this group of laser scanners, the maximum measuring range to highly reflective targets has been raised to more than 500 m; indeed, in some cases, it can be very substantially more. All of the instuments use pulse-ranging based on the time-of-flight measuring principle. It will be noted that two of the principal suppliers – Optech and Riegl - are also the manufacturers of airborne laser scanners that measure over similar distances in terms of the flying heights at which they are operated. Optech The Optech company's activities encompass laser ranging and scanning devices operating in many different environments - spaceborne, airborne and terrestrial. Within this context, the ILRIS (Intelligent Laser Range Imaging System) was developed originally by Optech for the Canadian Space Agency (CSA) in the late 1990s as a combined ranging and imaging device for use on-board spacecraft. In its original form, as the ILRIS-100, it used pulse ranging operating in both the visible (green) and infra-red parts of the spectrum to scan a scene and produce simultaneous range, angular and intensity data at ranges up to 500 m. When processed, this data was combined to form a range and intensity data set that could be produced either in grey scale or colour coded form that was 2k x 2k pixels (4 Megabytes) in size. In June 2000, Optech introduced its tripod-mounted ILRIS-3D version of the instrument (Fig. 16). 28 This had been developed specifically for topographic and open-cast mining applications on the one hand and for industrial applications, especially the measurement and modelling of industrial plants and facilities, on the other. Thus the instrument was designed and constructed with long-range capabilities from the outset. Using a Class 1 laser rangefinder emitting its infra-red radiation at λ = 1550 nm, it had a maximum range of 800 m with a range resolution of 1 cm, even with a target having only 20% reflectance and a range of 350 m with a very low-reflectance (4%) target such as a coal stockpile. A measuring rate of 2,000 points per second could be utilized while scanning over a 40 x 40 field of view using two internal deflection mirrors, producing a camera-type configuration. The measured data was written on interchangeable flash cards which were then transferred to a PC for post-processing. An infra-red interface allowed a palmtop computer to be used for set-up and control purposes. The main case of the ILRIS-3D instrument also contained a bore-sighted small-format digital camera giving a 640 x 480 pixel image. This camera also included an LCD viewfinder fitted to the back of the instrument that could be used both for set-up purposes and for the display of the captured data. Options included a differential GPS unit and an attitude measurement system. 29 Fig. 16 – An early model of the Optech ILRIS-3D laser scanner. Fig. 17 – The later ILRIS-36D laser scanner with its motorized pan and tilt base and its carrying handle. (Source: Optech) In 2004, a substantially upgraded model of the ILRIS-3D instrument was introduced. This provided an increased range (beyond 1,000 m to highly reflective targets); an improved accuracy; an integrated CMOS-based camera giving a 6 Megapixel image; and an integrated handle for carrying purposes. As noted above, the ILRIS-3D was a camera-type instrument with a fixed field- of-view (FOV) of 40 x 40. In order to cover a much larger area, Optech also introduced the ILRIS-36D version of the instrument, the first examples of which were shipped in May 2005. This instrument was equipped with a motorized pan and tilt base that allowed the scanner to cover a 360 x 230 field-of-view (Fig. 17). For this to be implemented, the motorized base unit moves the scanner unit of the ILRIS-3D with its 40 x 40 FOV in a series of steps that are measured by angular encoders. Each 40 x 40 scan patch or window overlapped on its neighbours by 5. However, in September 2007, a new profiling feature eliminated the need for these overlaps using multiple scan windows. The basic ILRIS-3D instrument continues to be developed. In October 2006, two new optional features were introduced. The Enhanced Range (ER) option increases the range still further by 40% for use in open-cast mining and large-area topographic surveys. The Motion Compensation (MC) option is designed to allow the ILRIS-3D instrument to be used from a moving or dynamic platform such as a boat or a vehicle and is based on the use of an additional integrated GPS/IMU system to provide the position and orientation data required for the motion compensation. Finally 30 in March 2007, Optech introduced a so-called Value Package (VP) version of the ILRIS-3D which retains the main capabilities of the instrument in respect of its range and accuracy of measurement, but in a less expensive form. For the processing of the measured ILRIS-3D data, Optech utilizes the Quick Terrain Modeler from the Applied Imagery company based in Maryland, U.S.A. and modules from the Polyworks software package developed by another Canadian company, InnovMetric Software from Quebec. Riegl This company builds a number of long-range laser scanners based on pulse ranging that are entitled the LMS-Zxxx series. Currently this series comprises four different models - the LMS-Z210ii, LMS-Z390i, LMS-Z420i and LMS-Z620 - having maximum measuring ranges of 650 m, 400 m, 1,000 m and 2,000 m respectively with objects having a reflectance of 80%. They will be treated here together since they all have similar design characteristics. For objects with a lower reflectance, the ranges measured by the four instruments will be much less - 200 m, 100 m, 350 m and 650 m respectively for a reflectivity value of 10%. The manufacturer's claimed accuracy of distance measurement is ±4 to 15 mm for a range of 50 m. All four LMS-Zxxx instruments utilize the same type of laser engine with a continuously rotating optical polygon that is placed in front of the laser rangefinder to produce the basic optical scan - as used also in Riegl's airborne laser scanning engines. The rangefinder itself uses a Class 1 laser operating in the near infra-red part of the spectrum either at λ = 905 or 1550 nm. However, with the ground-based systems, the rangefinder is placed in a vertical position pointing upwards instead of the horizontal position that is used with the airborne version (Fig. 18c). 