qgisraster
I have a raster with land use classes (see image). I want to count the pixel values (i.e. the classes) within the blue perimeter. In QGIS, there is a tool called 'Raster layer unique values report' which can be used to count pixel values. However, it is not possible to select a vector layer to only count the unique values within a specific polygon. As such, I get the pixel counts for the entire raster.
Is it possible to only count the pixel values from a raster within a polygon in QGIS?
Use to be that you had to use v.rast.stats in the GRASS Plugin, but now you can use the inbuilt Zonal Statistics in the Raster menu.
You have a polygon layer loaded and the raster. You run the menu item.
that will answer part a) of your question.
v.rast.stats and other tools will give you more detailed output, but probably a simple workflow for b) is to extract each polygon (use the Raster|Extraction|Clipper menu) and then run a histogram on each. Depends on the structure of your data, how often you have to do it, etc.
r.report from GRASS in the Processing toolbox lets you get a count if you set the Units to c.
Results in (this includes NoData represented as int 0):
+-----------------------------------------------------------------------------+
| Category Information | cell|
| #|description | count|
|-----------------------------------------------------------------------------|
| 0| . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |147600504|
| 10| . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 46945|
| 20| . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 34371070|
| 30| . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 38494918|
| 34| . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 3542216|
| 50| . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 37053253|
| 80| . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 24184039|
|110| . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 8622768|
|120| . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 4279049|
|122| . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 3126056|
|131| . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 786|
|133| . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 452832|
|136| . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 29765|
|137| . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 317|
|140| . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 1725632|
|147| . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 130470|
|153| . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 1153795|
|162| . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 125372|
|167| . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 176|
|175| . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 36144|
|181| . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 160465|
|188| . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 50|
|194| . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 4854|
|210| . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |257580769|
|220| . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 35232865|
|230| . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . | 6891335|
|-----------------------------------------------------------------------------|
|TOTAL |604846445|
+-----------------------------------------------------------------------------+
Taken from:
Best Answer
The
Zonal Histogram toolusing a polygon layer as input will give a new version of the polygon layer with a column for each raster value containing its countAlternatively, you can use PyQGIS to clip the raster by the mask polygon, and then access the pixel counts through its data provider: