[GIS] How to calculate the average value of a raster within polygons

grassqgis

I am using the GRASS plugin in QGIS. I am trying to calculate the average value of a raster dataset within a set of polygons.

I have created a new mapset and have imported both the vector and raster layers into the GRASS mapset.

I am now trying to calculate the set of statistics using the v.rast.stats tool.

It starts to run and then comes up with the error an error occurred while converting vector to raster and then 'raster map not found'.

v.rast.stats raster=secant_grass@zonalstats
vector=plots_grass@zonalstats
colprefix=g/Applications/GRASS-6.4.app/Contents/MacOS/scripts/v.rast.stats:
line 190: 632 Segmentation fault v.to.rast in="$VECTORFULL"
out="${VECTOR}_$TMPNAME" layer="$GIS_OPT_LAYER" use=cat type=area
--quiet An error occurred while converting vector to raster Raster map not found  nothing removed Finished with error

Any help would be greatly appreciated.

Regards

Best Answer

Use the zonal statistics plugin in QGIS - this gives you the count, sum and mean of a raster within a given polygon.

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Perform a hydrologic analysis on your data. Taking your first step of water bodies as a raster, you can then use that as a sinks raster. I'll specify the rest of this analysis in terms of GRASS as you mentioned that's the GIS system you're using:

Set up r.watershed (documentation) with the elevation data layer you'd like to analyze and the sinks raster you generated in the first step:

r.watershed elev=input_dem depression=input_sinks basin=output_basins \ 
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Where threshold is something appropriate for the scale of your data-- this should give you a map of each watershed: if you subtract that elevation from all the cells in that basin, you should get the vertical distance to the nearest water. You may need to iterate over smaller regions than your full raster to get good performance. You might find these tutorials (1, 2) also helpful in understanding how to use the command.

Mike helpfully mentioned an addon called r.watershed.distance which can be used to calculate this all in one go:

r.stream.distance -o dir=dirs stream=streams dem=elev \ 
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Which will result in an output close to what you're interested in:

This example was taken from the R.stream.* page, you can download the extension itself from the addon page.

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For me the workaround for this issue was setting the region extent and the cellsize (instead of leaving them blank)

EDIT: Setting the extent to be too large made the issue reappear

Best Answer

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