[GRASS-user] V.Select strange results

[Moritz Lennert]

On 25/05/09 18:00, Stefano wrote:
> Hi all.
> i'm looking to v.select (grass 6.4).
> 
> i've 2 paths done by v.net.path, i want to extract the diff path of
> these 2 paths using v.select, overlap and the reverse.
> before this operation i clean both of them using break and snap.
> 
> As you can see in the image there are two path, one blue and one
> green, these two path sheared pieces (u can't see the shared path,but
> is not imporant).
> so, i want find the differences of these two path.
> i do the select with a reverse command but the result is not right as
> i can excepted.
> 
> if u pay attention the black path (the diff path) is not completly
> right. in fact this path has to start and finish on the intersection
> of these two path (blue and green) but start and end before the
> intersection. so the black path has to be identical as the green path
> (the green path that u can see)
> 
> i think the problem is the lines of these 2 path (green and blue) that
> are not cut in the intersection, so the line goes across the
> intersection of these line.
> 
> how can i fix this problem?

v.select does not cut exactly at the edges of overlap, but selects the 
entire feature that overlaps, even if this feature goes beyond the 
overlap (i.e. it does an "overlap" and not an "intersect").

What you want is v.overlay, but unfortunately this has never been 
implemented for two line layers (never understood why).

Your best solution at this stage is to compile the current grass6 svn 
tree (6.5) with the --with-geos configure flag which will give you 
access to a much more wide array of tools than only overlap [1].

Moritz

[1] http://grass.osgeo.org/grass65/manuals/html65_user/v.select.html