[GRASS-user] Joining vectors by location

[Moritz Lennert]

On 21/11/11 19:24, Daniel Lee wrote:
> Alright, I just figured out a way to do it, albeit an unelegant way that
> requires using another program.
>
> I converted the rasters to vectors as usual. Then I did the following in
> order to preserve the original polygons:
> - Extracted the centroids by using v.select
> - Converted the centroids to points with v.type_wrapper
> - Do a join by location between the polygons in points in QGIS
>
> That worked, but it is VERY unelegant. Does anyone have a better idea?

You can use v.distance with dmax=0 to do such a "spatial join" between 
polygons and points:

v.db.addcolumn YourCentroids col='poly_cat int'
v.distance from=YourCentroids to=YourPolygons upload=cat column=poly_cat 
dmax=0

This should upload the category values of your polygons to the attribute 
value of your centroids. If you then want to join the two tables, you 
can use v.db.join.

Moritz