[GRASS-user] confused on categories created by topological cleaning of overlapping polygons

Markus Metz markus.metz.giswork at googlemail.com
Wed Nov 30 03:11:07 EST 2011

G. Allegri wrote:
> I'm still assessing the topological model and the build/clean effects on
> various combinations of synthetic geometries created by myself.
> This time I have created two overlapping polygons [1] [2] in Qgis, and
> imported in grass thorugh v.in.ogr.
> If I import the shapefile with the clean tool activated, I obtain 3 areas:
> two with one category and one layer, the other one (the overlapping zone)
> has two layers [3]:
> layer 1: category 1/2
> layer 2: category 1
> BUT, the two layers appear only if I display the categories with v.digit.

You can display all layers with d.vect layer=-1 or any specific layer
with d.vect layer=2. This is also possible in the wxGUI

> All the other tools list only one layer (the one in common with the other
> two areas).
This is not the tools, it's the wxGUI. The wxGUI only knows about
layers that have an attribute table attached to it. But wherever there
is a layer option, you can enter any number in the wxGUI, also numbers
not in the dropdown list

> This leads to various problems:
>  - the wxGUI (the identify tool inside the display window crashes)

This is a recently introduced bug because the wxGUI is no longer using
v.what and needs an attribute table (which is a completely optional
feature for grass vectors).

>  - to remove the layer 2, because layer 2 isn't selectable from the other
> modules (like v.category, etc.)

See above, of course layer 2 is selectable, just enter the number 2.
Or use CLI ;)

>  - to remove the second category from layer 1 I still have to use v.digit
> because neither v.edit nor v.category let me choose the category number I
> want to mantain/remove.

v.category layer=2 option=del cat=-1 should successfully remove all
categories from layer 2.

> Is this the expected behaviour of overlapping regions cleaning?
If you mean that overlapping parts get an additional category in a new
layer, yes.

Markus M

> giovanni
> [1] image: http://www.giovanniallegri.it/share/test_polygon.png
> [2] shapefile: http://www.giovanniallegri.it/share/test_polygon.zip
> [3] http://www.giovanniallegri.it/share/cleaned_test_polygon.png
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