[GRASS-user] Single centroid/point for multiple polygons sharing the same category

[Johannes Radinger]

Hi GRASS users,

I am working on a polygon map of France that shows the single areas
belonging to a specific postal code. I want to extract for each area the
centroid (using v.extract), transform it to a point map (using v.type) and
finally add the X and Y coordinates to the attribute table for each postal
code region.

However, now I have the problem that some "postal code areas" have more
than one centroid, i.e. two or more closeby but not connected polygons
belong the same postal region and share a common category and entry in the
attribute table. So for these cases I get two or more points (sharing the
same cat) and of course I cannot calculate a single XY pair for that postal
code region.

Consequently, I need to find a method to get centroids of all the areas
sharing a common cat or I need to calculate the mean position of points
afterwards (using v.centerpoint). As far as I understood v.centerpoint all
points are used to calculate a mean point but not stratified for points
sharing the same cat!? FYI: I have around 2000 postal code areas of which
200 consists of more than a single polygon.

Any suggestions?

/Johannes
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