[Qgis-user] Zonal Statistics multiple classe

[Nikos Alexandris]

* Nikos Alexandris <nik at nikosalexandris.net> [2018-02-12 10:38:42 +0100]:

>* Nicolas Cadieux <nicolas.cadieux at archeotec.ca> [2018-02-11 23:37:13 -0500]:
>
>>Hi,
>>I have a raster (int16) with multiple class (ex 1 = grass, 2 = 
>>trees...) and a vector polygon file.  I want to have the zonal 
>>statistic (cum, min, max, sum...) for every class.  What is the best 
>>way to get this done in QGis 2.18.16 on Windows 10?
>>
>>Thanks,
>>Nicolas
>
>Nicolas,
>
>not a direct answer, yet if you have access to GRASS' tools, then you could look
>into v.rast.stats.html [0] (or even r.stats.zonal [1] if you have a
>raster version of your "polygons").
>
>If I understand correctly, you want to get statistics per polygon, not
>per class. I.e., min, max, sum and the rest, for each class inside
>each vector polygon.
>
>v.rast.stats will derive the statistics in question from a raster map
>for the parts covered by the vector map. Note, internally, the vector map is
>converted to a raster.
>
>If you have two rasters, then your base map is the "multiple classes map".
>Your cover map will be the "rasterised vector polygons", which you consider as
>the zones to compute the statistics in question within. Very useful,
>here, is the `-r` flag: "Create reclass map with statistics as category
>labels".


Yet, I have to correct myself here. It's the other way around. 
The cover map is the source of numbers to compute the statistics.
Thus, here it will be the "multiple classes map".

The base map will be the "polygons" map.

Nikos

>
>[0] https://grass.osgeo.org/grass74/manuals/v.rast.stats.html
>[1] https://grass.osgeo.org/grass74/manuals/r.stats.zonal.html



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Nikos Alexandris | Remote Sensing & Geomatics
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