How does Survex translate between GPS and UK grid?

[Tarquin Wilton-Jones]

Hi folks,

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)

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.

However, just wanted to know if it currently copes with continental
drift, and whether or not it will convert back to GPS nicely (we used a
±2cm differential GPS to get the fix, aligned to ETRS89) when exported
to KML.

Cheers,

Tarquin