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Lecture 07

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Lecture 07 Powered By Docstoc
					Surveying and Digitizing
        Primary Data Sources
   Measurements
       Field → surveying
       Lab (not covered here)
   Remotely sensed data
       already secondary?
   Creating geometries
       Definitely in the realm of secondary data
       Digitizing
       Scanning
                   Surveying
   Measurements and measurement
    techniques
       Distances
       Angles
       Position determination
   Applications
       Traversing and mapping
       Construction and earthwork
       Boundary surveys
     Definition of Surveying
   General
       To inspect, view, scrutinize, or examine
       To determine condition, situation, or value

   Specifically
       Science and art of determining relative
        positions of points above, on, or beneath
        earth surface
              Uses of Surveying

   Locate/map resources
   Engineering design
       Layout construction or engineering projects
       Verify performance
   Acquire reliable data
   Provide control
       Usually for location
         History of Surveying
   Early applications
       Boundary location
       Construction
       Mapping
   Early surveys limited by technology
       Crude and inconsistent methods
       Development of sighting devices,
        standards, …
          History of Surveying (2)
   Industrial revolution improved surveying
       Advances in available materials
       Improvement in tools

   Electronics revolution  fundamental
    advances
       Electronic distance and angle measurement
       Satellite surveying
       Enhanced processing
Specific Types of Surveying
   Property (cadastral) surveying
   Control surveying
   Mapping surveying (planimetric or
    topographic)
   Photogrammetric surveying
   Construction (engineering) surveying
   Route surveying
   Hydrographic surveying
        Surveying Measurements
   Two quantities measured in surveying
     Lengths
     Angles




   All measurements are imperfect
     Errors
     Mistakes
         Measurement Errors
   Sources of errors
       Natural            Personal
       Instrumental
   Types of errors
       Systematic
       Random
   Terms used in describing errors
       Precision
       Accuracy
    Idea of Relative Position
   Question: Have the points moved?
   Answer: Relative to what?
   References
       Needed for expressing location of points,
        lines, other objects
   Datums provide references in surveying
       Horizontally
       Vertically
Reference Ellipsoids
   Basic Concept

              a = semi-major axis
              b = semi-minor axis

              f = flattening
                        b a b
                f  1 
                        a    a
              e = eccentricity
                  a 2  b2
             e             2f  f 2
                     a
        Example Reference Ellipsoids

  Ellipsoid Equatorial Axis   Polar Axis   Association
Clarke, 1866 12,756,412.8 m 12,713,167.6 m NAD27 datum
GRS80       12,756,274 m    12,713,504.6 m NAD83 datum
WGS84       12,756,274 m    12,713,504.6 m GPS
ITRS        12,756,272.98 m 12,713,503.5 m ITRF

 GRS = Geodetic Reference System
 WGS = World Geodetic System
 ITRS = International Terrestrial Reference System
Ignoring Earth Curvature
   Distance


           8000.000m ( 5 miles)


                 1000 km

                998.95 km
   Ignoring Earth Curvature (2)
      Level line

Horizontal plane
                    1 mile (1609 m)


                                      8 inches ( 20 cm)

           Level surface
Ignoring Earth Curvature (3)
   Triangle geometry

                         Sum of Interior
            75mi2          Angles =
        (48,000 acres)
       19,800 hectares    180° 00' 01"
    Digitizing and Scanning
   Instruments
   Georeferencing
   The process and problems associated
    with it
   Automation
   Formats
Why Do We Have To Digitize?
   Existing data sets are general purpose,
    so if you want something specific you
    have to create it

   In spite of 20+ years of GIS, most stuff
    is still in analog form

   Chances are somebody else has
    digitized it before; but data sharing is
    not what it should be
                 Digitizer

   Digitizing table
    10” x 10” to 80” x 60”
    $50 - $2,000
    1/100th inch accuracy



   Stylus or
    puck with control buttons
The Digitizing Procedure
   Affixing the map to the digitizer

   Registering the map

   Actual digitizing
       In point mode
       In stream mode
                   Georeferencing
        at least 3 control points      Entered:
                                         Tic 1: 11° 15' N
         aka reference points or tics            30° 30' E
                                         Tic 2: 11° 15' N
        easily identifiable on the map          73° 30' E

        exact coordinates need to be known
              Digitizing Table Coordinates                  East of Greenwich
 Origin:
  X = 4 in.                                                71°       72°        73°
  Y = 5 in.
                                                     11°                              11°


                                             South

                                                     12°                              12°
Tic Points

                                                           71°       72°        73°
            Digitizing Modes
   Point mode
       most common
       selective choice of points digitized
       requires judgment
       for man-made features

   Stream mode
       large number of (redundant) points
       requires concentration
       For natural (irregular) features
    Problems With Digitizing
   Paper instability
       Humidity-induced shrinking of 2%-3%
   Cartographic distortion, aka
    displacement
   Overshoots, gaps, and spikes


   Curve sampling
     Errors From Digitizing
   Fatigue
   Map complexity
       ½ hour to 3 days for a single map sheet
   Sliver polygons

   Wrongly placed labels
                         5        6 7   8
    Digitizing             Costs




   Rule of thumb: one
    boundary per minute
    ergo:
    appr. 62 lines
    = more than one hour
        Automated Data Input
             (Scanning)
   Work like a photocopier or fax machine
   Three types:
       Flatbed scanners
            A4 or A3
            600 to 2400 dpi optical resolution
            $50 to $2,000
       Drum scanner
            practically unlimited paper size
            $10k TO $50k
       Video line scanner
            produces
             vector data
    Requirements for Scanning
   Data capture is fast but preparation is tedious
   Computers cannot distinguish smudges
   Lines should be at least 0.1 of a mm wide
   Text and preferably color separation

                          300

   AI techniques don’t work (yet?)
   Symbols such as  are too variable for
    automatic detection and interpretation
    Semi-automatic Data Input
      (Heads-up Digitizing)
   Reasonable compromise between
    traditional digitizing and scanning

   Much less tedious

   Incorporating your intelligence
        Criteria for Choosing
             Input Mode
   Images without easily detectable line
    work should be left in raster format

   Really dense line work should be left as
    background image –
       unless it is really needed for automatic GIS
        analysis; in which case you would have to
        bite the bullet
      Conversion from Other
           Databases
   Autocad .dxf and dBASE .dbf are de facto
    standards for GIS data exchange

   In the raster domain there is no
    equivalent; .tif comes closest to a
    “standard”

   In any case: merging data that originate
    from different scales is problematic – in
    the best of all worlds; there is no
    automatic generalization routine

				
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posted:4/21/2013
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