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I want to aggregate or dissolve a set of input polygons based on 3 fields - Authority, Category, Site_Name. So where these values are the same, there should be one output feature. If the polygons are not adjacent, a muti-part feature should be output (File Geodatabase). If they are adjacent, then I expect them to be merged. However this is not happening. I've tried both Aggregator and Dissolver, individually and together. The boundaries may not be shared, but I believe the polygons overlap, so why aren't they being merged?

In the attached image, the blue fill is the input, and the hatching is the aggregated/dissolved polygons.

Also, there are 2 blue polygons on top of each other in the middle, but in the output there's no polygon visible.

To start with, the Aggregator won't dissolve polygon. It will generate a multi-polygon feature.

If the polygons are adjacent, it would be expected that they would be merged into a single object or three objects in this case (the one in the middle, the one on top and the one on the bottom). But you should not use the GroupBy, unless adjacent polygons have similar attributes.

In FME2018 there have been modifications to the Dissolver transformer, which might perform better than the older one, so that is something to try.

There are a few possible solutions if the areas are not adjacent. You could try a Snapper transformer, to snap vertices together. Also you could try the SliverRemover (AreaGapAndOverlapCleaner in FME2018) to make sure that the boundaries will be identical. Another option would be to use an Intersector and an AreaBuilder (use the snapping options) to get new polygons and Dissolve the result. A last option would be to use the HullReplacer. In that case you need to have a good look at the settings of the Alpha Value.

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