How to eliminate gaps in the ArcGIS union operation?

Question

I have a layer that has nested polygons in them. I want to aggregate the polygons. I do this using the Union function.

I use the "No Gaps" option when I perform the union.

However, when I overlay my unioned layer on top of the original, I can see small gaps in the polygon. Has anyone encountered this before?

We also thought that doing a topology would work better than the union. Except when we overlay the polygon count is different.

What causes these gaps? And how do I remove the gaps?

Thanks!

Answer 1

Without looking at your specific data, I'd say there are three possible answers here.

1. Are what you describe as gaps actually sliver polygons filling the gaps? ArcGIS does not move the polygon sides together to fill the gaps, it produces new sliver polygons to fill them. In order to fill the gaps, you have to start editing, and then merge the sliver-gap filling polygons with one of the neighboring polygons.
2. The other possibility is that the gap you want to fill is not entirely surrounded by other polygons. There are sometimes separations between polygons with one side of the gap simply bordering on empty space, therefore not allowing ArcGIS to produce a filling sliver polygon.
3. Another possibility is, your input is in geodatabase, and you're outputting to shapefile. Geodatabase features use higher precision coordinates, which will sometimes result in gaps between polygons when converting to lower precision shapefiles.