31 The pulses are then directed towards the ground and its objects at the appropriate vertical angle by the rotating optical polygon. Thus the angular scan takes place within the vertical plane instead of the plane containing the cross-track scan of the airborne versions of the laser scanning engine. The angular scanning range of the ground-based laser engine is 80 - which is normally implemented to provide ±40 above and below the horizontal plane when the rangefinder is set pointing vertically upwards in its normal operating position. For the required horizontal angular rotation, the upper part of the instrument containing the rotating optical polygon head is moved in steps in azimuth through the full 360 rotation against the fixed lower part of the instrument that contains the laser rangefinder. This angular coverage of 360 x 80 places the instrument in the hybrid class of laser scanners - as defined above in Section 3.1.1. The rotational steps in azimuth within the horizontal plane - which define the intervals between successive vertical scan lines - can be set at different angular intervals, for example, between 0.004 and 0.075 in the case of the LMS-Z420i instrument. The maximum measuring rates that can be achieved using pulse ranging with these LMS-Zxxx models lies between 8,000 and 12,000 points per second, although often lower rates will be used in actual practice. The measured data will be transmitted via the instrument‟s built-in Ethernet interface and recorded on a laptop computer. It can then be processed using Riegl's own RiSCAN Pro software package that comes bundled with the laser scanners. A GPS-Sync option is available for the LMS-Zxxx series which allows each set of measurements to be time-stamped with respect to GPS time. This allows the integration of the laser scanner with a GPS/IMU system to provide position and orientation data, as required for dynamic scanning operations. 32 Fig. 18 (a) – Riegl LMS-Z210i laser scanner sitting on a tribrach. (Source: Riegl) Fig. 18 (b) - This example of a Riegl LMS-Z390i laser scanner is shown with an additional mount having a horizontal axis around which the main part of the instrument can be rotated and operated at an oblique angle to the vertical – if that is required. This particular example is also fitted with a calibrated small-format digital frame camera producing coloured images. (Source: Riegl) Fig. 18 (c) – A diagram showing the general layout of the Riegl LMS-Zxxx series of terrestrial laser scanners (Source: Reigl; re-drawn by M. Shand) The Riegl laser engine with its rotating optical polygon head can be mounted on a conventional survey tribrach and operated with its main optical axis set in a vertical position using the tribrach footscrews (Fig. 18a). However the four LMS-Zxxx models can all be fitted optionally with a 33 mount with two vertical posts that provides an additional horizontal rotation axis that allows the scanner engine as a whole to be inclined at an oblique angle to the vertical axis (Fig. 18b). The various models can also be equipped optionally with a small-format CCD digital frame camera that sits on a mount on top of the main scanner engine (Fig. 18b & c). Currently users are offered a choice between the Nikon D70s or D100 models (providing a 6.1 Megapixel image), the D200 model (giving a 10.2 Megapixel image) or the D300 model (giving a 12.3 Megapixel image). If a still larger-format image is required, then the Canon EOS-1Ds Mark II camera with its 16.7 Megapixel frame image is offered as an alternative. The resulting colour images are transmitted via a USB interface to the laptop computer where the image data can be fused with the range data from the laser scanner. Fig. 19 (a) – The external design arrangement of the Riegl LPM-321 laser profiler with the double transmitting and receiving optics of the rangefinder attached to the vertical pillar; the sighting telescope mounted on top of the rangefinder unit; and the camera attached to the side of the 34 rangefinder; and (b) a photograph of the instrument. (Source: Riegl) Besides the LMS-Zxxx series of scanners, over the last decade, Riegl has also manufactured a series of long-range laser profile measuring systems. In 2008, it introduced the latest model in the series, which is called the LPM-321 profiler. This has a very different mechanical and optical design to that of the LMS-Zxxx scanners with the laser rangefinder unit mounted on the horizontal (trunnion) axis of the instrument, supported by a single pillar (Fig. 19a). The rangefinder can measure ranges up to a maximum distance of 6,000 m to targets with 80% reflectivity (without the use of a reflector) and can be operated to measure profiles either manually or in an automated mode. It also offers a full waveform digitizing capability in the same manner as an airborne laser scanner. In its automated mode of operation, the LPM-321 can measure up to 1,000 points per second. The instrument has a sighting telescope with up to 20x magnification that sits on top of the rangefinder and can have a calibrated digital camera fitted to the side of the rangefinder (Fig. 19b). I-SiTE The I-SiTE company was founded in 1999 as a subsidiary of the Maptek company based in Adelaide in South Australia. Maptek is the developer of the Vulcan 3D geological modelling software that is used extensively in the mining industry world-wide. The I-SiTE company was formed specifically to support and develop Maptek's early interest and activity in laser scanning. The I-SiTE Studio software for use with ground-based laser scanned data was an early product from the company, often sold to users together with Riegl terrestrial laser scanners as a bundled package. 