How does Survex translate between GPS and UK grid?

[Olly Betts]

On Fri, Oct 02, 2020 at 07:57:30PM +0100, Tarquin Wilton-Jones wrote:
> Just wanted to check how Survex handles it when I supply fixed points in
> either GPS or UK grid, and then try to output/export coordinates between
> them.
> 
> 1. Does Survex use Helmert transforms to get between them, or OSTN15?
> (or OSTN02)

Survex uses PROJ for all the heavy lifting here, so that's the place to
look for details.  I think how it converts probably depends on the pair
of coordinate systems it is converting between.

> 2. Does Survex translate between WGS84 and ETRS89 (based on the survey
> date) before converting to grid, or does it treat them as the same?
> 
> I know right now, the difference for #1 is about 2.5 m on average, and
> for #2 it is about 79 cm in 2020, both of which are more accurate than a
> typical GPS, so I will not be surprised if you use basic Helmert and
> equate WGS84 and ETRS89. In fact I fully expect you to.

We don't currently pass the survey date to PROJ (because our code
was written for an older version of the PROJ API which didn't allow
passing the date and nobody's update it yet), so coordinate systems
that vary with time will presumably be effectively pinned at some
arbitrary date.

Cheers,
    Olly