Raster Math and no data

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08-09-2012 06:42 AM
New Contributor
I have created a raster from a polygon and am trying to add the cell values of this raster to an existing underlying DEM.  I have tried "Plus" in Raster Math and Combine and Weighted Sum in Spatial Analyst Overlay tools.  Each time it removes the cells of the underlying DEM when there is no data for the raster I am adding. Why wouldn't any of these processes have an option to count no data from one or either of the rasters as ZERO?   I want to keep all the cells from the existing underlying DEM, but only add the cell values of the raster I created where they overlay. 

The only thing I can think of now is to create a polygon that encompasses the raster I created from the original polygon, union them so I can then assign a value of zero to the polygons where there was no data before, and then rerun my polygon to raster.... what a hassle.  Any other ideas?

Thanks,

Ryan
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Regular Contributor III
Hi Ryan,

This is a common question.  You can find the solution via Con(IsNull) command in the raster calculator.  For details, see this Support Services blog, How to add different-sized rasters with NoData Values ,  or you can see other forums with the same question such as this one.

http://forums.arcgis.com/threads/33347-Raster-Extents-Calculator-Problems

When you convert the polygons to raster, you should set the extent to be the same as the DEM and the same pixel size as the DEM.  Then Con(IsNull) will give you no problems.

Best,
Eric
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Esri Regular Contributor
also if you are using ArcGIS 10, you could use CellStatistics and set the extent to Union of..

see http://forums.arcgis.com/threads/59895-Combine-Rasters-using-Con-Expression-Error?highlight=cellstat...

Steve
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