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 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
- 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 shapefile in QuantumGIS
qgis chart_lines_gt5km/chart_lines_5km_crop.shp
Done!