With ArcGIS zonal statistics as a table I can easily calculate the mean value from a raster contained within a polygon from a shapefile. I now have a Mac, and therefore I cannot access ArcGIS anymore. Actually, I am trying to use open source software only (e.g. R and QGIS).
I thought that "zonal statistics" from QGIS would do the trick, however, an essential feature for my needs is missing: in ArcGIS, you can specify the column by which you want to aggregate the values. For example, if in your shapefile you have 100 polygons, you may not want to just calculate the mean value from the raster in each polygon. Rather, you may want to know the mean value of a group of polygons. In my case, one column ("veg_class") classifies all polygons as "tropical", "temperate" or "boreal" vegetation.
How can I, in QGIS or any other software, select the column to aggregate the statistics so, in the end, I obtain one mean per veg_class (i.e. 3) instead of per polygon?