[GRASS-user] Removing inner lines in merged vector map

[Hamish]

Arne wrote:
> I have merged three CORINE vector maps for coniferous, broad-leaved and
> mixed forests in order to create one general map for all forests as I
> need to calculate the length of the forest edges. 

the problem is fractal and has no answer. you need to think of a way to
work by area, or if you can absolutely confirm the coverage is digitized
at the same scale throughout (almost all are not, even if all the digitizing
was done by the same person for the same project), you could work with
relative proportions.

lots of software and people will give you numbers for the question, but
really that number doesn't mean much at all (unless you are a pure
mathematician).   defining margins is a tricky business..

see 
http://en.wikipedia.org/wiki/How_Long_Is_the_Coast_of_Britain%3F_Statistical_Self-Similarity_and_Fractional_Dimension

"[Mandelbrot's] discussion implies that it is meaningless to talk about
the length of a coastline; some other means of quantifying coastlines are
needed."


(we should make this a FAQ...)

 
> After using v.patch (with flag e) to merge the maps I have tried my
> best to remove all lines lying inside of boundaries so that only
> the outer boundaries would be left - with no luck. The main problem
> should be that the adjacent boundaries from the three maps have only
> been put next to another and not really "merged" (this is not a problem
> concerning double boundaries). Does anyone know how to cope with this?

try the v.dissolve module. note that works on map features with categories,
and boundaries often do not have category numbers by default, so you'll
have to deal with that first (v.category).


good luck,
Hamish