qgisstatistics
I have a vector layer of habitats. Each feature belongs to a category and has its area calculated. I'm trying to find out the total area of each type of habitat.
I've looked at QGIS "Statistics by categories" which gives everything but the Sum of each category.
The GroupStats plugin looks like it would do it, but I can't get the "calculate" button to activate.
I'm new to QGIS tools/scripts etc but happy to try to modify an existing tool but haven't yet found where they are or how to do this.
Use 'extract' to overlay polygon features from a SpatialPolygonsDataFrame (which can be read from a shapefile using maptools:readShapeSpatial) on a raster, then use 'table' to summarise. Example:
> c=raster("cumbria.tif") # this is my CORINE land use raster
> summary(spd)
Object of class SpatialPolygonsDataFrame
[...]
> nrow(spd) # how many polygons:
[1] 2
> ovR = extract(c,spd)
> str(ovR)
List of 2
$ : num [1:542] 26 26 26 26 26 26 26 26 26 26 ...
$ : num [1:958] 18 18 18 18 18 18 18 18 18 18 ...
So my first polygon covers 542 pixels, and my second covers 958. I can summarise each of them:
> lapply(ovR,table)
[[1]]
26 27
287 255
[[2]]
2 11 18
67 99 792
So my first polygon is 287 pixels of class 26, and 255 pixels of class 27. Easy enough to sum and divide and multiply by 100 to get percentages.
I haven't calculated mode values before (most frequently occurring value), but at least for quantile values you could try to use the plugin I coded (LecoS). Although not primarily intended for this use it is able to calculate some "zonal statistics" for polygon overlays.
How to:
numpy and scipy for your System and QGIS. On Windows choose the OSGEO4W advanced Install and select python-numpy and python-scipy. On Linux consult your linux package manager/compiler.
Please pay attention that the plugin is still experimental and calculation might fail sometimes! If you want a clear procedure suited for your needs than i would advise you to look into numpy and also R with the raster and sp packages.
I am also happy to see detailed bug reports and i will do the best i can to fix them. However if the calculation fails for you, you better lookout for other tools as well.
Best Answer
You are correct, Group Stats plugin is exactly what you need.
See the example below. You need to drag and drop your category field into Rows, and your area field and the function sum into value. In my image, "Freguesia" is my Category, and I used the area function instead of a field.
The beauty of this plugin is that you can do the same with more than one category by using rows and columns.