35 Fig. 20 - An example of an I-SiTE model 4400 laser scanner. (Source: I-SiTE) However, after two or three years of development, I-SiTE brought out its own Model 4400 laser scanner (Fig. 20). This is a hybrid-type scanner having a motorized rotation of 360 in azimuth and 80 angular coverage in the vertical plane. The stated angular accuracy is ±0.04. The instrument's rangefinder is based on a Class 3R laser which emits its pulses at λ = 905 nm with a power of 10 mW and measures its ranges at a maximum rate of 4400 points per second. The maximum range with highly reflective surfaces is 700 m, This value will of course decrease somewhat to 600 m with rock and concrete surfaces with 40 to 50% reflectivity and go down to 150 m in the case of black coal surfaces with only 5 to 10% reflectance. The claimed accuracy in range is ±5 cm at a range of 100 m. The intensity values of the return signals after reflectance can also be measured and recorded. Furthermore the I-SiTE 4400 instrument also incorporates a digital panoramic line scanner equipped with a Nikon f = 20 mm lens that produces a 37 Megapixel linescan image that is acquired concurrently during the laser scanning/ranging operation. The measured data is transferred via an Ethernet interface to be recorded on an external PC tablet computer. The corrected image is automatically rendered on to the 3D surface produced by the range and angular 36 data using the I-SiTE Studio software. The instrument also incorporates a viewing telescope with 14x magnification that can be used for alignment and back-sighting operations. Slightly different versions of the instrument are available - (i) the 4400LR for use in the longer ranges encountered in topographic and open-cast mining applications; and (ii) the 4400CR for use in the close ranges encountered in police and forensic applications. Measurement Devices Ltd. This company, based in Aberdeen and York in the U.K., has specialized in the manufacture of laser-based measuring systems since its foundation in 1983. Its products include a wide variety of hand-held laser distance measuring instruments for use in forestry, agriculture, etc.; various fan- beam laser systems for positioning ships and ensuring collision avoidance at sea; and laser scanning devices for use underground within cavities and voids. Nearer to the subject of this chapter, the company has also produced its Quarryman instrument that has been used extensively for many years in quarries and open-cast mines world-wide to measure profiles across rock faces employing laser ranging techniques. 37 Fig. 21 – On the left is the MDL LaserAce laser scanner; on the right is the Quarryman model. (Source: MDL) A re-developed version of this instrument was introduced in 2004 in the form of the LaserAce Scanner, (Fig. 21), which is designed to generate point cloud data in the manner of the other laser scanners being discussed in this section. The LaserAce Scanner uses a rangefinder that is mounted on the instrument‟s horizontal (trunnion) axis supported by two pillars or standards in the manner of a theodolite telescope. The rangefinder utilizes a Class 1 semi-conductor laser emitting its pulses at λ = 905 nm in the near infra-red part of the spectrum. This can measure a maximum range of 700 m to a moderately reflective target and 5 km with a prism reflector with a measuring resolution of 1 cm and an accuracy of ±5 cm. The instrument can be operated manually as a total station using a telescope that is mounted on top of the rangefinder. Alternatively it can be operated in an automated (robotic) mode as a laser scanner using its built-in motors, in which case, it can measure objects at a rate of 250 points per second. The corresponding angles are measured using horizontal and vertical angular encoders. The full angular coverage provided by the instrument is 360 x 135. However specific areas can be measured through the prior definition of a rectangle or polygon by the operator using the instrument's built-in numeric keyboard. The measured range and angular data is stored on a standard interchangeable flash card. The customized version of this instrument that is oriented towards MDL's traditional quarrying and open-cast mining market is called the Quarryman Pro. The instruments use the MDL Logger software on Psion PDA and Pocket PC computers to record the measured data and the company's Laser Cloud Viewer for the processing 38 of the data. Further processing to form contours and sections and to compute volumes can be carried out using MDL's Face 3D Modeller software. Trimble In June 2007, Trimble introduced its VX Spatial Station (Fig. 22). This instrument is constructed and operates on an entirely different concept to that of the company‟s GS and GX series of scanners described above. Basically it can be viewed as an automated (robotic) total station equipped with an imaging and scanning capability. The instrument is equipped with a conventional surveyor's telescope rather than a scanning mirror. The total station capability of the instrument that is used for positioning and traversing provides a ±1 second angular accuracy and a ±3 mm distance accuracy with a maximum range of between 300 to 800 m without a prism and 3 km with a single prism. The scanning is carried out at a rate of 5 to 15 measured points per second with a scanning range of up to 150 m. The tiny on-board digital frame camera is fitted to the underside of the telescope with its optical axis parallel to it. This calibrated camera produces HDTV images that are 2,048 x 1,536 pixels in size and can be recorded in JPEG format at a maximum 5 FPS on the instrument's removable data collector unit. Obviously the instrument is basically a total station that is designed to provide surveyors with laser scanning and imaging capabilities over limited areas or specific objects such as buildings, rather than carrying out the systematic scanning of large areas or objects that the Trimble GS/GX series is designed specifically to perform. 