Recommended Field Procedures for RTK GPS Surveys Government of by nikeborome

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									   Technology and Environment Directorate - Road and Traffic Engineering Branch



Document No. 67/08/50                                                Issue Date 04/2005




             RECOMMENDED FIELD PROCEDURES FOR
                 REAL-TIME KINEMATIC (RTK)
           GLOBAL POSITIONING SYSTEM (GPS) SURVEYS



This document is owned and controlled by the Survey and Mapping Manager.

The Senior Geodetic Surveyor is the delegated custodian.

All comments for revision should be submitted to the Senior Geodetic Surveyor.




Authorisation

As Executive Director Technology and Environment Main Roads Western Australia, I
authorise the issue and use of this standard.



         ................................……….…
         EXECUTIVE DIRECTOR TECHNOLOGY AND ENVIRONMENT

         Date: _____/ _____/ _____



No controlled copies are issued for this standard.
The latest copy of this standard is located on the Main Roads Internet site.




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REVISION STATUS RECORD

 Page Latest                Latest                       Revision
  No Revision              Revision                Description/Reference
        No                   Date
   5     1                 07.06.04   Inserted description and reference to VRS

    6           1          07.06.04      Corrected reference to section 4.2 to 4.4
                                               Inserted reference to 67-08-53
    7           1          07.06.04      Sections 4.4, 5.1 Standard A changed to
                                                     Standard 67-08-15
                                      Inserted comment re relevance of section 5.1.1
                                                        to VRS users
    9           1          07.06.04    Section 5.1.2, third dot point, inserted “This
                                       restriction does not apply to VRS users”; 6th
                                           dot point, inserted “or VRS network”

   10           1          07.06.04   2nd dot point: Replaced “RTK GPS Survey
                                      Work Instruction A” with “section 6 of this
                                      document”; 6th dot point: Replaced “RTK GPS
                                      Survey Work Instruction A” with “section 7 of
                                      this document”; 7th dot point: removed phrase
                                      in brackets; 8th dot point: inserted a clause for
                                      VRS users and added “Date” to “start and stop
                                      time of receivers”; Inserted “VRS users must
                                      ensure they are using a GPS antenna that is
                                      approved by their VRS network provider” in
                                      third last paragraph.
   11           1          07.06.04        Minor typographic corrections applied.
                                      Sections 7 Standard A changed to Standard 67-
                                        08-15; “Note that this option is not available
                                            for VRS” inserted at bottom of page.
   12           1          07.06.04       Section 7: GDA97 corrected to GDA94.
                                               Ausgeoid web address updated.
   16                                                Appendix A inserted




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                                                        CONTENTS

1.       PURPOSE.............................................................................................................. 4

2.       SCOPE .................................................................................................................... 4

3.       REFERENCES ...................................................................................................... 4

4.       DEFINITIONS AND ACRONYMS..................................................................... 4

         4.1       Acronyms .................................................................................................... 4

         4.2       Description of RTK GPS Surveying ......................................................... 5

         4.3       The Need for Quality Control in RTK GPS Surveys .............................. 6

         4.4       Methods of RTK GPS Survey ................................................................... 7

5.       RTK GPS SURVEY PROCEDURES .................................................................. 7

         5.1       Survey Design ............................................................................................. 7

                   5.1.1 Reference Receiver and Antenna Set-up and Operation ............ 7

                   5.1.2 Roving Receiver and Antenna Set-up and Operation ................. 9

6.       RECOMMENDED RTK GPS QUALITY CONTROL
         PROCEDURES .................................................................................................... 11

7.       VERTICAL DATUM TRANSFORMATIONS ................................................ 13

         7.1       The gravimetric method .......................................................................... 13

         7.2       The geometric method.............................................................................. 14

         7.3       The gravimetric-geometric method ........................................................ 15

8.       LIAISON .............................................................................................................. 15




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1.       PURPOSE

         The purpose of this Work Instruction is to provide guidelines for survey procedures
         that will help ensure the accurate collection of field data using Real-Time
         Kinematic (RTK) Global Positioning System (GPS) survey techniques.

