count features in polygon shapefile A that fall within each feature of polygon shapefile B

Question

I have two polygon shapefiles, each of which contain multiple (possibly overlapping) polygons.

Is there some way, using GRASS, to calculate the number of polygons belonging to shapefile A that fall within each of the polygons that exist in shapefile B? Preferable would be a method that doesn't require rasterization, but I'm open to going down that route if necessary.

(I'm aware that this can be done in R, using the over() function, but these are large files I'm referring to, and so I'd prefer to do it in GRASS.)

Answer 1

To continue webrian's suggestion:

v.select ain=poly_a b_in=poly_b oper=within out=a_within_b

Now the output vector "a_within_b" will contain those polygons from a that were totally within b. But be careful: when you import a shapefile into GRASS with overlapping polygons, the overlaps will be split up by the topology "clean" operation, so you'll have more, smaller polygons in the GRASS vector than in the original shapefile.

Now to count the number of polygons, you might (depending on the database GRASS is using) be able to do:

echo 'SELECT count(cat) FROM a_within_b' | db.execute

But this raises the question: why not do this right in a spatial database from the start? Import both shapefiles into spatialite, and run:

SELECT count(poly_a.pk_uid) FROM poly_a, poly_b 
WHERE ST_Within(poly_a.Geometry, poly_b.Geometry);

Answer 2

Have a look at the v.select module in GRASS GIS. There is no need to rasterize.