[geos-devel] Precision/tolerance questions

[Sandro Santilli]

On Thu, Jul 30, 2015 at 11:03:55AM +0200, Hugo Mercier (ML) wrote:
> Hi all,
> 
> I am not very familiar with the way GEOS can handle precision models ...
> 
> If I am correct, having a "fixed" precision model, i.e. a regular grid
> allows to define a tolerance where (point) coordinates will be snapped
> before computations.

This is actually not always true. Input coordinates are tipically NOT
snapped/rounder, while coordinates that are generated by internal
operations will be. This is to avoid duplicating the work when the
input is already rounded.

> Does it mean also predicates work with a tolerance, like the
> intersection test between a point and a line will take a distance into
> account ?

Nope, predicates are exact.

> Similarly on this page
> https://trac.osgeo.org/postgis/wiki/ToleranceDiscussion
> 
> I can read "Adding a PrecisionModel to the GEOS functionality uses a
> precision grid to "snap" all coordinates, which could eliminate some
> rounding and robustness issues with some functions. "
> What rounding and robustness issues are they ? and which functions ?

Constructive functions can raise an exception when unable to deal with
numerical instabilities. Intersection/Difference/Union...

--strk; 

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