I am trying to add three rasters together. I'm trying to figure out where I am going wrong so I am going to list out what I am doing:

  • I have Raster_1, Raster_2 and Raster_3.
  • Raster_1 and Raster_2 were set to the extent of Raster_3
  • I used the Map Algebra tool to write "Raster_1 + Raster_2 + Raster_3" = Raster_4
  • Raster_4 only has areas where all three previous rasters intersect.
  • I thought that by setting the extent of Raster_1 and Raster_2 to Raster_3 this wouldn't be an issue.
  • I went to check that I had done this correctly but the extents had gone back to their previous extents.
  • I reset Raster_1 and Raster_2 to the extent of Raster_3 but they are not the same. I have no idea why they would not be the same if that's how I am setting them. The setting I am using is:

the rectangular extent of Raster_3

What am I doing wrong?


In raster calculations if at least one of your data has no data values, the result will be given a no data, therefore calculation happens only for those cells you have data value in all three rasters. So setting the same extent is not enough.

  1. use the Con((IsNULL(Raster_x),0,RAster_x) raster calculation on all your three rasters in order to change the no data values into a distinct number, I arbitrarily used 0, as you said your next step is to sum the rasters, therefore a cell that is zero won't affect your operation, if you were to multiply the rasters though, 1 would be a better choice. Make sure you set the extent to the largest raster.
  2. Do your raster sum operation
  • One thing to be careful is , you won't be able to tell between the 'zeroes' from your original data and the ones from the new raster operation at step 1 in my answer. The other option is to set a large signed integer for the no data cells at step 1, for example -999999 (This works if all your data is >=0) and to later select all negative cells from the resulting raster after step 2 and add +999999 to them. – yanes Oct 2 '15 at 17:12
  • All of my data is 0.1 to 1. For me to even be evaluating this data, it needs to be at least 0.1. Does this still matter? I'm doing a test run of the above now. – giscard78 Oct 2 '15 at 17:22
  • in that case, setting zero to all your no data will be fine, you'd know any zero in the three rasters means no data after doing step 1. It shouldn't be a problem. – yanes Oct 2 '15 at 17:30
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    Yes, I will upload screen shots but it will be later on tonight when I have access to imgur. – giscard78 Oct 5 '15 at 18:23
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    This problem ended up being solved using a tool that counts the overlapping polygons, no conversion to raster required. Just thought I would let you know! Thank you for your help, I appreciate it! – giscard78 Oct 7 '15 at 10:45

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