Why might I be losing data when computing Map Algebra using the Raster Calculator?

Question

Greetings,

I am trying to compute Map Algebra using the Raster Calculator in ArcGIS 10.5 to find the mean of a set of raster (.tif) layers. The raster layers have an 8-bit radiometric resolution so I have 256 ‘shades’ of data in each raster layer. However, after the computation, my new composite raster only has 252-254 possible data values. I kept the default parameters in the Environment settings, so I am wondering whether I missed in the Environment, or is this a glitch with ArcMap 10.5?

Thank you,

Brent

SteveLynch · Answer

Brent

Time for some debugging 🙂

Subtract the RasterCalculator output from the CellStats output and find a pixel where they are not zero. Find the value at this location for all your input rasters and calculate the mean. What do you see?

-Steve

deleted-user-sBBlTZilBs4n · Answer

Hi Steve,

Yes, all of the rasters have the same spatial reference, 1km cell size, and origin as they were retrieved from the same dataset. Sorry, I should have mentioned that some cells from the input rasters do contain missing data, so might this issue be compounded after the computations?

Cell Statistics was well-suited for this computation, but I am still curious as to why there is data loss with Map Algebra.

Thank you!

Brent

SteveLynch · Answer

Have you tried CellStatistics

and with regards the RasterCalculator issue, do all the rasters have the same spatial reference, cell size and origin?

and, for example,  (256+256+200)/3 = 237 which means that the output could have a value less than 256. Maybe I'm not understanding what you are saying.