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I have found some unexpected behavior of ArcGIS union tool. My assumption was that this tool will never add any new vertex to the output layer, but this was not the case.

I have run the ArcGIS union tool on only one layer GADM (world admn01) and found that in output layer there are around 866 new vertices. I think this tool make tiles of data and then process. After processing there are extra vertices which are basically created because of tiles are not completely deleted.

Is there anybody else with the same problem/experience?

I am using ArcGIS 10.0 on Windows7 64bit

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5

You are spot on, this is called "tiled processing of large datasets", as explained here

To improve the performance and scalability of feature overlay tools such as Union and Intersect, operational logic called adaptive subdivision processing is used. The use of this logic is triggered when data cannot be processed within the available amount of physical memory. To stay in the bounds of physical memory, which greatly improves performance, processing is done incrementally on subdivisions of the original extent. Features that straddle the edges of these subdivisions (also called tiles) are split at the edge of the tile and reassembled into a single feature during the last stage of processing. The vertices introduced at these tile edges will remain in the output features. Tile boundaries also may be left in the output feature class when a feature being processed is so large the subdivision process is unable to put the feature back to its original state using the available memory.

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