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 Instrumental
Types of errors Systematic Random
Terms used in describing errors Precision Accuracy
Personal
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
b = semi-minor axis
f = flattening
aba
ab1f
a = semi-major axis
e = eccentricity
222
f2fa
bae
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
Ignoring Earth Curvature
8000.000m ( 5 miles)
8000( 5
+ 0.25”).006m miles
998.95 km
1000 km
Distance
Ignoring Earth Curvature (2)
1 mile (1609 m)
8 inches ( 20 cm)
Level surface
Horizontal plane
Level line
Ignoring Earth Curvature (3)
75 mi2(48,000 acres)
19,800 hectares
Sum of Interior Angles =
180° 00' 01"
Triangle geometry
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 table10” x 10” to 80” x 60”$50 - $2,0001/100th 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 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
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 labels5 86 7
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
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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