Surveying and Digitizing

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Surveying and Digitizing
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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

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Surveying

• Measurements and measurement techniques
• Distances
• Angles
• Position determination
• Applications
• Traversing and mapping
• Construction and earthwork
• Boundary surveys

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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

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Uses of Surveying

• Locate/map resources

• Engineering design
• Layout construction or engineering projects
• Verify performance

• Acquire reliable data

• Provide control
• Usually for location

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History of Surveying

• Early applications
• Boundary location
• Construction
• Mapping
• Early surveys limited by technology
• Crude and inconsistent methods
• Development of sighting devices, standards,

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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

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Specific Types of Surveying

• Property (cadastral) surveying
• Control surveying
• Mapping surveying (planimetric or topographic)
• Photogrammetric surveying
• Construction (engineering) surveying
• Route surveying
• Hydrographic surveying

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Surveying Measurements

• Two quantities measured in surveying
• Lengths
• Angles
• All measurements are imperfect
• Errors
• Mistakes

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Measurement Errors

• Sources of errors
• Natural
• Instrumental
• Types of errors
• Systematic
• Random
• Terms used in describing errors
• Precision
• Accuracy

• Personal

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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

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Reference Ellipsoids Basic Concept
a semi-major axis
b semi-minor axis
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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
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Ignoring Earth Curvature

• Distance

(? 5
miles
.006m
8000
0.25)
8000.000m (? 5 miles)
1000 km
? 998.95 km
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Ignoring Earth Curvature (2)

• Level line

Horizontal plane
1 mile (1609 m)
8 inches (? 20 cm)
Level surface
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Ignoring Earth Curvature (3)

• Triangle geometry

Sum of Interior Angles 180 00' 01"
75 mi2(48,000 acres)19,800 hectares
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Digitizing and Scanning

• Instruments
• Georeferencing
• The process and problems associated with it
• Automation
• Formats

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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

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Digitizer

• Digitizing table10 x 10 to 80 x 6050 -
  2,0001/100th inch accuracy
• Stylus or puck with control buttons

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The Digitizing Procedure

• Affixing the map to the digitizer
• Registering the map
• Actual digitizing
• In point mode
• In stream mode

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Georeferencing

• at least 3 control pointsaka reference points or
  tics
• easily identifiable on the map
• exact coordinates need to be known

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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

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Problems With Digitizing

• Paper instability
• Humidity-induced shrinking of 2-3
• Cartographic distortion, aka displacement
• Overshoots, gaps, and spikes
• Curve sampling

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Errors From Digitizing

• Fatigue
• Map complexity
• ½ hour to 3 days for a single map sheet
• Sliver polygons
• Wrongly placed labels

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Digitizing Costs

• Rule of thumb one boundary per minute
  ergoappr. 62 lines more than one hour

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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
• producesvector data

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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
• AI techniques dont work (yet?)
• Symbols such as ? are too variable for automatic
  detection and interpretation

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Semi-automatic Data Input(Heads-up Digitizing)

• Reasonable compromise between traditional
  digitizing and scanning
• Much less tedious
• Incorporating your intelligence

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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

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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