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

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(text cleanup)
Line 2: Line 2:
 
  r.in.gdal
 
  r.in.gdal
  
* georectify map using GUI georectifier or i.points + i.rectify
+
* Georectify map using GUI georectifier or i.points + i.rectify
* switch to in target location
+
* Switch to target location
* we now have a registered raster map containing a 300 dpi scan of our A0 sized map
+
* We now have a registered raster map containing a 300 dpi scan of our A0 sized paper map
  
* check how many categories
+
* Check how many categories (map colors)
 
  r.info chart
 
  r.info chart
* 8 colors (categories)
+
: 8 colors
  
* zoom to the map's bounds
+
* Zoom to the map's bounds
 
  g.region rast=chart
 
  g.region rast=chart
  
* display map
+
* Display map (command line version)
 
  d.mon x0
 
  d.mon x0
 
  d.rast chart
 
  d.rast chart
  
* query map values
+
* Query map values (command line version)
 
  d.what.rast
 
  d.what.rast
 
* black is category 1
 
* black is category 1
  
* make a reclass map, with just the black lines
+
* Create a reclass map, with just the black lines
 
  r.reclass in=chart out=chart_black << EOF
 
  r.reclass in=chart out=chart_black << EOF
 
  1 = 1 black
 
  1 = 1 black
 
  EOF
 
  EOF
  
* thin the lines to be 1 pixel wide
+
* Thin the lines to be 1 pixel wide
 
  r.thin in=chart_black out=chart_black.thinned
 
  r.thin in=chart_black out=chart_black.thinned
  
* convert raster lines to vector lines
+
* Convert raster lines to vector lines
 
  r.to.vect -s in=chart_black.thinned out=chart_lines
 
  r.to.vect -s in=chart_black.thinned out=chart_lines
  
* remove any dangles smaller than 1km
+
* Remove any dangles smaller than 1km
 
  v.clean in=chart_lines out=chart_lines_cleaned tool=rmdangle thresh=1000
 
  v.clean in=chart_lines out=chart_lines_cleaned tool=rmdangle thresh=1000
  
* connect broken lines
+
* Connect lines which were broken at dangles
 
  v.build.polylines -q in=chart_lines_cleaned out=chart_polyline
 
  v.build.polylines -q in=chart_lines_cleaned out=chart_polyline
  
* add category numbers to lines
+
* Add category numbers to lines
 
  v.category in=chart_polyline out=chart_polyline_cat type=line
 
  v.category in=chart_polyline out=chart_polyline_cat type=line
  
* create a DB table to hold line length values
+
* Create a DB table to hold line length values
 
  v.db.addtable chart_polyline_cat columns="length_km DOUBLE PRECISION"
 
  v.db.addtable chart_polyline_cat columns="length_km DOUBLE PRECISION"
  
* upload line lengths to DB table
+
* Upload line lengths to DB table
 
  v.to.db chart_polyline_cat type=line option=length units=k column=length_km
 
  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)
+
* 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"
 
  v.extract in=chart_polyline_cat out=chart_lines_5km type=line where="length_km > 5"
  
* display result over raster chart
+
* Display result over raster chart (command line version)
 
  d.vect chart_lines_5km color=red width=2
 
  d.vect chart_lines_5km color=red width=2
  
* export as a shapefile
+
* Export as a shapefile
 
  v.out.ogr in=chart_lines_5km dsn=chart_lines_gt5km
 
  v.out.ogr in=chart_lines_5km dsn=chart_lines_gt5km
  
* view shape file in [http://qgis.org QuantumGIS]
+
* View shape file in [http://qgis.org QuantumGIS]
 
  qgis chart_lines_gt5km/chart_lines_5km_crop.shp
 
  qgis chart_lines_gt5km/chart_lines_5km_crop.shp

Revision as of 00:41, 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.
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
qgis chart_lines_gt5km/chart_lines_5km_crop.shp