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Summarize Raster Statistics according to polygon boundaries

August 25 2009 | 2 comments
Categories: ArcGIS Methods, Data Modeling

I have a raster elevation file and a polygon SHP file, and I would like to know the mean elevation according to each spatial unit in my SHP file. I see that this can be done through zonal statistics, but I don't have zones in my raster file, but rather only a SHP file. Later I will repeat the same process for aspect, and slope.

Thank you for your help.

Mapping Center Answer:

You should be able to use the Polygon to Raster tool (ArcInfo License) to convert your shapefile to a raster.  One of the tool parameters is the value field from your polygons that will contain the zone ID.  The one thing to watch out for are multipart polygons which will not work, so you may need to use the Multipart to Singlepart tool first.

Though it may be more useful to use the Raster to Point tool.  This will take longer, but then you can use the Identity tool to assign the polygon IDs to these points. That will allow you to do a more meaningful presentation of what average elevation means by including a standard deviation, and a cell count.  To do that use the Summary Statistics tool.  You could get the same information with rasters, though you'd have to produce three raster datasets, and optionally use the Combine tool to have a similar attribute table for a single raster dataset.

Zonal Statistics posted by Patrick Whelley on Apr 27 2010 5:58AM
Use Zonal Statistics (or Zonal Statistics as Table) to summarize the values of a raster based on the area covered by polygons. Statistics are calculated for each object, so you might have to combine statistics from multiple objects to get the answer you want for each unit, but that can be done (in excel if need be).
Good suggestion! posted by Aileen Buckley on Oct 28 2010 9:45AM
Thanks, Patrick!

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