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Hi,

I have some points (boundary stones) that should be aligned on the contour of boundary polygons. Since the field location accuracy is known to be inaccurate, I want to snap the points to the contour of the polygons.

I supposed that AnchoredSnapper was the transformer to do this. And it seems to work fine. However, when I further apply a PointOnAreaOverlayer transformer, although all the points visually seem to be on the polygon contour, some points have 0 _overlaps.

when I check the output of PointOnAreaOverlayer some snapped features have _overlaps = 0

I was careful to link the snapped output of AncoredSnapper to PointOnAreaOverlayer.

What am I missing ? workbench

Your process is correct.

 

First thing I would do is inspect that failed feature and take a look at it's coordinates (select that feature and then look at the Feature Information panel of Visual Preview), compare them to the corresponding ones on the area feature.

 

Most likely the point are snapped to the edge, so technically wont be inside the polygons

Most likely the point are snapped to the edge, so technically wont be inside the polygons

I assumed this as well, but I can't reproduce it. You can check if it does touch the border using the SpatialRelator.

@timotheebecker​ If you could include a small sample dataset and workspace so the community can take a closer look at the issue you've encountered. I could reproduce the issue

OK @markatsafe. Here is the test data and workbench.

You'll see I am able to solve the problem with a bufferer transformer. But to my understanding, it should work without this.

@timotheebecker​ Thanks very much for including your workspace and the sample data. This helps us see the issue more clearly.

There are some interrelated behaviours that cause your issue - that SpatialRelator apparently does not return the correct intersection result:

  • SpatialRelator does not have a tolerance and the intersections must be exact.
  • PointOnAreaOverlayer doesn't have a tolerance so points have to lie exactly on the line to get a match.
  • The AnchorSnapper does insert vertices on the the Anchor features when using segment snapping

So when a point is near the middle, say, of a line segment, the slight difference in the calculated location on the line and the actual point location is enough to throw off the PointOnAreaOverlayer, SpatialRelator or SpatialFilter. Inserting a point on the areas boundary will help SpatialRelator/Filter do it's work.

For this problem you can use either:

  • AnchorSnapper, but use the Anchor areas as the input to the SpatialRelator/Filter Supplier port. This will use inserted points on the areas and allow SpatialRelator/Filter to do it's work .
  • NeighborFinder, with a small tolerance and test for a reasonable '_distance" value
  • Intersector, Snapper, TopologyBuilder are other options for adding vertices to the areas, where the points are, so SpatialRelator/Filter can do their work

You don't really need the PointOnAreaOverlayer. I've attached a revised example (FME 2021.2)

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