Moritz, thank you very much for your help. I understand what you mean and I'll try next time I have a similar problem. But in my special case now, the additional steps might be a little complicated. My hope was that there could be a switch or option to tell GRASS if I need the grouped or the single area values. For now, I'll calculate areas in QGIS which does not care about cats. However, thanks again.
Best regards, Uwe -----Ursprüngliche Nachricht----- Von: Moritz Lennert [mailto:mlenn...@club.worldonline.be] Gesendet: Mittwoch, 29. Juni 2016 17:17 An: Uwe Fischer <gisfi...@t-online.de>; grass-user@lists.osgeo.org Betreff: Re: [GRASS-user] area calculation after dissolve 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
