[PROJ] NTv1/NTv2/CTable2 and west-oriented longitude shifts

[Kurt Schwehr]

Why keep the old convention for GeoTIFFs?  I believe the sooner things
become more uniform, the better.

On Mon, Dec 9, 2019 at 1:33 PM Even Rouault <even.rouault at spatialys.com>
wrote:

> Hi,
>
> Another messiness of NTv2 I (re)discover. As NTv2 was designed for Canada,
> it
> uses a "west longitude is positive" convention.
> That applies to the extent of the grid, the order of the columns... and
> the
> longitude shift values.
> NTv1 also uses that convention.
>
> Our venerable CTable2 format uses a more classic convention of negative
> values
> for west longitudes and use a west-to-east column ordering, but has also
> inherited that annoying characteristics that the delta longitudes are
> postive
> to the west... So this affects the +proj=deformation . Kristian, are you
> aware
> of that ? But as you hijack CTable2 to store values in mm/yr, you probably
> compensated that issue with the negative sign.
>
> So what about GeoTIFF storage for horizontal grids ? Do we want to
> perpetuate
> that tradition of positive longitude shift meaning correction to the west
> ? Or
> use the "natural" convention: positive longitude offset values are
> corrections
> to the east ?
> It seems that at the very least I should add a new metadata item:
> positive_value=west/east to document the convention.
>
> Perhaps for grids converted from NTv2, I will keep the original value and
> set
> positive_value=west so that it is obvious we didn't alter the values.
> People wanting to use the "natural" convention will be strongly encouraged
> to
> use positive_value=east (which will be the default)
>
> Thoughts ?
>
> Even
>
> --
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