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List:       qgis-user
Subject:    Re: [Qgis-user] Aggregation of polygons into vector grid
From:       Sebastian Schubert <schubert.seb () googlemail ! com>
Date:       2011-10-13 12:38:57
Message-ID: 4E96DBE1.3060705 () googlemail ! com
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On 05/10/11 07:10, Micha Silver wrote:
> 
> On Tue, 2011-10-04 at 16:55 +0200, Sebastian Schubert wrote:
>> Hi,
>>
>> I have a map with several polygons i, each having a certain value x_i. I 
>> want to calculate the average value of x in a rectangular grid. Each 
>> cell j of this grid includes (parts of) several polygons with A_ij being 
>> the area of the part of polygon i being in grid cell j. With that the 
>> average value in grid cell j is
>>
>> x_j = sum_i A_ij x_i / (sum_i A_ij)
>>
>> I was able to create a grid with Vector > Research Tools > Vector grid. 
>> Now each cell of this grid should include the average value of all 
>> polygons inside the cell (as described above).
>>
>> Any idea how to do that?
>>
> 
> You probably want to look at the GRASS module v.rast.stats

Thanks for the hint. This, however, does exactly the opposite.
Statistics of raster based values controlled by a vector file. I have
the values in the vector polygons and I want to calculate the statistics
based on the pixels of a raster map.

What I did was basically convert the vector map to a very fine raster
file and then use the function
r.resamp.stats
to calculate the values for the wanted coarse resolution.

Cheers
Sebastian


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