Tracking Dissolve changes in ArcMap?

Question

How would one to check to see what was dissolved using the Dissolve tool in Arcmap 10.2?

Other than manually checking to see what polygons were dissolved by toggling back and forth between the dissolve input and output layers is there a method, tool, or way to visually verify what was dissolved?

Answer 1

With just the dissolved data, no, you cannot. With both the input and dissolve result, two ways come to mind immediately:

• Use a Select by Location. Select features from your original layer that are identical to your dissolved layer, then invert the selection. These will be the polygons in your original layer which were dissolved.
• Spatial Join your original layer to your dissolved layer and use a field mapping on one of the fields to provide a count statistic. Any dissolved features will have a count greater than one. Features that were not dissolved will have only a 1.

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If you want to go a step further, you can't keep track of which polygons contributed without some further work, since those attributes aren't created by the tool when it aggregates. I suppose you could do it with some kind of math statistic, since you can aggregate field values with some basic stats (mean, min, max, etc.).

The simpler solution is to use another overlay tool like intersect, union, or a spatial join between the original data and the dissolve results. You get a bunch of duplicate geometries or a straight many-to-one relationship for a join and have to flatten that back to a single table/feature set. Perhaps with one attribute that concatenates all the input feature IDs; see Summary table with a field that shows components of each summarized record for an example of this.