39 Fig 22 – The Trimble VX Spatial Station. (Source: Trimble) III.2 – Dynamic Terrestrial Laser Scanners The subject of vehicle-borne terrestrial laser scanners is an exciting one to explore. It forms only a part of a much wider subject area – namely that of conducting surveying and mapping operations from dynamic vehicular platforms. Up till now, this wider subject area has been concerned mainly with the imagery acquired from these platforms using multiple video and digital cameras in combination with the data acquired concurrently for direct geo-referencing purposes by integrated GPS/IMU units. This is then followed by the photogrammetric evaluation of the imagery on a highly automated basis to deal with the enormous number of small-format images that are collected during these operations. Initially this activity was a pioneering research effort carried out in the late 1980s by the Center for Mapping of the Ohio State University using its GPSVan (Bossler & Toth 1996; Toth & Grejner-Brzezinska 2003; Toth & Grejner-Brzezinska, 2004). It was joined somewhat later by an independent but parallel project at the University of Calgary using its similar VISAT van. Other similar projects were undertaken by various European universities and institutions such as the EPFL, Lausanne (Photobus) and the ICC, Barcelona 40 (GeoVan). These various research efforts have resulted in the establishment of a number of commercial companies carrying out mapping from vehicles using the purely camera-based technology, especially in North America, with companies such as Lambda Tech, Transmap, Facet, DDTI, Mandli (U.S.A.) and Geo3D (Canada). These are concerned principally with the collection of geospatial data for road inventory purposes. The development and use of laser scanners on vehicular platforms either instead of or in conjunction with cameras has taken place somewhat later. An early research effort was the Vehicle-borne Laser Mapping System (VLMS) of the Centre for Spatial Information Science of the University of Tokyo (Manandhar & Shibasaki 2001, 2003). The camera-based GeoVan of the ICC, Barcelona was transformed into the Institute‟s Geomobil project (Talaya et al 2004). Both of these projects featured multiple cameras and laser scanners for data collection purposes. Only very recently, over the last two or three years, have these largely research-oriented projects resulted in the introduction of vehicular-based systems using laser scanners that operate on a commercial basis. Now vehicular-based systems are available for purchase that can be used routinely by service providers. There appears little doubt that this is merely the start of what should become a commonly available service – at least in the more advanced industrialized countries. In this introduction to this section, the use of laser scanners on vehicles for non-topographic purposes should also be mentioned – in particular, their use in navigation and collision avoidance systems. If these come to be mass-produced and used widely, then undoubtedly this will have a strong influence, especially with regard to the costs of laser scanners that can be mounted on vehicles. A further mention should also be made of the influence of the DARPA Grand Challenge 41 (DGC) and Urban Challenge (DUC) contests that have hastened the development of these navigation and collision avoidance technologies for use in unmanned vehicles. There is a striking difference between the imaging sensor configurations used in the first DGC held in 2004 and in the recent DUC held in 2007, respectively. In 2004, digital cameras represented the primary sensors to observe the object space around the vehicles and stereo techniques were widely used for object space reconstruction. Laser profilers, predominantly SICK models, were only used to supplement the image sensing capabilities. The situation was reversed by the 2005 DGC, when the winner, the Stanley vehicle from Stanford University, used five SICK roof-mounted laser scanners to map the area in the front of the vehicle and cameras were only used at high speed to look ahead for objects that were farther away than 40 m. Fig. 23 – (a) The Velodyne HDL-64E laser scanner. (Source: Velodyne) Fig. 24 - Diagram showing the main external features of the Velodyne HDL-64E spinning laser scanner (Drawn by M. Shand) Recognizing the potential of laser scanning, manufacturers developed dedicated laser scanners for 42 the 2007 DUC. The Ibeo system provided laser profiling capabilities in four planes, thus replacing four SICK units. Then, more importantly, the introduction of the Velodyne laser scanner, which was used by the winner, the Boss, a fully autonomous Chevy Tahoe vehicle from Carnegie Mellon University, and by most of the top ten participants, represented a major technological breakthrough in 2007. The Velodyne HDL-64E High Definition Lidar (Fig. 23) is based on using 64 laser units covering a 26.8 vertical spread, thus eliminating the need for any vertical mechanical motion (Fig. 24). The system sports high horizontal rotation rates of the laser sensors around the vertical axis of the unit, at up to 15 Hz, with an angular resolution of 0.09. The Class 1 laser operates at the wavelength of 905 nm with a 10 ns pulse width. The ranging accuracy is claimed to be less than 5 cm for 50 m and 120 m with reflectivities of 10% and 80%, respectively. The data collection rate of more than one million points per second is simply amazing. In summary, these unmanned vehicles (Figs. 25 & 26) have made extensive use of the laser scanners and integrated GPS/IMU systems which are the building blocks of the vehicular systems that are being developed for topographic applications (Ozguner et al 2007). 