Transcript Document

Digitizing and Scanning
Primary Data Sources
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Measurements
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Remotely sensed data
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Field
Lab
already secondary?
Creating geometries
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Definitely in the realm of secondary data
Digitizing
Scanning
Why Do We Have To Digitize?
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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
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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
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Affixing the map to the digitizer
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Registering the map
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Actual digitizing
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In point mode
In stream mode
Georeferencing
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Entered:
at least 3 control points
Tic 1: 11° 15' N
30° 30' E
aka reference points or tics
Tic 2: 11° 15' N
73° 30' E
easily identifiable on the map
exact coordinates need to be known
East of Greenwich
Digitizing Table Coordinates
71°
72°
73°
11°
11°
12°
12°
South
Origin:
X = 4 in.
Y = 5 in.
Tic Points
71°
72°
73°
Digitizing Modes
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Point mode
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most common
selective choice of points digitized
requires judgment
for man-made features
Stream mode
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large number of (redundant) points
requires concentration
For natural (irregular) features
Problems With Digitizing
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Paper instability
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Humidity-induced shrinking of 2%-3%
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Cartographic distortion, aka
displacement
Overshoots, gaps, and spikes
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Curve sampling
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Errors From Digitizing
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Fatigue
Map complexity
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½ hour to 3 days for a single map sheet
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Sliver polygons
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Wrongly placed labels
5
6 7
8
Digitizing Costs
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Rule of thumb: one
boundary per minute
ergo:
appr. 65 lines
= more than one hour
Automated Data Input
(Scanning)
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Work like a photocopier or fax machine
Three types:
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Flatbed scanners
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Drum scanner
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A4 or A3
600 to 2400 dpi optical resolution
$100 to $2,000
practically unlimited paper size
$10k TO $50k
Video line scanner
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produces
vector data
Requirements for Scanning
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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
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AI techniques don’t work (yet?)
Symbols such as  are too variable for
automatic detection and interpretation
Semi-automatic Data Input
(Heads-up Digitizing)
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Reasonable compromise between
traditional digitizing and scanning
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Much less tedious
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Incorporating your intelligence
Criteria for Choosing
Input Mode
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Images without easily detectable line
work should be left in raster format
Really dense line work should be left as
background image –
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unless it is really needed for automatic GIS
analysis; in which case you would have to
bite the bullet
Conversion from Other
Databases
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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