Overlay layers creates duplicate records

Question

I'm using the Overlay Layers tool clip records within a reserve boundary. I'd like to think that it functions in a similar way to the Clip tool in ArcGIS Pro, but the results suggest otherwise. When I overlay polygons of prairie fragments with the boundaries of rural reserves, the resulting layer looks really good. Only the prairie fragments that are located within the rural reserve boundaries are present and prairies that extend across the reserve boundary are cut to retain only the area inside the boundary.

Unfortunately when I look at the data table I see many instances of duplicate records. These records generally contain a small polygon in one record, and a larger polygon (that overlaps the smaller one) in the other record. The biggest problem for me is that the two records each show the area of the larger polygon. Thus, for every duplicate, the area is double counted.

I'm after total area of prairie within the reserves, so these duplicates throw off my estimate considerably.

I brought the same data into ArcGIS Pro and used the clip tool to clip the prairies to the boundaries of the rural reserves. No duplicate records. Using the Clip tool in ArcGIS Pro, I found there to be 34,082 acres of prairie within the reserves. Using Overlay Layers in AGOL yielded 53,368 acres of prairie. Clearly the duplicate records yield an inaccurate result.

Am I using Overlay Layers correctly? I understand that it will split records based on the boundaries in the overlay layer, but I didn't think it would duplicate them. It should create two separate records, not smaller polygons nested within larger polygons, right?

AmandaRing · Answer

Hi @Rich_VanB,

Can you confirm how you are getting the area totals?

As @clt_cabq mentions, when using the Intersect option in the Overlay Layers tool, it behaves much more like the Intersect tool in ArcGIS Pro, in that the result will include the attributes from both input datasets. It looks like the HECTARES field in your screenshot is a field that is from one of your initial datasets; this is not a field that will be dynamically updated by the tool. It is simply an attribute that will be carried over to the results.
However, the result should include a field named something like 'Area in Square Miles' that is generated by the tool. Can you confirm whether this field contains the correct area calculations?

Cheers,

clt_cabq · Answer

I imagine the Hectares field in your feature class isn't dynamic - you will have to recalculate that. If you read the help documentation for Overlay Layers you will see you have 3 overlay types, Intersect seems to be the default. So overlay layers performs more like either the Intersect or Union tools in Pro, not like Clip.

Rich_VanB · Answer

Interestingly enough, when I dissolve the internal boundaries of my overlay features (rural reserves), the result matches my output from ArcGIS Pro. Overlay Layers uses the outer boundary of the dissolved reserves to retain only the prairie patches located within. It cuts the prairie patches at the boundary as expected and provides the correct area.

Going back to the original analysis with the undissolved reserve records, it appears that the duplicate records are generally split correctly. The real problem lies with the duplicate data assigned to each. In the screen shots below, you'll see one portion of the split record selected in the first image and the other portion selected in the second image. Even though the two records are clearly different sizes, the data in the table is duplicated for each. Both the FID and the area in hectares are identical.

Shouldn't Overlay Layers create new records with accurate data for these features that are split during the overlay process?

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