43 Fig. 25 – The Velodyne HDL-64E spinning laser scanner (circled at top) together with a Riegl scanner placed in front and to the left of it, both mounted on the Stanford University Junior vehicle (a Volkswagen Passat) that finished second in the DARPA 2007 Urban Challenge. (Source: Stanford Racing Team) Fig. 26 - A combination of a Velodyne scanner and twin Riegl scanners mounted on another DARPA Urban Challenge vehicle. (Source: ) The account that follows this introduction will discuss (i) commercially available systems; (ii) custom-made systems; and (iii) some representative research systems that utilize vehicle-mounted laser scanners for topographic applications – in much the same way as has been done with the airborne laser scanning systems covered in Chapter Two, with which they share many features in common. III.2.1 Commercial System Suppliers Optech As already mentioned above, Optech has developed a special Motion Compensation (MC) version of its ILRIS-3D laser scanner for use on dynamic or moving platforms. This system has been used, for example, by the Sineco company in Italy for surveys of an open cast mine and for road surveys conducted from a moving van (Zampa & Conforti 2008). Besides the ILRIS-3D laser scanner unit that generates the range, intensity, angle and time data, the overall system includes an Applanix POS/LV 420 GPS/IMU sub-system that provides the position and orientation data against time. The Polyworks software package is used by Sineco to carry out the subsequent processing of all the captured data. 44 Fig. 27 – The LYNX Mobile Mapper scanning unit. (Source: Optech) Towards the end of 2007, Optech has also released a completely new product, called the LYNX Mobile Mapper (Fig. 27). This is a purpose-built spinning laser profiling system designed specifically for attachment to standard vehicle roof racks with mounts for two laser scanners and two (optional) calibrated frame cameras as well as the system IMU and GPS antenna. The third dimension comes from the vehicle motion. Although the scanners used in the LYNX system also utilize a Class I laser as the basis for their laser rangefinders, they are very different units to those used in the ILRIS-MC, having a maximum range of 100 m; a full 360 angular coverage; a pulse measuring rate of 100 kHz; and a scan rate of 9,000 rpm (150 Hz). The system control unit with its embedded navigation solution is based on the Applanix POS/LV 420 sub-system and can control up to four laser scanners simultaneously using the laptop computer attached to the unit. The Applanix POSPAC software is used to process the POS/LV data while Optech supplies its own LYNX-Survey and LYNX-Process software for final post-processing. Optech has announced that the LYNX systems are already being supplied to the Infoterra mapping company based in the U.K. 45 and to the Italian Sineco company which already operates an ILRIS-MC system as described above. 3D Laser Mapping This company, which is based in Nottingham in the U.K., has developed its portable StreetMapper system specifically for use on moving vehicles in close collaboration with the German systems supplier, IGI, which produces the LiteMapper airborne laser scanning system (Hunter et al 2006; Kremer & Hunter 2007). For the StreetMapper, IGI supplies its TERRAcontrol GPS/IMU system, which is derived from its AEROcontrol unit, together with its own hardware and software solutions for the control of the laser scanners and data storage. The control unit is housed in a cabinet that is mounted inside the vehicle. IGI also contributes its TERRAoffice software (derived from its AEROoffice package) for the processing of the IMU data, while the differential GPS data is processed using the GrafNav package from the Waypoint division of NovAtel based in Canada. The TerraScan/ TerraModeler/ TerraMatch suite of programs from Terrasolid in Finland is utilized for the processing of the laser scan data and its transformation into the final 3D model data. The multiple laser scanner engines are supplied by Riegl, between two and four of its LMS-Q120 units (with their 150 m range) being fitted on to a roof rack together with the IMU and the GPS antenna (Figs. 28 and 29). Either video or digital still cameras can be supplied to generate higher quality images that supplement the laser scanned data. Touch screen displays installed on the dashboard of the vehicle are used for the display of the captured data (Fig. 30). A StreetMapper system has been used extensively by Reality Mapping, a service company based in Cambridge, England, to carry out corridor surveys along roads for highway asset management and to capture street level data in city centres in the U.K. 46 Fig. 28 – This StreetMapper van is equipped with its roof rack supporting four Riegl laser scanners – two sideways-facing; one upwards-facing and one downwards-facing – and two small-format cameras; together with the IMU and GPS antenna of the position and orientation system supplied by IGI. (Source: 3D Laser Mapping) 47 Fig. 29 – Another example of a StreetMapper van with