Removing unwanted nodes

[Susan Huse]

> 
> I want to create a vector file from a raster.  I scanned in a map with
> lines marking different vegetation units.  I brought it into GRASS and
> reclassified the map into two categories 1=line 0=no line.  I then used
> r.thin to thin the lines to one pixel.  Afterwards, I ran r.line and chose
> areas.  It created a vector file containing polygons made up of several
> lines with their corresponding nodes.  I want to remove the nodes in order
> to have one line surrounding each area.  The GRASS manual suggested I use
> v.trim however it didn't improve the situation.  I looked in v.digit but I
> didn't see anything that would work.  Any suggestions???
> 
> Melissa Cowoski
> Stanford University
> cowoski at pangea.stanford.edu
>

If v.trim didn't remove the nodes, I suspect they are necessary for the
lines definition.  Every time two area edges intersect there should be 
a node.

Do your vegetation units share common boundaries?  If they do then you
won't be able to get rid of the extra nodes:
For example:

 -------*--------
|       |        |
|   A   |   B    |
|       |        |
 -------*--------

In this case you will need to have the two nodes (*).  Occaisionally you
can have separate areas for each with both area edges tracing the middle
boundary.  I have seen this from ArcInfo files, but it is NOT a good idea
to leave it that way in GRASS.  The topology is dealt with very differently.

Do you really need to have only one line surrounding each area??


Sue Huse
CEDR - REGIS
UC Berkeley