[GRASS-user] Apply "v.transform" in polygons with overlays

[José María Michia Roberts]

José María Michia Roberts:
> [...]
> many elements disappear after importing the layer,
> and more elements disappear after applying "v.transform"
> [...]

In reply to myself: I now remember that I had solve this by adding
"-c" to "v.in.ogr", so the output layer is not cleaned and all source
elements are imported. But "v.transform" don't have such option. ¿May
be this implemented? ¿May be useful?

Hamish:
> try running v.split on the largest of the polygons.
>
> search the mailing list archives for "the florida coastline
> problem".

I've found a thread named "speeding up v.in.ogr". It is a bit complex.
I gave him a look, but I think it not applies to my case, since I have
many polygons, with many small overlaps. Also, each polygon have a
database record, so I need to preserve at least an numeric field (an
ID), so split the polygons seems to be problematic to me.

On the other hand, revisiting the man page for "v.transform" I've
noted the ST_Affine PostGIS function. If I'm right, by taking the
transformation matrix reported by "v.transform" (applied to any
element) and entering their values in ST_Affine, I could transform a
whole PostGIS table (within a PostGIS table, polygons can be
overlapped). I tried that way, but I get an incorrect result. I
suspect the problem is in the transformation matrix values reported by
GRASS. I will refer to this in a new thread.

Thanks all!
José