With the Zonal statistics plugin, you can analyze
the results of a thematic classification. It allows you to calculate several values
of the pixels of a raster layer with the help of a polygonal vector layer (see
figure_zonal_statistics). Choosing a color band, the plugin generates output
columns in the vector layer with an user-defined prefix and calculates for each
polygon, statistics on pixels that are within.
The available statistics are :
Figure Zonal Statistics
Zonal statistics dialog