[postgis-devel] error in ST_ModEdgeSplit

Sandro Santilli strk at keybit.net
Wed Feb 11 03:28:49 PST 2015

On Wed, Feb 11, 2015 at 12:02:13PM +0100, RĂ©mi Cura wrote:
> Hey,
> I get an error with ST_ModEdgeSplit that looks like aa precision related
> issue.
> When trying to split an edge with a point (the point is from
> ST_ClosestPoint, so _very_ close to the edge),
> st_modeedgesplit complains that the point is not on the edge.

This is a known issue. I've mixed feeling about what the function
should do because "ST_" prefixed functions supposedly come from
the standard that makes no mention of "precision".

The toTopoGeom function handles that by first snapping the edge against
the point retrived by "ClosestPoint". Note that such snapping might
move the edge in a way that it could end up colliding against another
edge. This may happen if the intersection node you found is close to
another existing node and between two indicent edges almost parallel.
There's no primitive function handling such case, which would involve
updating partially merging the two edges into one.


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