         It is acknowledged that the suggested survey procedures will inevitably lead to a
         decrease in the efficiency of currently adopted RTK GPS surveying techniques.
         However, if contractors and consultants to MRWA wish to ensure quality of RTK
         GPS surveys, it is strongly recommended these practices should be adopted at all
         times.


2.       SCOPE

         This Work Instruction is issued to all consultants and contractors who propose to
         use RTK GPS survey methods for work to be submitted to MRWA, unless
         otherwise approved by the Survey and Mapping Manager.

         Advice and further information can be obtained by contacting the Senior Geodetic
         Surveyor of the Road and Traffic Engineering Branch.


3.       REFERENCES

         The following documents relate to this Work Instruction:

          Real-time Kinematic GPS Standard 67-08-15
          Real-time Kinematic GPS Validation Procedures 67-08-53
          Survey and Mapping Specification 67-08-43, Digital Ground Survey
          Survey and Mapping Specification 67-08-46, Quality Statement
          Survey and Mapping Specification 67-08-36, Road Reference Marks
          Survey and Mapping Specification 67-08-37, Minor Control
          ICSM – Standards and Specifications for Control Surveys version 1.5 2002
            (SP1)
          Geocentric Datum of Australia Technical Manual
          Featherstone, W.E. and M.P. Stewart (1999) RTK GPS Evaluation, Contract
            Report to Main Roads Western Australia, Perth, Australia, 50pp.


4.       DEFINITIONS AND ACRONYMS

4.1      Acronyms

         The following acronyms used in this document have the specific meanings
         indicated.

         AGD               - Australian Geodetic Datum
         AHD               - Australian Height Datum
         AUSGeoid          - National geoid model
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         BM                -   Benchmark
         GDA               -   Geocentric Datum of Australia
         GPS               -   Global Positioning System
         GRS80             -   Geodetic Reference System 1980
         ICSM              -   Inter-governmental Committee on Surveying and Mapping
         MGA               -   Map Grid of Australia
         RM                -   Reference Mark
         RRM               -   Road Reference Mark
         RTK               -   Real-time kinematic
         SSM               -   Standard Survey Mark
         VRS               -   Virtual Reference Station
         WGS84             -   World Geodetic System 1984


4.2      Description of RTK GPS Surveying

         RTK GPS surveying is the process of determining and recording three-dimensional
         coordinates of unknown points using an RTK GPS system (ie. instrumentation and
         software/firmware). The RTK-GPS-derived coordinates must be transformed to the
         appropriate horizontal and vertical datums.

         RTK GPS systems comprise a reference receiver and antenna set up over a point
         whose three-dimensional coordinates (geodetic latitude, longitude and ellipsoidal
         height) are known with respect to a geocentric datum. The reference receiver
         telemeters appropriate information to a roving receiver whose antenna is situated
         directly and steadily above an unknown point. The coordinates of the unknown
         point, and associated internal quality indicators, are computed in „real-time‟ by the
         roving receiver and recorded by some form of data logging device.

         A recent development is the Virtual Reference Station (VRS) concept. A VRS
         network is a series of continuously operating GPS receivers located across a region
         at spacings of between approximately 10 and 100km. All data from these stations
         are transmitted, in real time, back to a master control, which can use the resultant
         information to compute a GPS error model over the entire region. Such an error
         model overcomes the limits imposed on long range RTK positioning by the growth
         of systematic errors with baseline length (this growth of error is predominantly due
         to the atmosphere). Therefore, the presence of a VRS network allows long range
         RTK positioning. A further advantage is that a VRS network can support many
         roving users simultaneously. Surveyors no longer need set up their own reference
         stations and, in fact, only require a single mobile receiver to complete a survey,
         whereas in the past they required a mobile and a base.