a protective cover mounted over its roof rack and contents. The windows allow the pulses from the laser scanners to be transmitted and received and the camera images to be exposed. (Source: 3D Laser Mapping) Fig. 30 – Two of the touch screen displays that are mounted on the dashboard of this StreetMapper van. (Source: 3D Laser Mapping) III.3.2 Custom-Built & In-House Operated Systems Tele Atlas This company, which is based in Ghent, Belgium, is a leading supplier of digital road map data for use in vehicle navigation and location-based systems. It operates fleets of vans that continually acquire data for the revision of its digital map database. In this context, between 15 to 20% of the information on roads contained in the database needs to be revised annually. For this purpose, Tele Atlas has a fleet of 22 camper vans (in an eye-catching orange colour!) operating throughout Europe. Each van is equipped with six digital cameras, a GPS receiver that uses the Fugro OmniSTAR service; a gyro unit; and a distance measuring device attached to the rear wheel of the van for use when the GPS signals are lost. The resulting data is processed and analyzed in data centres located in Poland and India. 48 Fig. 31 – A Toyota mini-van of Tele Atlas North America equipped with a roof rack on which the system‟s cameras, laser scanners and GPS antenna are mounted. (Source: Tele Atlas) Fig. 32 – The inside of the mini-van showing the system control unit mounted at the rear and the display monitor at the front beside the dashboard. (Source: Tele Atlas) In North America, a fleet of nine smaller vans is used. Each van is equipped with a roof rack containing the cameras, GPS antenna, etc. as before. However each system also features twin laser scanners that are pointed to the side of the van to continuously collect street-level range data as the van travels forward (Figs. 31 & 32). The laser scanners are supplied by the Swiss SICK company, which is well known as a supplier of short-distance laser scanners that are used for navigation and safety (warning) purposes on vehicles (such as fork-lift trucks) and potentially dangerous equipment (such as cranes and cutting and bending machines) being operated in factories. However SICK and its two German subsidiaries and partners, LASE GmbH and Ibeo, also manufacture laser scanners that can measure over longer distances and some of these models are used in the Tele Atlas mobile mapping vans that are being operated in North America (Figs. 33 & 34). As discussed above, these SICK and Ibeo laser systems have also been used extensively on the unmanned vehicles taking part in the DARPA Grand and Urban Challenge competitions (Ozguner et al 2007). 49 Fig. 33 – Showing cameras and a laser scanner mounted on the roof rack of the Tele Atlas mini- van that is being used as a mobile mapping system. (Source: Tele Atlas) Fig. 34 – A close-up photo of one of the SICK laser scanners that is being used in the mobile mapping system. (Source: Tele Atlas) Terrapoint Another interesting custom-built vehicle mapping system employing a laser scanner is the so- called SideSwipe system devised by the Canadian Terrapoint company for use in road surveys and mapping in Afghanistan (Newby & Mrstik 2005). This system is a modified version of one of the company‟s ALMIS-350 airborne laser scanners already discussed in Chapter Two. The laser scanner was mounted rigidly on a pole attached to the side of an open-backed pick-up truck (Fig. 35) which also contained the systems‟ two computers; the one used for route navigation, the other for system control and logging purposes. The system also included a high-resolution video camera that was integrated with the GPS/IMU system. The latter comprised an HG1700 tactical-grade IMU from Honeywell that is connected to a NovAtel GPS receiver. The operational procedure involved first driving down the road with the laser scanner pointing forwards and tilting slightly downwards, so providing a horizontal scan or swath with a 60 angular coverage. During a second pass along the same stretch of road, the scanner was rotated and reset to scan vertically from the side of the vehicle producing side-scan coverage that extended from 5 to 100 m to the side of the vehicle. The same set-up was used on a third pass along the same section of road, but pointing to the other side (Fig. 36). The three pass solution provided the data for the mapping of the highway and the corridor on both sides of the road. 50 Fig. 35 - The truck-mounted SideSwipe laser scanner system of Terrapoint has been used for mapping purposes along main highways in Afghanistan. (Source: Terrapoint) Fig. 36 - A diagrammatic representation of the three-pass solution that is used to provide coverage both of the road and the terrain on both sides of the highway (Source: Terrapoint). Based on this successful initial activity, Terrapoint has developed its TITAN (Tactical Infrastructure & Terrain Acquisition Navigator) system (Glennie 2007), designed to overcome the limitations of the multiple passes that were required with the SideSwipe system and the limited accuracy of its tactical-grade IMU. It features an equipment pod containing the system‟s laser scanners, IMU, GPS receivers and digital cameras that is mounted on a hydraulic lift attached to the floor of an open-back pick-up truck (Fig. 37). The system comprises an array of four Riegl laser scanners; a higher-grade IMU from iMAR coupled to a NovAtel GPS receiver; and up to four digital video or frame cameras (Fig. 38). The data collected by these various instruments is passed via cables to the data logging computers installed within the truck‟s cabin. Terrapoint uses its own 51 software package called CAPTIN (Computation of Attitude and Position for Terrestrial