[PROJ] Implementing HS2 NTV2 grid in PROJ

[Shane]

Hi,

     First post to this group and a relative novice to PROJ, beyond 
using existing transformation strings.  I'm looking to implement 
transformations to and from HS2 NTv2 and OSTN02 and OSTN15, and also 
matching vertical shifts, using Proj (current v7 build, working in C++), 
see 
https://www.thepwi.org/technical_hub/presentations_for_tech_hub/190709_summer_nr_tech/07_hs2_engineering_coordinate_systems__j_turner 
for a description of the HS2 NTv2 grid which replaces the HS2 Snakegrid 
implementation.  HS2 grid is registered under EPSG:9300: 
https://epsg.org/crs_9300/HS2-Survey-Grid.html 
<https://epsg.org/crs_9300/HS2-Survey-Grid.html?sessionkey=wbkmhx76b2> 
And the compound CRS is registered under EPSG:9306: 
https://epsg.org/crs_9306/HS2-Survey-Grid-HS2-VRF-height.html 
<https://epsg.org/crs_9306/HS2-Survey-Grid-HS2-VRF-height.html?sessionkey=oryxvd4m1b>

I have the NTv2 GSB files for HS2 NTv2 (HS2TN15_NTv2.gsb and 
HS2TN02_NTv2.gsb) and will be transforming between 3d coordinates, where 
from my understanding height and grid position will be dealt with 
separately.  I also have vertical shift files in the format lat, long, 
height which I'm guessing I'll need to convert to a GTX file as 
described here; https://vdatum.noaa.gov/docs/gtx_info.html Not sure 
whether PROJ uses ASCII or binary versions of the GTX file, but no 
issues building either as needed.

What I'd like to achieve is a PROJ string or pipeline I can use within 
my software to perform the required transformations, i.e. OSTN02 & 
OSGM02 and OSTN15 & OSGM15 to HS2 survey grid plus VRF and back again.

While I can probably work my way through this, as someone new using GSB 
and GTX files in PROJ, any help or suggestions would be greatly 
appreciated.  I was hoping it would be available in epsg.io but it seems 
not as yet,

Shane




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