         RTK receivers are implicitly of geodetic quality and use dual-frequency carrier
         phase measurements as the primary GPS observables to compute positions.

         Fundamentally, RTK GPS systems measure the three-dimensional vector
         (nominally in the WGS84 geocentric Cartesian coordinate system) from the
         reference station to the unknown point. The computed three-dimensional vector is
         added to the three-dimensional coordinates of the reference station to determine the
         position of the unknown station. Therefore, the determined position of the
         unknown station is dependent on
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         a) the accuracy of the coordinates of the reference station, and
         b) the accuracy of the computed three-dimensional vector.

         The coordinates of the unknown station can be transformed to any local geodetic
         datum, provided that the transformation parameters are known. These parameters
         must be input to the RTK GPS system in order to perform a „real-time‟
         transformation, or applied at a post-processing stage.

         The appropriate horizontal transformation methods for RTK GPS surveys in
         Western Australia are described and discussed in the Geocentric Datum of Australia
         Technical Manual.

         The appropriate vertical transformation methods for RTK GPS surveys in Western
         Australia are described and discussed in Section 5.7.

4.3      The Need for Quality Control in RTK GPS Surveys

         The status of current GPS technology, in terms of algorithm development and
         hardware, is such that RTK GPS internal quality control indicators, provided by the
         proprietary software/firmware, cannot be used solely to validate GPS positioning
         accuracy.

         RTK GPS is a surveying technique that essentially offers three-dimensional
         radiations from a known point. As with any survey technique, to independently
         estimate the accuracy of the coordinates computed by radiation, redundancy in the
         form of additional observations is required.

         In the case of RTK GPS surveys, the only rigorous technique for independently
         estimating the uncertainty in computed coordinates is to make a separate
         observation at every unknown point. This could use RTK GPS corrections received
         from another reference station and at a different sidereal time to ensure that the GPS
         satellite geometry and multipath effects have changed.

         In the case of pick-up surveys (section 4.4), it is not possible to perform this type of
         quality control procedure. Therefore, fixed-point surveys (section 4.4) must be used
         in conjunction with the approach described above.

         From experience and a number of repeat RTK GPS surveys conducted on the RTK
         GPS Testing and Benchmarking Facility at Curtin University of Technology, a RTK
         GPS surveying methodology has been developed that delivers a reasonable
         probability that accurate results will be achieved.

         However, it is essential to point out that the accuracy of any GPS-derived positions
         is spatially and temporarily dependent. Therefore, there is no guarantee that just
         because a RTK GPS system delivers results of a particular standard on the RTK
         GPS Testing and Benchmarking Facility (67-08-53) that it will in all other surveys.




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         In addition, these methodologies do not guarantee that all points will be within a
         certain level of accuracy (eg. ambiguities can be fixed incorrectly, multipath can
         cause height variation of 50mm or greater). Furthermore, a problem with using
         only one independent check measurement is that it is impossible to discern between
         correct and incorrect points at definite levels of accuracy and significance.

         Accordingly, the burden of proof rests with the surveyor to ensure that the positions
         are accurate to the desired standards at the time and place of the RTK GPS surveys.

         Any one or a combination of the RTK GPS survey procedures suggested in Section
         5 of this Work Instruction should allow the user to derive survey quality control
         indicators independently of the internal quality control indicators given by RTK
         GPS receiver firmware.


4.4      Methods of RTK GPS Survey

         For the purpose of this Work Instruction and Real-time Kinematic GPS Standard
         67-08-15, two modes of RTK GPS survey are defined.

         1. Fixed-point surveys - coordination of marked points, survey monuments or
            ground features that can be accurately resurveyed. These marks are permanent
            or semi-permanent such that they may be resurveyed at some time in the future.

