Trace vector contours from a scanned map

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Revision as of 23:37, 11 December 2006 by HamishBowman (talk | contribs)

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  • Import a scanned map image. To save processing later you might want to mask out obvious text and other noise in a paint program first.
  • georectify map using GUI georectifier or i.points + i.rectify
  • switch to in target location
  • we now have a registered raster map containing a 300 dpi scan of our A0 sized map
  • check how many categories chart
  • 8 colors (categories)
  • zoom to the map's bounds
g.region rast=chart
  • display map
d.mon x0
d.rast chart
  • query map values
  • black is category 1
  • make a reclass map, with just the black lines
r.reclass in=chart out=chart_black << EOF
1 = 1 black
  • thin the lines to be 1 pixel wide
r.thin in=chart_black out=chart_black.thinned
  • convert raster lines to vector lines -s in=chart_black.thinned out=chart_lines
  • remove any dangles smaller than 1km
v.clean in=chart_lines out=chart_lines_cleaned tool=rmdangle thresh=1000
  • connect broken lines -q in=chart_lines_cleaned out=chart_polyline
  • add category numbers to lines
v.category in=chart_polyline out=chart_polyline_cat type=line
  • create a DB table to hold line length values
v.db.addtable chart_polyline_cat columns="length_km DOUBLE PRECISION"
  • upload line lengths to DB table chart_polyline_cat type=line option=length units=k column=length_km
  • extract line features longer than 5km (cleans out the noise)
v.extract in=chart_polyline_cat out=chart_lines_5km type=line where="length_km > 5"
  • display result over raster chart
d.vect chart_lines_5km color=red width=2
  • export as a shapefile
v.out.ogr in=chart_lines_5km dsn=chart_lines_gt5km
qgis chart_lines_gt5km/chart_lines_5km_crop.shp