[GRASS5] How to patch vector maps?

David D Gray ddgray at armadce.demon.co.uk
Fri Dec 14 15:22:28 EST 2001

Hi Markus,

This is one function in the vector library that could do with improvement.

I find that there are *far* fewer problems if you work with polylines 
when intersecting. Attempts to patch polygon maps result in loss of some 
lines, principally simple islands. Then v.spag. This usually introduces 
many small lines of 2-3 points that are hard to find and eliminate. So, 
maybe backup the dig_att list, convert polygon data file to lines with 
v.area2line or v.polylines, then v.patch and v.spag, clean up with 
v.spag -i and v.clean, and change back to polygons with v.line2area or 
v.polylines. restore the dig_att file and build.

I was in fact pondering this very problem today. When I get back into 
coding in earnest in the new year, I was thinking that the various 
vector processing modules could do with re-arangement (for the 5.1 
release of course). v.patch and v.spag (intersection) should be 
combined, possibly with various options, and v.clean and v.spag -i also 
should be combined into one module.


Markus Neteler wrote:

> Hi,
> at time I try to understand how to intersect vector maps
> topologically correct. What I did (using the Spearfish dataset,
> latest version from the web site):
> v.patch in=soils,fields out=soilsfarms.patch
> # here v.support would print tons of warnings as expected
> v.spag -si soilsfarms.patch
> v.support soilsfarms.patch op=build
> Still many warnings appear - is intersecting not possible
> without getting attribute problems? Or am I on the wrong
> roads?
> Thanks,
>  Markus
> _______________________________________________
> grass5 mailing list
> grass5 at grass.itc.it
> http://grass.itc.it/mailman/listinfo/grass5

More information about the grass-dev mailing list