Subtracting two elevation layers that don't overlap
I have two elevation layers (that don't overlap in all areas) that I want to compare. When I compare (subtract) the two layers the areas that don't overlap don't show up. Can I set it up so that if there is no data in one layer then it treats that as zero. Eg first layer has no data, second layer has value (depth) of 0.3m. When I subract the two layers I would like that to show as -0.3. ie the depth at that spot decreased by 0.3m.