Trace vector contours from a scanned map

• 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.

r.in.gdal

• 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

• 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)

d.what.rast

• black is category 1

• Create a reclass map, with just the black lines

r.reclass in=chart out=chart_black << EOF
1 = 1 black
EOF

• Thin the lines to be 1 pixel 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

• Remove any dangles smaller than 1km

v.clean in=chart_lines out=chart_lines_cleaned tool=rmdangle thresh=1000

• Connect lines which were broken at dangles

v.build.polylines -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

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

• View shape file in QuantumGIS

qgis chart_lines_gt5km/chart_lines_5km_crop.shp