[postgis-users] Topology precision

Sandro Santilli strk at kbt.io
Fri Jul 6 13:34:48 PDT 2018

On Fri, Jul 06, 2018 at 11:25:16AM +0200, Marc-Antoine Nüssli wrote:
> Hi there,
> First, I want to say a big thanks to Sandro Santilli and all people who
> work on the topology features. It looks like it becomes now mature enough
> to be used for production applications and this is really great to manage
> geo-data much more consistently than with simple geometries (especially
> when you deal a lot with territorial partitions like in my case)

Thanks for the feedback!

> I have a question regarding the precision value that we have to provide
> when creating a topology. I am not sure to understand its purpose.
> Somehow, I would have expected that it would be used when computing new
> nodes positions, for example resulting from an intersection, but from my
> tests it seems it is not the case (see my example below)
> Do you have some links or other infos that would describe how & when this
> global topology precision parameter is used?

Using it for nodes position is planned. Doing so will need introduction
of the concept of fixed-precision operations which are still unavailable
in PostGIS. Such operations would first enforce snapping of input
geometries to a fixed-precision grid and then ask GEOS to perform the
operation using that grid.

Right now PostGIS Topology only uses the "precision" value to snap
incoming lines and vertices to existing nodes and vertices when using
toTopoGeom or the underlying TopoGeo_addPoint, TopoGeo_addLinestring
and TopoGeo_addPolygon and not overriding the global value.


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