I'm adding two rasters that have different extent. The result is a raster with the extent of the smaller one, which is not what I want. What do I need to do to have the output raster as big as the biggest of the input rasters? I have tried to increase the extent of the smaller one with "Copy Raster" and in 'Environments' I have selected the biggest raster as 'Extent' and 'Snap Raster'. This has not worked unfortunately.

Am I doing anything wrong? Is there any other way to do what I'm trying to do?

NOTE: I need to extend the smaller raster and set all those new cells to 0.

  • Just to clarify, does the smaller raster always fall entirely within the larger raster?
    – Hornbydd
    Dec 13, 2017 at 17:56
  • Yes, it does fall entirely within the larger raster. Dec 13, 2017 at 20:17

3 Answers 3


One approach is this:

  • Reclassify all the cells in your larger raster to zero using the reclassify tool.
  • Add your smaller raster to your new large zero raster using the Plus tool.
  • 1
    The first step was only to set up the background of zero values that the OP was looking for.
    – Hornbydd
    Dec 14, 2017 at 9:09

This basically worked for me but I had to first get the NoData cells into the larger extent in order for the IsNull to work. Someone elsewhere pointed out that using the Export dropdown command from the (small) raster layer in ArcMap and setting the Output Extent to display does this nicely. (Making sure the display is larger than the desired expanded raster.) I could check this by setting the NoData symbology to a set color. The Con(IsNull) worked but I did not add the small and larger rasters, but just used the small raster where it was not null.

  • Did you find a way to set this larger boundary in code?
    – Gigamosh57
    Feb 11, 2022 at 0:05

The problem is likely to come from the presence of nodata values, which "absorb" other values in a computation. In you case, you could use the raster calculator (geoprocessing > toolbox > spatial analyst > map algebra), define the output extent as the union of the inputs, and then apply :

Con(IsNull("your_small_raster"), "large_raster", "large_raster" + "your_small_raster")

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