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

[Paolo Ruzza]

Thanks for the suggestion, but if I think that if I follow the procedure
which you have indicated I will only be able to partially solve my problem.
What about those parts of the polygons that are not boundaries? These might
well overlap with certain cells of the raster grid...

thanks,

Paolo




2013/7/9 Moritz Lennert <mlennert at club.worldonline.be>

> 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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