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Digitizing and Scanning

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?.

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Digitizing and Scanning

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  1. Digitizing and Scanning

  2. Primary Data Sources • Measurements • Field • Lab • Remotely sensed data • already secondary? • Creating geometries • Definitely in the realm of secondary data • Digitizing • Scanning

  3. 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

  4. Digitizer • Digitizing table10” x 10” to 80” x 60”$50 - $2,0001/100th inch accuracy • Stylus or puck with control buttons

  5. The Digitizing Procedure • Affixing the map to the digitizer • Registering the map • Actual digitizing • In point mode • In stream mode

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

  7. 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

  8. Problems With Digitizing • Paper instability • Humidity-induced shrinking of 2%-3% • Cartographic distortion, aka displacement • Overshoots, gaps, and spikes • Curve sampling

  9. Errors From Digitizing • Fatigue • Map complexity • ½ hour to 3 days for a single map sheet • Sliver polygons • Wrongly placed labels

  10. Digitizing Costs • Rule of thumb: one boundary per minuteergo:appr. 65 lines= more than one hour

  11. 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 • producesvector data

  12. 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

  13. Semi-automatic Data Input(Heads-up Digitizing) • Reasonable compromise between traditional digitizing and scanning • Much less tedious • Incorporating your intelligence

  14. 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

  15. 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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