topic Re: Dissolve and summarized union giving different area calculations in ArcMap Questions

Dissolve and summarized union giving different area calculations

tnels16 — Fri, 23 Jul 2021 00:17:11 GMT

I am trying to get the area coverage of a layer in my map, and the two methods I've used to do so are producing different results.

I used dissolve to create one feature in the layer and get the SHAPE_Area off the attribute table, and then I unioned the layer and summed the areas from that result. The dissolve is giving me 654901660.917106 m^2, and the summed union is giving me 1316663489.162411 m^2. Everything is using the same projected coordinate system. Anybody know what is happening?? This is driving me crazy...

Re: Dissolve and summarized union giving different area calculations

DanPatterson — Fri, 23 Jul 2021 02:12:28 GMT

are the polygons overlapping anywhere?

Re: Dissolve and summarized union giving different area calculations

tnels16 — Fri, 23 Jul 2021 04:34:46 GMT

There were before I did the dissolve and union, but I did that to get rid of them.

Re: Dissolve and summarized union giving different area calculations

DanPatterson — Fri, 23 Jul 2021 09:02:12 GMT

How Union works—ArcGIS Pro | Documentation

Union (Analysis)—ArcGIS Pro | Documentation

you might want to run the Multipart to singlepart tool as well and use the AddGeometryAttributes tool to confirm shape area values. Check the latter result against a polygon that had no overlaps or adjoining polygons to confirm the process

Re: Dissolve and summarized union giving different area calculations

tnels16 — Fri, 23 Jul 2021 10:24:46 GMT

At what step do you run that tool? There is already geometry added to the layers.

Re: Dissolve and summarized union giving different area calculations

tnels16 — Fri, 23 Jul 2021 18:39:57 GMT

The multipart to singlepart tool didn't seem to help, but I was able to identify some overlap in the union, and feel more confident that the dissolve tool is giving the correct area now. Thank you!