Summarize Raster Statistics according to polygon boundaries

Question: 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.

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.

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