I have 80 Landsat images, and I have calculated 5 vegetative indices for each one and then created a fuzzy overlay of the indices. So I have 80 images that are fuzzy overlays. My question is, would it be practical to do image differencing using these fuzzy overlays? For instance, a NDVI value of 0.7 means the same thing in all image dates, but does a fuzzy overlay value of 0.7 mean the same thing between images? Or is it being scaled in each image according to the rest of the values?