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

[Jose M Michia Roberts]

2013/2/26 Markus Metz <markus.metz.giswork en gmail.com>:
> BTW, v.transform simply transforms coordinates, it does not do any
> cleaning. v.transform should actually work without topology, building
> topology on the output is optional (in GRASS 7).
> [...]
> For this particular problem (georectifying overlapping polygons) there
> is probably an easier solution outside GRASS. Maybe QGIS supports
> coordinate transformation/rectification using GCPs?

Looks like you're right. I will try a newer version of GDAL / OGR. I
will also update GRASS, to use "v.rectify", so I can check the values
of the transformation matrix, and explore the possibility of working
without topology.

In QGIS, I could not find an equivalent function. Maybe it exist, but
I have looked very little. I know there is a plugin for GCP based
transformations, but it is for rasters. Anyway, some of the above
alternatives will surely solve my problem.

2013/2/26 Moritz Lennert <mlennert en club.worldonline.be>:
> -1
>
> I'm strongly opposed to that option. As you mention, this would
> fundamentally undermine the data logic in GRASS.
>
> To answer José's issue: If you want to keep on working in GRASS, you should
> not try to do so with overlapping polygones.

Thanks Moritz, I hope fully understand all implications of this topic
someday! For now, I promise to not propose something like that
anymore.

> If all you want to do in GRASS is v.transform, you should be able to import
> with cleaning. Wherever there is overlap, you will get new polygones which
> have all the category numbers of the original polygones. You can then
> v.transform the file and if you need to get the original polygones you
> should be able reconstruct them based on the common category numbers.

I considered doing this at first. But when I checked the maps
imported, I could not find all the information necessary to perform
the task. I will follow your instructions if only as an exercise to
learn a little more.

Thank you very much!
Jose