[GRASS-user] Overlaying vector with raster-computational problems

Paolo Ruzza paolo.ruzza12 at gmail.com
Tue Jul 9 04:21:41 PDT 2013

Dear GRASS users,

I am confronted with a "non-standard" problem and I would appreciate any
help that you can give me. I have several vector files (polygons) and an
arbitrary raster grid, only with category numbers associated to it. I would
like to overlay the vectors onto the raster grid in order to query it and
extract the categories of the raster grid which overlap with the polygons
of each vector file. I am not interested in getting any info from the
vector polygons, I just want know where the vector and the raster overlap.
I have already thought of a way of doing this-I guess it would naturally
come to mind of every GRASS user. This would involve converting the vectors
into a rasters and then query the raster grid and the rasterized vectors
with r.stats. This would be very easy to do, but  my vectors have very
small islands, which can, of course, be converted into raster cells, if I
set them my region to a very fine resolution for. However, if I went down
this route, this would: i) massively slow down vector to raster conversion
ii) massively slow down the r.stats querying process. Just to give you an
idea of the magnitude of my problem I am using several vector layers
(hundreds) at global level and a raster grid at a 2 arc-minutes resolution.
I am kind of stuck and I am not sure if there is any computationally
efficient strategy to do what I want. Any help would be greatly appreciated!


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