On 09/07/13 13:21, Paolo Ruzza wrote:
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!
One possible option I see allowing you to keep your original resolution
is to use v.to.points (type=boundary and dmax sufficiently small) to
transform your polygon boundaries to points, then v.to.rast to transform
that to raster.
Moritz
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