Union for similar polygons results in thousands of slivers

[DebbieH]

I have two similar polygons of flood hazards, and the new one has changed a quite a bit. Including the projection. In order to show the similarities and differences, a Union is performed. Areas with no change are indicated, as well as increases or decreases in the flood hazard.

However, the Union results in thousands of slivers where there is supposedly no change to the flooding, I am assuming due to the differences in projection. When I project both shapefiles to the same projection, there isn't much of a change. Is there any solution to this, or is it just not possible to get a Union without all the slivers?

[DanPatterson]

I would only Union when both files are in the same projection in any event. 

Are you trying to save doing a projection?

[DebbieH]

Yes, I agree with that - they are in the same projection when I Union. The problem is that the projection is changed by other groups before I get the files - there is nothing I can do about it...

[DanPatterson · Accepted solution]

Then the union is as expected.  

I just hope you aren't using a Web Mercator projection in any event.

Look at the cluster tolerance and no-gaps optional parameters for other information.

Union (Analysis)—ArcGIS Pro | Documentation

[DebbieH]

Thanks - that is what I thought... (no, not using a Web Mercator projection).