[GRASS5] How to patch vector maps?
David D Gray
ddgray at armadce.demon.co.uk
Fri Dec 14 15:22:28 EST 2001
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:
> 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
> grass5 mailing list
> grass5 at grass.itc.it
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