vectors: lines to polygons (2)
David D Gray
ddgray at armadce.demon.co.uk
Thu Jun 3 16:36:04 EDT 1999
Agustin Lobo wrote:
> I have imported a vegetation map of Europe.
> It seems that it was digitized as LINES, not
> as polygons, which is not very useful for a vegetation map.
> Therefore, I have now a vector map in which the lines should be
> appended to make polygons. I've checked the ascii
> file and it is not just
> that each polygon is made out of one single line
> (i.e., that the last point is not equal to the
> first one), so it's not just a question of
> copying the first point at the end of the polygon an
> replacing "L" by "A" in the ascii vector format.
> It is more complicated as lines have to be combined
> into polygons.
> The question (and I suspect the answer...):
> is there an automatic procedure to combine the
> lines into polygons?, i.e., if we have:
> B A E
> B C E
> D C F
> (where each letter is a line)
> transform to 2 polygons
> Thanks (to both mailing list users and mailing list keepers)
I'm doing similar work just now with digitised polygons imported as
I think you will need to perform the following stages:
1. Convert the lines to area edges with v.line2area.sh . It runs
interactively at the console.
2. [Maybe] Just check in v.digit to make sure all the lines are
correctly snapped. Snapped nodes are a different color from unsnapped
3. Use v.trim to remove the excess nodes - ie so that each area edge is
just a single line. Run this with the threshold value set to 0. This
creates a new vector file.
( I think this is the critical stage you are looking for :)
4. Run v.support on the new file -
and then the topology should be correct, and you can label the polygons
Hopes this helps
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