[PROJ] Vertical Transformations?

[Martin Desruisseaux]

Le 20/02/2019 à 20:06, Even Rouault a écrit :
>
> For horizontal datum transformations, EPSG has generally both
> grid-based and Helmert-based transformations. Which are both declared
> as valid. Generally the grid-based one will have a better declared
> accuracy, so might be prefered. But that doesn't invalidate the
> Helmert-based one. Failing if the grid is not available would be a
> PROJ specific policy, and not always desirable.
>
I support this position. Both are valid transformations, differing in
their accuracy and some others considerations (performance, etc.). If
PROJ reports the accuracy of coordinate operations, users can decide if
it is good enough for their need. For example if a map is displayed on a
screen at a scale where each pixel is 1 km large, the accuracy provided
by grids is unperceptible and the Helmert-based alternative may have
performance advantages (in some cases) for this rendering purpose. An
application may want to switch between the two methods depending on the
scale of rendered map.

    Martin