QGIS – Calculate Average Raster Value within Polygons by Type Using QGIS

qgisrastervectorzonal statistics

I have seen How to calculate the average value of a raster within polygons?, which helps me a bit but not completely.

I have a raster of 16000 by 12000 (country-size) and a vector which indicates different types of landuse. I would like to get the average value of the raster per type of landuse (Forest, Recreation, Roads, Farmlands, etc.)

The post linked above says to use the 'Zonal Statistics' tool. Unfortunately I can't seem to get this to work for a layer with a lot of different polygons.

I could create separate layers for each landuse type and use that for the analysis, but I rather have something quicker.

Maybe an option is also to rasterize the vector and then use the 'Raster Layer Zonal Statistics'tool?

Best Answer

The solution would be to Dissolve (Processing Toolbox -> Vector Geometry) your polygon layer using the landuse as the "Dissolve field(s)". By doing this you will have one polygon layer per landuse type. Then you can apply the Zonal Statistics (Processing Toolbox -> Raster Analysis) which is going to give you a single value (for the selected parameter to compute, mean for instance) for each landuse.

Related Solutions

[GIS] How to calculate raster statistics for polygons

I was struggling to do exactly the same thing, but for various reasons I'm committed to using QGIS. I tried using v.rast.stats using the GRASS plugin and also via the Sextante plugin. The latter approach failed, because it seems to attach the stats to a temporary vector layer which it then deletes. The GRASS plugin worked, but it doesn't deal with overlapping polygons.

After doing some digging around (in the source of the promising-sounding ZonalStats plugin), I found that QGIS actually has zonal statistics methods built into the API, and these also have Python bindings. So as long as you are only looking for count, sum and mean statistics for your polygon features, the Python Console (Plugins > Python Console) is currently the easiest way to attach the stats to the polygon attribute table.

Select your raster layer in the TOC, and type the following into the console (it grabs the source file name of your raster layer)

>>> rasterfile = qgis.utils.iface.mapCanvas().currentLayer().source()
Select your vector layer, and execute the following command in the console (it grabs the vector layer itself)

>>> vectorlayer = qgis.utils.iface.mapCanvas().currentLayer()
Execute the following three commands in the console (they pass the vector layer and raster file to QGIS' built-in zonal stats calculator)

>>> import qgis.analysis

>>> zonalstats = qgis.analysis.QgsZonalStatistics(vectorlayer,rasterfile)

>>> zonalstats.calculateStatistics(None)

The results will be appended as extra fields in the polygon layer.

Zonal Statistics

Note that if you want to append a prefix to the new fields (rather than just having them as sum, mean and count), then you would replace the first command in step 3 with the following (assuming you wanted to use 'zonal_' as the prefix:

>>> zonalstats = qgis.analysis.QgsZonalStatistics(vectorlayer,rasterfile,"zonal_")

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

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

Best Answer

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[GIS] How to calculate raster statistics for polygons

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

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