[postgis-users] How to get an interpolated point on a linestring and make sure ST_Intersects returns True?

Sandro Santilli strk at keybit.net
Wed Oct 7 01:20:46 PDT 2015

On Wed, Oct 07, 2015 at 09:18:21AM +0200, Alessandro Pasotti wrote:

> I'm trying to get a point on a linestring such as ST_Intersects with
> the original linestring will return True but it is failing in some
> real life situations (postgres 9.3, postgis version: 2.1 USE_GEOS=1
> Is this due to rounding errors?

It's due to the constrained nature of floating point numbers.
There's an infinite set of points laying in a line, but just a finite
set of them are representable.

The only way to be guaranteed a point is on a line is to pick a vertex
of that line. Snapping the input line to the closest point is what PostGIS
topology does, for example.


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