selecting edges in vectors

[Ronald Wiemer]

> 
> I'd like to do the following:
> 1. Assume I have a raster thematic image (land cover types).
> 2. I vectorise with r.poly
> 3. Now I would like to select those vector lines that are
> boundaries between 2 given classes, i.e., lines that are
> edges between type 4 and type 5.
> 
> Can this operation be done in Grass with vector files?
> If not but you are bith an ArcInfo and Grass user, could this
> operation be done with AI?
> 
> Thanks!
> 
> Agus

What you can do is reclass the thematic image, in a way that you have
a map showing type 4 and a map showing type 5.
Then apply r.grow on one of the maps.
with r.mapcalc you can then extract lines that belong to both type 5 
and type 5.
This is of course not as accurate as it has to be, because the lines 
are shifted 1 cell towards the category that has not been "grown"


Ronald Wiemer
R.O.B. / Archis
Kerkstraat 1
NL 3811 CV Amersfoort
The Netherlands

Tel: ++ 31 33 4634233
Fax: ++ 31 33 4653235