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.
Best regards,
Uwe
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
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:mlennert@club.worldonline.be]
Gesendet: Mittwoch, 29. Juni 2016 17:17
An: Uwe Fischer <gisfisch@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