digitizing and scanning. primary data sources measurements field lab remotely sensed data already...
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Digitizing and Scanning
Primary Data Sources Measurements
Field Lab
Remotely sensed data already secondary?
Creating geometries Definitely in the realm of secondary
data Digitizing Scanning
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°
South
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. 65 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 $100 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