Difference between revisions of "Trace vector contours from a scanned map"

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(v.digit, grid lines)
(Clean vector map: v.overlay)
Line 44: Line 44:
  v.overlay ain=scanned_lines bin=buffered_grid operator=not
* Clean by hand in with the digitizing tool
* Clean by hand in with the digitizing tool

Revision as of 19:43, 12 December 2006


  • 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 target location
  • We now have a registered raster map containing a 300 dpi scan of our A0 sized paper map

Convert to vector map

  • Check how many categories (map colors)
r.info chart
8 colors
  • Zoom to the map's bounds
g.region rast=chart
  • Display map (command line version)
d.mon x0
d.rast chart
  • Query map values (command line version)
black is category 1
  • Create 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 raster cell wide
r.thin in=chart_black out=chart_black.thinned
  • Convert raster lines to vector lines
r.to.vect -s in=chart_black.thinned out=chart_lines

Clean vector map

  • Remove any dangles smaller than 1km
v.clean in=chart_lines out=chart_lines_cleaned tool=rmdangle thresh=1000
  • Remove known grid lines
v.overlay ain=scanned_lines bin=buffered_grid operator=not
  • Clean by hand in with the digitizing tool
or the QGIS GRASS vector editor
  • Connect lines which were broken at dangles
v.build.polylines -q in=chart_lines_cleaned out=chart_polyline

Remove short line segments

  • 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
v.to.db 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 (command line version)
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