Understanding Union analysis output using single feature class with overlapping polygons in ArcGIS Pro

Question

I have a single polygon Feature class with about 33,000 shapes. I'm looking for overlaps.

I want to be able to say "Shape x overlaps with Shape y and Shape z"

So the UNION analysis tool claims to find the overlaps, give all the attributes of the clashes give the Feature Class IDs of both the shape and the overlapping shape(s), and all the attributes as well. Sounds great... but doesn't work.

So I have a concrete example with 3 polygons overlapping each other. They are visibly and physically on top of each other. Let's call them Shape A B and C.

In my union analysis, Shape A B and C come up with a total of 248 overlaps in one form or another which is fine, since the shapes are large, and there are many other overlaps too.

But now I am trying to find the specific instance of where Shape A overlaps with Shape B and Shape C... and I can't.

I have looked up the Object IDs for each shape

103 Shape A
709 Shape B
226 Shape C

The union Analysis produces a feature class that gives me all the original attribute fields as I would expect, and one new field called "FID_myFeatureClass". So I have a ObjectID and a FID_myFeatureClass and a Shape Ref.

So I search for Shape Ref, and I find 248 rows with Shape A or Shape B or Shape C. OK. I search for Object ID=103 or Object ID=709 or Object ID=226 and nothing. No matches. I search for FID_myFeatureClass=103 or FID_myFeatureClass=709 or FID_myFeatureClass=226 and ... nothing

So basically, a union analysis says "This shape with the original object ID of x does overlap with something"

... but doesn't say what.

Not very useful....What am I missing?

ArcGIS Pro 2.6.0

Answer 1

So the UNION analysis tool claims to find the overlaps, give all the attributes of the clashes give the Feature Class IDs of both the shape and the overlapping shape(s), and all the attributes as well. Sounds great... but doesn't work.

Sounds like you're misunderstanding what the Union tool actually does. What it actually claims to do is:

Union can run with a single input feature class or layer. In this case, instead of discovering overlap between the polygon features from the different feature classes or layers, it will discover the overlap between features within the single input. The areas where features overlap will be separated into new features with all the attribute information of the input feature. The area of overlap will always generate two identical overlapping features, one for each of the features that participates in that overlap.

If you're looking for where A, B, and C overlap, Union will not give you an output with a single area-of-overlap feature listing A, B, and C. Instead it will give you 3 identical features, one for A, one for B, and one for C.