Inertial Navigation) to carry out the post-processing of the measured GPS/IMU data. Terrapoint has entered into a partnership with the Neptec Design Group based in Ottawa, Canada, under which Neptec will support the further development of the TITAN system through the supply of its analytical software algorithms. Neptec will also license the TITAN technology to produce vehicle- mounted laser scanner systems designed specifically for use by military and homeland security agencies. Fig. 37 – The open back of this pick-up truck forms the base of the hydraulic lift supporting the equipment pod of the Terrapoint TITAN mobile mapping system. (Source: Terrapoint) Fig. 38 – A block diagram of the TITAN system that shows the relationship of its various hardware components to one another. (Source: Terrapoint) III.3.3 Research Systems As noted above, several vehicle-based laser scanning systems have been constructed and operated 52 by university departments for research purposes. Two of these will be discussed in some more detail as exemplars of such systems. University of Tokyo The Centre for Spatial Information Sciences at the University of Tokyo has been undertaking research into Vehicle-borne Laser Mapping Systems (VLMS) under the leadership of Prof. Shibasaki and with the collaboration of the Asia Air Survey Co. Ltd. since 1999 (Inaba et al 1999). The first prototype system comprised four laser scanners, three of which scanned vertically while the fourth carried out a horizontal scan. The resulting laser scan data was supplemented by the image data captured by four CCD cameras, each producing small-format (640 x 480 pixels) images. A GPS receiver allied to an IMU and a shaft-driven precision odometer produced the required position and orientation data (Manandhar & Shibasaki 2000). The system has been steadily improved, using laser scanners with higher frequencies and greater angular ranges. Furthermore the digital frame cameras have been replaced by pushbroom line scanners that use the forward motion of the vehicle to produce continuous linescan imagery (Manadhar & Shibasaki 2003, Zhao & Shibasaki 2003). More recent developments are reported in detail in a paper by Zhao & Shibasaki 2005). The current laser scanners are LD-A models produced by Ibeo. These have a maximum range of 100 m, a scan frequency of 20 Hz and an average error in range of ±3 cm. Each of the pushbroom line scanners is equipped with a fish-eye lens giving an angular field of 180 and captures its image data at the rate of 80 Hz. The laser scanners and pushbroom line scanners are all mounted on the roof rack of the GeoMaster vehicle with their scanning planes oriented at different angles to reduce the occlusions caused by trees and other obstacles (Fig. 39). Tests were carried out to map building facades in the Ginza district of Tokyo. 53 Fig. 39 – Diagram showing the layout and orientation of the six pushbroom line scanners, three laser scanners and the GPS antenna mounted on the roof of the GeoMaster vehicle used by the Centre for Spatial Information Sciences of the University of Tokyo. (Source: Univerity of Tokyo; re-drawn by M. Shand) Institut Cartogràfic de Catalunya (ICC) The Cartographic Institute of Catalonia (ICC) is the organization that is responsible for the official topographic mapping and cartographic activities in Catalonia, Spain. The Institute has been developing its own mobile mapping system since 2000. Initially this comprised its Geovan equipped with two small-format (one Megapixel) digital frame cameras producing monochrome images together with an Applanix based GPS/IMU position and orientation system (Talaya et al 2004). In September 2003, a laser scanner was added to the system, after which it was known as the Geomobil system (Fig. 40). The system‟s laser scanner is the Reigl Z-210 model that can collect data at rates up to 10 kHz with a vertical angular coverage of 80 and a horizontal coverage 54 of ±166.5 (Alamus et al 2004, Talaya et al 2004). As reported in a further paper by Alamus et al (2005), the system now has one IMU and two GPS receivers, the one a dual-frequency model for position determination, the other a single-frequency model to help improve the heading angle (azimuth) determination – forming part of the POS sub-system. The Applanix POSPAC software is used to process the GPS data. The Geomobil vehicle also has a distance measurement indicator (DMI) attached to one of the van‟s rear wheels to provide measurements of the distance traveled by the van. A further development is the installation of new digital colour cameras pointing both forward and backwards from the van (Fig. 41). The Geomobil system is being used to make a geo- referenced inventory of the ground floor facades (in colour) of all the buildings in the city of Barcelona, covering 3,800 individual streets amounting to a linear distance of 1291.5 km. Fig. 40 – The ICC Geomobil van with its roof rack on which the cameras, laser scanner and GPS antennas are mounted. (Source: ICC) Fig, 41 – The specially constructed roof rack of the Geomobil with the digital cameras mounted at the rear corners of the frame, video cameras at the sides, the GPS antennas in the middle of the frame and at the front right, while the Riegl Z-210 laser scanner is placed at the front centre. (Source: ICC). 55 References & Further Reading Static Terrestrial Laser Scanners Blais, F., 2004 – Review of 20 Years of Range Sensor Development. Journal of Electronic Imaging. Vol. 13, No. 1: p. 231-240. Bohm, J. & Haala, N., 2005 – Efficient Integration of Aerial and Terrestrial Laser Data for Virtual City Modelling Using Lasermaps. Proceedings ISPRS WG III/3, III/4, V/3 Workshop “Laser Scanning 2005”, Enschede, The Netherlands, September 12-14, 2005: p.192-197. Ingensand, H., 2006 – Metrological Aspects in Terrestrial Laser-Scanning Technology. 