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I am using Zonal Statistics as Table to obtain the Max height from a DEM for each polygon (then Joined to the polygons). For some reason, a lot of the polygons get NULL returned. All the polygons returning Null appear to be between 1 - 4 cells in size (given the DEM scale). Other same sized polygons are however returning the expected result. 154 out of 3131 polygons have NULL values

I have used the Identify tool to check that I do have non-null values in the underlying DEM for a bunch of these.

Screenshot of Identify tool output showing DEM value and NULL from Zonal Statistics

The problem appears to be the FID is not being created for the Zonal Statistics table for some polygons. I have checked a couple of other small polygons and the values obtained from Zonal Statistics appear correct. What should I be looking for/testing? How can I correct this?

I am using ArcGIS Desktop v10.6 with Advanced License.

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You could try resampling the raster to a finer resolution. Not sure why this works but accourding to the help section for version 10.3 (not present in 10.6):

If the zone input is a feature dataset with relatively small features, keep in mind that the resolution of the information needs to be appropriate relative to the resolution of the value raster. If the areas of single features are similar to or smaller than the area of single cells in the value raster, in the feature-to-raster conversion some of these zones may not be represented.

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    I had the same problem. Especially features that were very thin had null values. So I think when they do not cross the centroid of the raster cell, you get NULLS. – blabbath Apr 26 at 5:47

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