[Qgis-developer] Snapping tolerance

Martin Dobias wonder.sk at gmail.com
Sun Dec 14 23:46:42 PST 2014

Hi Andrea

On Sat, Dec 13, 2014 at 3:39 PM, Andrea Peri <aperi2007 at gmail.com> wrote:
> Hi,
> perhaps I dont understand well what this mean.
> Infact we usually editing always in native reference system, never
> editing in reprojected SRS.

Then you do not need to be worried :-)

But I guess that using the project reference system instead of the
> layer srs for snap operations
> could cause the snap will affect from an error due to the reprojection
> operation.

This has always happened in QGIS snapping: if you digitize in a different
CRS, the snapped coordinates may suffer from reprojection errors (snapped
coords are reprojected from layer coords to map coords and then eventually
again to layer coords). We are talking about numerical errors because of
the finite precision -

If this is true, this could mean that the editing result could change
> moving from a PC where the precision in due to some local NTv2 data
> (more precision in reporjection), to another pc where the NTv2 are not
> available.
> And also it could depend from the trasformation choose for the
> reprojection.

This problem is not really specific to snapping. If you misconfigure your
reprojection, you may digitize with incorrect coordinates regardless of any

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