I am looking for a tool in QGIS 2.18 similar to the Combine tool in ArcGIS for rasters. I have four different raster layers that I would like to combine into a single raster that has values that represent the unique combinations of the source rasters. My source raster layers have 2-6 different classes. E.g. geology types 1-6, topographic position class 1-3, Aspect class 1-2. So they have values that range from 1-6. I would like to use this to stratify my study area by these unique combinations so that I can then randomly select sample sites.
This other question (Seeking tool in QGIS similar to ArcGIS Combine?) finds a solution that tells you how much of the area has different combinations, but I would like the result to be either a raster or vector layer so that I can then use it for other spatial analyses.
I can probably do it by converting them to vector layers by doing some unions and then some field calculations. But I didn't want to have to stuff around with invalid geometries that always seem to crop up when working with vectors and this seems a clunky way to go about it.