How does Survex translate between GPS and UK grid?

[Olly Betts]

On Mon, Oct 05, 2020 at 07:21:58PM +0100, Tarquin Wilton-Jones wrote:
> Hi Olli,

Hi Tarkwin,

> >> 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.
> 
> I think it can be summarised by this:
> https://gis.stackexchange.com/questions/364871/why-does-pyproj-give-a-different-point-location-compared-to-ordnance-survey-when
> 
> By default, Proj will use Helmert transformations. It can use OSTN15,
> but it will not in the case of Survex, because you most likely do not
> install the OS's OSTN15 package, since it is non-free (from an open
> source perspective).
> 
> Technically, you could make it work. Legally the Survex project cannot.
> Maybe this is something users can install for themselves, but obviously
> this has limitations on systems where everything is pre-compiled (Windows).
> 
> Unless I missed something.

https://www.ordnancesurvey.co.uk/business-government/tools-support/os-net/for-developers
says "This software is licensed as per the terms of the BSD Licence"
(with a link to the 2-Clause BSD Licence).  It seems that includes the
models as well from a quick poke inside a download

They are quite large though, so bundling them doesn't seem ideal.

Cheers,
    Olly