After applying the Diff operator of the raster calculator in arcgis to view changes in land use, would like to see the areas of increased and areas of decreased. This is possible using the raster calculator?
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2 Answers
To answer your question yes, this is possible but it is important to consider a few other things to make sure you are getting proper results.
In theory this is a simple subtraction problem, however there are a few things that need to be accounted for in order to obtain the correct results. First and foremost the two rasters need to be orthogonal and concurrent. Meaning that they have the same extent and that the there cell centers line up. This will guarantee that you are subtracting the same cell from the different time steps.
Additionally it is important that you subtract the old raster from the new raster to simplify the interpretation of the results. This will ensure that negative cells coincides with what would be considered erosion and positive cells coincide with what would be considered deposition.
Another complicating detail to consider but this seems a bit overkill at least for your question (and what sort of magnitude of change you are trying to see) is uncertainty in the rasters that you are using in your calculation. There are many methods out there to quantify uncertainty in a raster.
If looking at volumetric change is something that you do a lot of, I would encourage you to look into the Geomorphic Change Detection (GCD) Software. Depending on what version of Arc you use it has a plug-in version and an Add-In version. GCD puts tools to guarantee orthogonality/concurrency, tools to calculate uncertainty and many other tools that make calucating change between rasters in one place for you, is very well documented, and provides different methods to report your results. Plus it is open source and free!
What kind of data are you comparing? Since diff apparently returns the absolute value difference, you might just subtract one raster from another to get the kind of information you're looking for.