         2. Pick-up surveys – coordination of features that have no monumentation or
            ground features that cannot be accurately resurveyed. The distinguishing
            feature of pick-up surveys is that individual points cannot be reoccupied as part
            of a field-validation procedure.

         All coordinates derived from either mode of RTK GPS survey must be computed
         with respect to geodetic survey control on the appropriate geodetic datums.


5.       RTK GPS SURVEY PROCEDURES

5.1      Survey Design

         In practice, most surveys must be executed as a combination of the fixed-point
         surveys and pick-up surveys described in Section 4.4. It is strongly recommended
         that, where practical, 10% of the points surveyed be fixed points. These points
         should be used for independent quality checks described in Section 6.

         As per Real-time Kinematic GPS Standard 67-08-15, the fixed points must be
         occupied once every 30 minutes during the RTK GPS surveys and must be situated
         within one-kilometre of the survey area. In the case of RTK GPS surveys that use
         more than one RTK GPS base-station, it is preferable that these fixed points are
         situated such that they can be occupied from each base station in order to provide
         checks between the surveys.

         VRS users may ignore the following section (5.1.1) as VRS reference receiver
         operation is outside the user‟s control.
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5.1.1 Reference Receiver and Antenna Set-up and Operation

      Reference GPS receivers and antennae (aka RTK GPS base-stations) must be
       established such that the roving-to-reference GPS receiver separation will be no
       greater than 5km (ideally 2km) at any stage of the survey.

      Reference GPS antennae must be set up over a control point of known quality
       (either an existing SSM or a new geodetic control station established in accordance
       with Survey and Mapping Specification 2300/05/06, Minor Control, or Survey and
       Mapping Specification 2300/05/07, Road Reference Marks).

      The coordinates of the control point must be known in three dimensions (geodetic
       latitude, longitude and ellipsoidal height) with respect to the GDA94. If these
       control coordinates have been transformed from other geodetic datums, the
       transformation technique and parameters must be specified.

      The position of any existing control points must be verified by check measurements
       to RMs before they are adopted and used.

      The appropriate height of the antenna‟s phase centre must be measured, recorded,
       and input into the reference or roving GPS receiver as required by the RTK GPS
       equipment and software/firmware being used.

      There should be no obstructions to the GPS satellite signals intruding into the space
       formed by a cone of 15-degrees elevation above the horizon of the reference GPS
       antenna.

      The site of the reference GPS antenna must be uncontaminated by probable sources
       of multipath and electrical interference (eg. strong radio transmitters or high-voltage
       power lines).

      The reference receiver must be correctly configured to generate and transmit RTK
       GPS corrections compatible with the roving RTK GPS receiver(s).

      The GPS data-sampling rate should be as fast as possible (preferably one-second).

      A standard log sheet (see example in Appendix A) must be completed for each
       reference station occupation, which includes the following information.
        Name, identification number and type of control point (eg. SSM, RRM);
        Receiver make, model and receiver serial number;
        Antenna make, model and serial number;
        Firmware/software make and version number;
        Antenna set-up information (orientation, antenna height measurements [true
          vertical or slope height], antenna set-up diagram);
        Start and stop time of the receiver;
        Approximate sky visibility plot of the reference site;
        Three-dimensional coordinates of the reference point (including datums), and
          any transformation methods and parameters used;

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          Any unusual site features (including any possible sources of multipath and
           electrical interference);
          Any unusual occurrences during the survey (eg. power failure, inclement weather
           conditions).


5.1.2 Roving Receiver and Antenna Set-up and Operation

        The roving GPS receiver must be configured with the appropriate height of the
         roving GPS antenna‟s phase centre, which must be updated if the roving GPS
         antenna height is changed during the survey.

        The GPS data-sampling rate of the roving GPS receiver should be as fast as
         possible (preferably one-second) and must be the same as that of the reference GPS
         receiver.

        The roving-to-reference GPS receiver separation must be no greater than 5km
         (ideally 2km) at any stage of the survey. This restriction does NOT apply to VRS
         users although VRS users must not operate outside the area pre-specified by their
         service provider.

