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I'm getting a lot of errors like this:

GEOSUnaryUnion: TopologyException: found non-noded intersection between LINESTRING (147.254 -42.8705, 147.254 -42.8705) and LINESTRING (147.254 -42.8705, 147.254 -42.8705) at 147.25355728849621 -42.870521684000678

I am calling ST_MakeValid(ST_Union(ST_SnapToGrid(mpoly))) on an aggregate of all rows that match my filter.

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    You want to call ST_MakeValid before ST_Union. If union runs into an invalid geometry, it's more likely to throw a topo exception – LR1234567 Oct 6 '15 at 6:04
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    Turns out this isn't enough. – Danielle Madeley Oct 8 '15 at 0:01
  • yeah I ran MakeValid before Union and I still got this error – Theo F Feb 28 at 16:43
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There is a similar question at How best to fix a non-noded intersection problem in PostGIS?. Some have had success by performing ST_MakeValid, ST_Buffer or ST_SnapToGrid to the geometry before to performing the union. I see you tried SnapToGrid. Have you tried something like:

ST_Union(ST_MakeValid(mpoly))

or

ST_Union(ST_Buffer(mpoly, 1e-5))

The presentation PostGIS: Tips for Power Users outlines some potential sources of trouble when unioning geometries.

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