[PROJ] Some questions about UK CRS

[Peter Gagliardi]

Hi everyone,

I have a separate question about some CRS in the UK that a client wanted us
to support.

1. For EPSG:7405 OSGB36, it looks like it tries to open
`OSTN15_OSGM15_GB.txt` and fails. From
https://www.ordnancesurvey.co.uk/business-government/tools-support/os-net/for-developers,
the developer pack includes a `OSTN15_OSGM15_DataFile.txt`, is this
related, or are they two different files for two different purposes?

2. I see that EPSG:7952 and EPSG:7953 are not listed in the database. From
the developer pack, it looks like this isn't a projection, but rather a
horizontal shift + a vertical shift. Our client provided .gsb and
.gtx files for us to use, but now... how do I specify both horizontal and
vertical shift at once? I tried using the following:

cs2cs +proj=pipeline +step +proj=hgridshift +grids=OSTN15_NTv2.gsb
+step +proj=vgridshift +grids=OSGM15.gtx +to
EPSG:4979

However, I get the error:
proj_create_operations: source_crs is not a CRS
Rel. 6.3.1, February 10th, 2020
<cs2cs>:
cannot initialize transformation
cause: generic error of unknown origin
program abnormally terminated

which sounds like I'm not specifying enough information about the CRS, but
I'm not quite sure what the correct input string would look like. Also, I
am fine with using a WKT instead if it makes for a more accurate
description of the transformation.

I appreciate any help you can provide on learning how to implement the
above.

Thank you,
Peter Gagliardi
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