        There should be no significant obstructions to the GPS satellite signals intruding
         into the space formed by a cone of 15-degrees elevation above the horizon of the
         roving GPS antenna.

        The site of the roving GPS antenna must be uncontaminated by probable sources of
         multipath and electrical interference (eg. strong radio transmitters or high-voltage
         power lines).

        Current RTK GPS corrections must be received from the reference receiver or VRS
         network for each point coordinated and for the entire time that measurements are
         collected to determine that point‟s position.

        Each site to be coordinated must be occupied for a minimum of five (5) continuous
         GPS epochs (preferably 20 or more). Importantly, this occupation period must be
         the same as that used for the RTK GPS equipment and firmware/software validation
         (Section 5.1)

        A minimum of five (5) GPS satellites (preferably 6) must be observed for each
         point coordinated and for the entire time that measurements are collected to
         determine that point‟s position.

        The Position Dilution of Precision (PDoP) must be less than 6.0 for each point to
         be coordinated and for the entire time that measurements are collected to determine
         that point‟s position.

        The roving receiver must indicate that an ambiguity-fixed GPS position solution
         has been achieved for each point to be coordinated and this must be maintained for
         the entire time that measurements are collected to determine that point‟s position.

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        All other proprietary software/firmware quality indicators must be satisfied for each
         point to be coordinated and these must be maintained for the entire time that
         measurements are collected to determine that point‟s position.

        The proprietary quality indicators used must be described in the project report and
         included with the results submitted (Section 6).

        During each survey, fixed-points (beginning of Section 4.2) must be resurveyed
         once every 30 minutes. These fixed-points must be at a distance no greater than
         1km from the area in which the survey is being conducted. The resurveyed
         positions must be validated (including documentation and justification) according
         to one or a combination of the procedures in section 6 of this document.

        If the resurveyed position disagrees with the previously determined position by
         more than the positional tolerance set by MRWA, the RTK GPS system must be re-
         initialised, the fixed-point resurveyed and checked, and all points surveyed since the
         previous survey of the fixed point must be resurveyed.

        All horizontal positions must be referenced to the GDA94 (or MGA94). If these
         coordinates have been transformed, the transformation technique and parameters
         must be specified.

        All vertical positions must be determined with respect to the AHD.

        The geoid modelling technique(s) used to derive the AHD heights from GPS
         ellipsoidal heights must be documented and justified according to one or a
         combination of the procedures in section 7 of this document.

        A standard log sheet must be provided for each GPS roving receiver, which
         includes the following information.
         Receiver make, model and receiver serial number;
         Antenna make, model and serial number;
         Firmware/software make and version number;
         VRS Users only: Name of VRS network provider.
         Antenna set-up information (antenna height measurements [true vertical or slope
            height], antenna set-up diagram);
         Date, Start and stop time of the receiver;
         Number of satellites, PDoP and number of epochs used for each point surveyed
         Any unusual site features (including any possible sources of multipath and
            electrical interference);
         Any unusual occurrences (including time) during the survey (eg. power failure,
            loss of radio link, unscheduled re-initialisation, inclement weather conditions).

         Where practicable, the GPS antennas should be of the same make and model. If
         not, the RTK GPS software/firmware must be able to make appropriate corrections
         for the different phase centres. VRS users must ensure they are using a GPS antenna
         that is approved by their VRS network provider.

         Where practicable, the roving and reference GPS antennae should be oriented in the
         same direction so that common phase centre variations cancel.
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         Where practicable, the roving GPS antenna should be stabilised over the unknown
         point using a bi-pod or tripod.


6.       RECOMMENDED RTK GPS QUALITY CONTROL PROCEDURES

         The minimum recommended procedures for quality control of RTK GPS surveys
         performed for MRWA are given below. These may be viewed to be complimentary
         to the ICSM SP1 (2002) RTK specifications.

