Surveying and Digitizing. Primary Data Sources Measurements Field → surveying Lab (not covered...
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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
b = semi-minor axis
f = flattening
a
ba
a
b1f
a = semi-major axis
e = eccentricity
222
f2fa
bae
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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 SystemWGS = World Geodetic SystemITRS = International Terrestrial Reference System
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Ignoring Earth Curvature
8000.000m ( 5 miles)
8000( 5
+ 0.25”).006m miles
998.95 km
1000 km
Distance
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Ignoring Earth Curvature (2)
1 mile (1609 m)
8 inches ( 20 cm)
Level surface
Horizontal plane
Level line
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Ignoring Earth Curvature (3)
75 mi2
(48,000 acres)19,800 hectares
Sum of Interior Angles =
180° 00' 01"
Triangle geometry
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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 60”$50 - $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 East of Greenwich
72°71° 73°
72°71° 73°
11°
12°
11°
12°
Sou
th
Tic Points
Origin: X = 4 in. Y = 5 in.
Digitizing Table Coordinates
Entered: Tic 1: 11° 15' N 30° 30' E Tic 2: 11° 15' N 73° 30' E
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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 labels5 86 7
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Digitizing Costs
Rule of thumb: one boundary per minute ergo:appr. 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 produces
vector 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 don’t 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