postgis
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.
It means that nodes cannot be in the same spatial space during intersection. I drew you a picture.
These are the same lines, but intersected.
If you noticed, if I had dropped the node in the middle (since it is not needed) that the bottom diagram would be impossible.
In my experience this problem is nearly always caused by:
The "nudge" approach of the ST_Buffer solutions lets you get away with #2, but anything you can do to resolve these underlying causes, like snapping your geometry to a 1e-6 grid, will make your life easier. The buffered geometries are usually fine for intermediate calculations like overlap area, but you'll want to be careful about retaining them because they can make your close-but-not-quite problems worse in the long haul.
PostgreSQL's exception-handling capability lets you write wrapper functions to handle these special cases, buffering only when needed. Here's an example for ST_Intersection; I use a similar function for ST_Difference. You'll need to decide if the buffering and potential return of an empty polygon are acceptable in your situation.
CREATE OR REPLACE FUNCTION safe_isect(geom_a geometry, geom_b geometry)
RETURNS geometry AS
$$
BEGIN
RETURN ST_Intersection(geom_a, geom_b);
EXCEPTION
WHEN OTHERS THEN
BEGIN
RETURN ST_Intersection(ST_Buffer(geom_a, 0.0000001), ST_Buffer(geom_b, 0.0000001));
EXCEPTION
WHEN OTHERS THEN
RETURN ST_GeomFromText('POLYGON EMPTY');
END;
END
$$
LANGUAGE 'plpgsql' STABLE STRICT;
Another benefit with this approach is you can pinpoint the geometries that are actually causing your problems; just add some RAISE NOTICE statements in the EXCEPTION block to output WKT or something else that will help you track down the problem.
Best Answer
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:or
The presentation PostGIS: Tips for Power Users outlines some potential sources of trouble when unioning geometries.