         Consultants and contractors to MRWA must provide documentary evidence that the
         specifications have been met in a way that expedites audit by MRWA.

         These procedures are only acceptable if a RTK GPS validation has been previously
         performed on the RTK GPS Testing and Benchmarking Facility at Curtin
         University of Technology using the techniques and software specified in Real-time
         Kinematic GPS Validation Procedures 67-08-53.

         Consultants and contractors to MRWA must note that conditions on the RTK GPS
         Testing and Benchmarking Facility may radically differ from the situation in the
         field, and certification will thus only validate the surveying methodology used by
         the contractor.

         In itself, this certification cannot be used to unequivocally justify the reliability of
         any other survey. Accordingly, the burden of proof is on the consultant or
         contractor to demonstrate to MRWA that quality control criteria in Real-time
         Kinematic GPS Standard 67-08-15 have been met for all points, either by direct
         analysis or unequivocal implication.


         The following survey designs and methodologies are presented in order of
         preference based on their perceived reliability. That is, the surveys that are most
         likely to yield coordinates with a reliable accuracy estimation are listed first. It is
         acknowledged that these approaches are not necessarily suited to all contract
         surveys for MRWA. Therefore, surveyors may use a hybrid of these approaches.
         Nevertheless, the quality control approach adopted by the contractor must be fully
         justified in writing to MRWA.


Independent repeat survey

         A number (preferably all) of the survey points are occupied independently from two
         different base-stations at different sidereal times to ensure independent satellite
         geometry. This technique requires fixed points (Section 4.4). A least squares
         adjustment should be performed for each point and 95% confidence error ellipses
         derived. For simplicity in the field, a simple misclosure calculation can be
         performed. Any positions that do not agree to within the specifications set for the
         survey must be reoccupied at third time and the outlying observation rejected.

         Note that this option is not available for VRS.
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Over-sample and look for outliers in post-analysis along a profile or surface

         Using this technique, points are picked up along a profile, using the point pick-up
         technique (Section 4.4). However, the profile is „over-sampled‟ to an extent that
         when the vertical and horizontal components of the profile are plotted, a simple
         filtering procedure can detect any gross errors. Therefore, if a specified sampling
         interval is 100m, actual point observations may be taken every 10m. The outlier
         detection function and the sampling interval would be defined by the predicted
         maximum gradient along the profile or surface.


Mark position of observed points approximately

         If observed points are marked approximately, say using spray paint, during an initial
         survey, then they can be revisited and checked using techniques similar to Section
         6.1. A degradation in accuracy in the horizontal position of the antenna is not as
         critical for MRWA specifications, whilst height is generally much less variable for
         small changes in the horizontal. Note, however, that errors in the horizontal can
         degrade the vertical component.


Repeat profiling

         Some profiles, such as road centre-line mapping can be repeated using point pick-
         up techniques (Section 4.4). Therefore, a series of profiles can be constructed
         which could then be used to construct a master profile with an associated
         uncertainty for that profile. To ensure a statistically significant result, the profile
         should be repeated at least three times.


Known-point re-initialisation to give a second independent coordinate estimate at each
point

         This technique involves moving from one point (say point A) to the next point (say
         point B) and taking a position fix. If the position fix at point B is accepted (as per
         the specifications), the coordinates of point B can be used for a known point
         initialisation over that point. Therefore, a second position fix is performed at point
         B, based on a known point initialisation. If re-initialisation fails or the results are
         significantly different (say >5cm in each component) to the previous results, the
         surveyor should wait for a longer period of time (up to 10 minutes) or re-initialise
         again. The survey proceeds to the next point where the same procedure is carried
         out. This procedure relies solely on internal quality indicators though the re-
         initialisation provides some check that ambiguities have been resolved correctly.