3rd IAC / 12th FIG Symposium, Baden, Germany, May 22-24, 2006: 10 pages. Kersten, Th., Sternberg, H. & Mechelke, K., 2005 - Investigations into the Accuracy Behaviour of the Terrestrial Laser Scanning System Trimble GS100. Optical 3D Measurement Techniques VII, Gruen, A. & Kahmen, H. (Eds.), Vol. 1: p. 122-131. Lichti, D., Gordon, S. & Stewart, M., 2002 – Ground-Based Laser Scanners: Operation, Systems & Applications. Geomatica, Vol. 56, No. 1: p. 21-33. Lichti, D., Brustle, S., & Franke, J., 2007 - Self Calibration and Analysis of the Surphaser 25HS 3D Scanner. Strategic Integration of Surveying Services, FIG Working Week 2007, Hong Kong SAR, China, May 13-17, 2007: 13 pages. 56 Mechelke, K., Kersten, T. & Lindstaedt, M., 2007 - Comparative Investigations into the Accuracy Behaviour of the New Generation of Terrestrial Laser Scanning Systems. Optical 3-D Measurement Techniques VIII, Gruen, A. & Kahmen, H., (Eds.), Zurich, July 9-12, 2007, Vol. I: p. 319-327. Mettenleiter, M., Hartl, F. & Fröhlich, C., 2000 – Imaging Laser Radar for 3-D Modelling of Real World Environments. International Conference on OPTO / IRS2 / MTT, Erfurt, Germany, 11th May, 2000: 5 pages. Pfeifer, N. & Briese, C., 2007 - Geometrical Aspects of Airborne Laser Scanning and Terrestrial Laser Scanning. International Archives of Photogrammetry & Remote Sensing, Vol. 36, Part 3, W52: p. 311-319. Schulz, T. & Ingensand, H., 2004 – Influencing Variables, Precision and Accuracy of Terrestrial Laser Scanners. INGEO 2004 & FIG Regional Central and Eastern European Conference on Engineering Surveying, Bratislava, Slovakia, November 11-13, 2004: 8 pages. Staiger, R., 2003 – Terrestrial Laser Scanning – Technology, Systems and Applications. 2nd FIG Regional Conference, Marrakech, Morocco, December 2-5, 2003: 10 pages. Sternberg, H., Kersten, T., Jahn, I. & Kinzel, R., 2004 - Terrestrial 3D Laser Scanning – Data Acquisition and Object Modelling for Industrial As-Built Documentation and Architectural 57 Applications. International Archives of Photogrammetry, Remote Sensing & Spatial Information Sciences, Vol. 35, Commission VII, Part B2: p. 942-947. Dynamic Terrestrial Laser Scanners Alamús, R., Baron, A., Bosch, E., Casacuberta, J., Miranda, J., Pla, M., Sànchez, S., Serra, A., & Talaya, J., 2004 – On the Accuracy and Performance of the Geomobil System. International Archives of Photogrammetry & Remote Sensing, ISPRS Comm. IV, Vol. 35, Part B5: p. 262- 267. Alamús, R., Baron, A., Casacuberta, J., Pla, M., Sánchez, S., Serra, A. & Talaya, J., 2005 – Geomobil: ICC Land Based Mobile Mapping System for Cartographic Data Capture. Proceedings 22nd International Cartographic Conference, Corunna, Spain: 9 pages. Bossler, J. & Toth, C., 1996. - Feature Positioning Accuracy in Mobile Mapping: Results Obtained by the GPSVan™. International Archives of Photogrammetry and Remote Sensing, ISPRS Comm. IV, Vol. 31, Part B4: p. 139-142. Glennie, C., 2007 – Reign of Point Clouds: A Kinematic Terrestrial Lidar Scanning System. InsideGNSS, Vol. 2, No. 7: p. 22-31. Hunter, G., Cox, C. & Kremer, J., 2006 - Development of a Commercial Laser Scanning Mobile Mapping System – StreetMapper. 2nd International Workshop, The Future of Remote Sensing, Antwerp, Belgium, 17-18 October 2006: 4 pages. 58 Inaba, K., Manandhar, D. & Shibasaki, R., 1999 – Calibration of a Vehicle-based Laser/CCD Sensor System for Urban 3D Mapping. Proceedings 20th Asian Conference on Remote Sensing (ACRS), Hong Kong, China: 7 pages. Kremer, J. & Hunter, G., 2007 – Performance of the StreetMapper Mobile LiDAR Mapping System in “Real World” Projects. Photogrammetric Week „07. (Ed. D. Fritsch): p. 215-225. Manandhar, D. & Shibasaki, R., 2000 – Geo-Referencing of Multi-Range Data for Vehicle-Borne Laser Mapping System (VLMS). Proceedings 21st Asian Conference on Remote Sensing (ACRS), Taipei, Taiwan: p. 974-979. Manandhar, D. & Shibasaki, R., 2003 – Accuracy Assessment of Mobile Mapping System. Proceedings 24th Asian Conference on Remote Sensing (ACRS), Busan, South Korea: 3 pages. Newby, S. & Mrstik, P., 2005 – LiDAR on the Level in Afghanistan: GPS, Inertial Map the Kabul Road. GPS World, Vol.16, No.7: p. 16-21. Ozguner, U., Redmill, K., Toth, C. & Grejner-Brzezinska, D., 2007 - Navigating These Mean Streets: Real-time Mapping in Autonomous Vehicles. GPS World, Vol.18, No.10: p. 32-37. Talaya, J., Bosch, E., Alamus, R., Serra, A. & Baron, A., 2004 – GeoVan: The Mobile Mapping 59 System from the ICC. Proceedings 4th International Symposium on Mobile Mapping Technology, Kunming, China, 29-31 March 2004: 7 pages. Talaya, J., Alamus, R., Bosch, E., Serra, A., Kornus, W. & Baron, A., 2004 – Integration of Terrestrial Laser Scanner with GPS/IMU Orientation Sensors. International Archives of Photogrammetry & Remote Sensing, ISPRS Comm. V, Vol. 35, Part B5: 6 pages. Toth, C. & Grejner-Brzezinska, D., 2003 – Driving the Line: Multi-Sensor Monitoring for Mobile Mapping. GPS World, Vol. 14, No. 3: p. 16-22. Toth, C., & Grejner-Brzezinska, D., 2004 – Redefining the Paradigm of Modern Mobile Mapping: An Automated High-precision Road Centerline Mapping System, Photogrammetric Engineering and Remote Sensing, Vol. 70, No. 6: p. 685-694. Zampa, F. & Conforti, D., 2008 – Continuous Mobile Laser Scanning. GIM International, Vol. 22, No. 1: p. Zhao, H. & Shibasaki, R., 2003 – A Vehicle-borne Urban 3D Acquisition System Using Single- row Laser Range Scanners. IEEE Transactions, SMC Part B: Cybernetics, Vol. 33, No. 4: p. 658- 666. Zhao, H. & Shibasaki, R., 2005 – Updating a Digital Geographic Database Using Vehicle-Borne Laser Scanners and Line Cameras. Photogrammetric Engineering & Remote Sensing, Vol. 71, No. 60 4: p. 415-424.