[GRASS-user] topo map to dem

[Hamish]

antonio rodriguez wrote:
> I apologize if this question has been posted before, but I couldn't
> find something searching through the mailing list.

it helps if you know the secret words, try searching for "r.thin".


> Is it posible to perform the task of extracting a DEM from a scanned 
> topographic map in GRASS? I mean like Autodem or R2V programs.

using r.resample or r.mapcalc, or maybe with "select by color" in a
graphics program, select only the lines you are interested in, make
every thing else white/NULL. You might have to use v.mkgrid + v.buffer
to get rid of any lat/lon grid lines the same color as isolines.

Once you only have raster versions of the isolines, run r.thin to get
rid of noise, then run "r.to.vect feature=line".

then add an elevation column to the vector map (v.db.addcol), v.digit or
QGIS to clean out junk, and "d.what.vect -e" to label lines with
elevation values.

Finally use v.surf.* or r.surf.contour to convert into a DEM.


I've done this with bathymetry charts and it takes some manual labor but
works pretty well.


Hamish