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Ambiguity check on initialisation

         An initial point is positioned, lock is lost deliberately and the point is re-positioned.
         If the coordinates agree to, say better than 5cm, points continuously picked up until
         lock is lost, in which case the procedure undertaken at the initial point is repeated.
         This method provides a secondary check that the receiver is using the correct
         ambiguities whilst picking up points. However, as conditions such as multipath and
         satellite geometry vary from point to point, this method provides no definitive
         external quality control.


Known control at either end of line

         Independent control checks can be obtained by observing over a known point, such
         as an SSM, at any stage of the survey. However, it should be understood that this
         technique in no way validates the other points in the survey because other points
         may have experienced loss of lock, incorrect ambiguity resolution or different
         satellite geometry or multipath conditions.


         Combinations or permutations of these survey configurations (Sections 6.1 to 6.7)
         may be adopted. The primary aim of each technique to perform some kind of
         independent reliability check on the RTK GPS results. As a rule of thumb, the
         more points that are observed when defining a profile or surface, or resurveyed
         when performing other types of survey, the more potential a contract will have for
         demonstrating quality control procedures have been applied.


7.       VERTICAL DATUM TRANSFORMATION

         Horizontal datum transformations need not be applied of the control coordinates are
         given on the GDA94, which is compatible with the WGS84 system utilised by GPS.
         However, the AHD will continue to be used in Australia, so there is the need to
         transform RTK GPS-derived ellipsoidal heights to the AHD. Most RTK GPS
         systems allow for the „real-time‟ application of the following techniques. If not, the
         consultant or contractor to MRWA must apply one of these techniques at a later
         post-processing stage.


7.1      The gravimetric method

         The geoid-WGS84-ellipsoid separation is interpolated from a pre-computed grid of
         values. The latest geoid model available is AUSGeoid98, which is referenced to
         the GRS80 ellipsoid and its relative accuracy is estimated to be between 1 and 3
         parts per million of baseline length. AUSGeoid98 and interpolation software can
         be downloaded free-of-charge from
         http://www.agso.gov.au/nmd/geodesy/ausgeoid/




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7.2      The geometric method

         WGS84 ellipsoidal and AHD heights can be used to determine discrete estimates of
         the geoid-ellipsoid separation. From these point geoid observations, the geoid
         height can then be interpolated to other points in the survey area. This interpolated
         value is then applied to the ellipsoidal height to give the AHD height. Over small
         areas (say, less than a few kilometres) and where the geoid is smooth, the
         interpolation can be achieved using a plane.

         In order to define the plane, GPS observations must be made at a minimum of three
         benchmarks that surround the survey area. If more than three benchmarks are used,
         a least squares solution should be used in preference.

         Alternatively, the geoid can be modelled using a low-order polynomial surface.
         However, this approach should be treated with some caution because spurious
         features can result as an artefact of using polynomial surfaces.

         There are some important precautions that must be taken when using geometrical
         interpolation to determine orthometric heights from GPS. These comprise:

         1. The geometrically derived geoid-WGS84-ellipsoid separation is limited to the
            combined accuracy of the GPS and AHD heights. If errors exist in each or both
            of these measurements, the geometrical geoid model will be distorted, which will
            degrade any subsequently interpolated GPS-derived AHD heights.

         2. Interpolation of the geometrically derived geoid can prove superior to a
            gravimetric geoid for some survey areas that are smaller than the resolution of
            the gravimetric geoid.

         3. Interpolation can only be applied in the area bound by the benchmarks used to
            define the geoid surface, otherwise less accurate and unreliable extrapolation is
            used for those areas outside the control points.

         4. Independent checks should always be used to ensure that the geoid has been
            accurately modelled. For example, in the case of a plane, four or more
            benchmarks should be used; three to define the plane and the remainder to
            provide an independent check. This will give the most realistic estimate of how
            well AHD heights can be recovered from GPS for each particular survey area.


