How does Survex translate between GPS and UK grid?

[Graham Mullan]

I have a similar query. I have a large dataset covering many caves and cave entrances. The set has been collated over a number of years, some data is in Survex files and some in Therion files. As far as I recall, there are three different EPSG co-ordinate systems buried in various parts of the data. Crunching the data in Therion produces a properly integrated 3d file with everything on it apparently in the right places. I need to know what the output co-ordinate system is as I want to import the 3d file into a GIS map.

Graham

-----Original Message-----
From: Cave-Surveying <cave-surveying-bounces at survex.com> On Behalf Of Tarquin Wilton-Jones
Sent: 02 October 2020 19:58To: cave-surveying at survex.com
Subject: How does Survex translate between GPS and UK grid?

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

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