I have a set of rasters (geologic risk) from which I want to extract zones with a specific color, clipping them with a polygon (administrative borders), outputting a shapefile.
I'm using QGis (and GDAL from command line).
I know I can use clipper, polygonize and show only features with the value I need, but:
- it takes ages to polygonize the raster, and I don't want much of the information in it;
- clipper doesn't work because the polygon border is larger than each raster, but fits inside the sum of all rasters
- to make things complicated, the zone I would like to extract don't have the same color value in all rasters (but do have the same rgb color)
How can I solve it in a fast way?