7.3      The combined gravimetric-geometric method

         Since the objective is to use GPS to provide AHD heights that are compatible with
         existing benchmarks, a combination of the gravimetric and geometric approaches
         proves superior to either method alone. This is because the geometrically derived
         geoid heights can account for any local biases in the gravimetric geoid with respect
         to the local benchmarks for each particular survey area and vice versa. It ensures
         that the GPS-derived AHD heights are compatible with the local control, which, in
         turn, allows full integration of GPS heights with the existing AHD heights.
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         The combined gravimetric-geometric method is outlined in the following scheme:

         1) Observe GPS ellipsoidal heights at a minimum of three AHD benchmarks that
            surround the survey area. Ideally, more than three AHD benchmarks should be
            observed with GPS in order to provide a check on this method, or to model the
            geoid using a low-order polynomial surface.

         2) Algebraically subtract gravimetric geoid heights from all GPS ellipsoidal
            heights.

         3) Calculate the „residuals‟ between the gravimetric and geometrical geoid
            heights.

         4) Interpolate and apply these „residuals‟ to the positions of the AHD heights that
            have been derived from GPS and the gravimetric geoid alone, as described in
            stage 2.

         5) Use the additional control points for quality assurance and to determine
            whether this combined method provides improved geoid heights (over the
            gravimetric geoid alone or geometrical interpolation alone) in the survey area.

         In the case where only one AHD benchmark is occupied, a constant bias may be
         applied to all other survey points.


8        LIAISON

        Consultants and contractors to MRWA are not to contact Curtin University of
        Technology regarding these RTK GPS survey specifications.

        All queries and requests should be directed to the Senior Geodetic Surveyor of the
        Road and Traffic Engineering Branch.

        Contact: Geoff Xanthis

        (08) 9323-4152 (direct)
        (08) 9323-4449 (fax)
        geoff.xanthis@mrwa.wa.gov.au




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Appendix A: Sample RTK GPS base-station log sheet




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              MAIN ROADS
               Western Australia

       REAL-TIME KINEMATIC (RTK) GPS BASE-STATION
     SAMPLE LOG SHEET                                                   Page 1

    Company name                      Observer Name (print)                  signature         Survey date




                                            Receiver Information

                                 Manufacturer                        Model                  Serial Number

  Base Receiver

  Base Antenna

  Base Receiver Firmware version number

                                         Control Point Information

            Name                            Identification number                  Type (eg SSM, RRM)



     GDA Coordinates

     Latitude                      Longitude                  Ellipsoidal height            Geoid height



     Coordinates as input into reference receiver (if different to above)

Tick as appropiate         Long. or Easting       Lat. Or Northing          Ellips.Height      Geoid height
 MGA

AGD84

 AMG

 Other
                                                                                             continued over




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                 MAIN ROADS
                  Western Australia

           REAL-TIME KINEMATIC (RTK) GPS BASE-STATION
         SAMPLE LOG SHEET                                               Page 2

If GDA coordinates were not used to for base station coordinates state below how the coordinate
used were derived eg source, transformation method, transformation parameters




Antenna plan (indicate to which points on the antenna                                   Site visibility
measurement were made)
                                                                                             0°
                                                                                 330°                 30°

                                                                         300°                                60°


                                                                                                              90°
                                                                                             60°
                                                                         240°                 30°            120°
                                                                                             15°
                                                                                 210°        0°       150°

         Antenna set-up                                                            Measurement type eg:
                                    tick      Heights: Before        Heights:      L1 phase centre
                                              logging (m)            After logging bottom of notch on ground plane
                                                                                   base of antenna
                                                                     (m)           corner of antenna base

Pillar                                        1

Tripod                                        2

Tribrach                                      3

Optical plummet                               4

Plumb bob                                     5
Antenna height input into receiver (m)

Transmission Started                                                   Time frame (eg UT,
                                                                       WAST)
Transmission Stopped                                                   Time frame (eg UT,
                                                                       WAST

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