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I am using ArcGIS Desktop 10.4.

I have 2 feature classes in the same GDB. One feature class is a subset of the other one (i.e. it's a direct copy, with a subset of features retained, the others deleted). All 190 features in both FCs are identical.

My assumption then is that a union between these two FCs should yield the same total number of features as the source FCs (i.e. 190) because their geometries are directly coincident. However, it doesn't, and a number of slivers are created in the union shapefile.

Can someone please explain why this happens?

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    Are there any partially overlapping features in the original feature class? If there are overlapping features, then the union tool will create multiple features for these areas. Eg, if there are two features that partially overlap, then the Union tool will create multiple features for these: at least 1 for the overlap area(s), at least 1 for the non-overlapping area(s) of the first feature and at least 1 for the non-overlapping area(s) of the second feature. With complex shapes, or more than two features involved in the overlap, the number of features created by Union can be much larger. – Son of a Beach Aug 8 at 4:10
  • Thanks for the reply! Great point, I forgot to think about that. There are 2 known overlaps within the data (I ran a self-intersect on the FCs). So that would mean there should only be 2 extra features in the union layer, right? (the non-overlapping areas shouldn't contribute to additional feature count, but the overlap polygons would).... However, I go from 190 polys in the source FCs to 243 in the union layer.... And most of the new slivers are on outer edges (not shared boundaries)... – JG86Aus Aug 8 at 4:35
  • It depends on the complexity of the shapes and the number of coincident overlaps, but the most likely outcome (for simple shapes with only two features overlapping) is 1 additional feature per overlap. – Son of a Beach Aug 8 at 4:38
  • It may be possible that the two features have differing underlying resolutions, even if you made a direct copy of the feature class. ie. one may have a resolution of 0.001 and one may have a resolution of 0.0001. This happens frequently when converting shapefiles to geodatabase FCs since the default for each format are not the same. – RGfromRWBJV Aug 16 at 21:04

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