[Proj] Realigning velocity grid values by Helmert shift - is that normal?

[Even Rouault]

Hi Kristian,

I have rather questions than feedback than geodesy business feedback ;-)

> In anticipation of a new velocity model being released soon,
> the original model is instead used by applying a Helmert shift to the
> gridded velocities.

What is the typical format into which you can store those gridded velocities: one of the 
existing supported grid formats ?

> The simplest
> solution is to just create a new gridded model where the correction has
> been applied, and using it with a "velocitygrid" operation.

So the parameters of this Helmert Transform are always the same for a given grid ? Or per 
year and/or country ? (this is probably explained in the paper...) If there's just one set of 
parameter for the grid, this would perhaps make sense to have a pre-processing script to 
correct the velocity grid.

> Another option
> would be to incorporate the Helmert shift into the velocitygrid operation
> so that the realignment can be performed when loading the grid.

Performed when *loading* the grid (ie correcting all values of the grid), or rather performed 
when selecting the particular velocity of the grid that applies to the coordinate being 
transformed ? The former could have performance implications if the grid is big and you need 
to transform only a couple of coordinates.

> A third
> option is to introduces some a form of nested operation in the pipeline
> framework.
> 
> I don't want to make things more complicated than they need to be, so I will
> only consider option two and three above if the concept of "realigning
> gridded velocity models by Helmert shift" is a somewhat common operation.

Option 3 sounds like a complication in the general framework. Would probably require some 
parenthesing syntax.

Even


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