On 29/06/16 16:33, Uwe Fischer wrote:
I have a polygon dataset which needs to be dissolved by a certain item.
That works fine. But when I calculate the area size for the new created
Polygons in the next step using v.to.db, I find that the system
summarizes all the polygons with the same value in the dissolve item,
even if they are remote from each other (I mean, they do not touch).
That might be correct if I want to handle them like a multipart feature,
but those are not present in GRASS as far as I know. What I need is the
area for each single polygon after removing the inside borders, no
matter what value the polygon has. How to calculate that?
I found that polygons with the same dissolve item also have the same cat
number, which might be the reason for the common area size. But I don't
know how to clear that without losing the connection to the database
table containing the dissolve item.
You could use v.category to assign individual cat values to each polygon
on a different layer (e.g. layer=2), add a table to layer 2 using
v.db.addtable and then v.to.db layer=2 to fill that table.
That way you will have one layer with the original (grouped) cat values
and the related attributes and a second layer with the new (individual)
cat values and the related attributes. If you need to combine the two
into one table, you could add into the layer 2 table a column for the
cat in layer 1 and fill that using, e.g. v.distance:
v.distance from=MyPolygons from_layer=2 to=MyPolygons to_layer=1 up=cat
col=cat_lay1
Moritz
_______________________________________________
grass-user mailing list
grass-user@lists.osgeo.org
http://lists.osgeo.org/mailman/listinfo/grass-user
