dragons8mycat at live.co.uk
Mon Sep 26 17:10:28 EST 2016
Having spoken with Ordnance Survey and spent the day with them on Friday, there is no current plan to release 2 versions of the NTv2 file, this is under discussion with EPSG and will require some further work.
The current (working) version works fine with QGIS and other OSGeo distributions when used in the srs.db format as the software recognises the direction of the transformation and translates accordingly, please see my blog to see how to implement this and verify.
I understand that there is an issue with the command line format Proj as describing the reverse direction is problematic due to the library only reading from projected to geographic, this is not an issue with the transformation, which works perfectly.
From: proj-bounces at lists.maptools.org <proj-bounces at lists.maptools.org> on behalf of Melita Kennedy <mkennedy2 at earthlink.net>
Sent: 26 September 2016 21:23
To: proj at lists.maptools.org
Subject: Re: [Proj] OSTN15
>Date: Mon, 26 Sep 2016 09:50:19 +0000
>From: Nicholas Duggan <dragons8mycat at live.co.uk>
>Subject: [Proj] OSTN15
>To: "proj at lists.maptools.org" <proj at lists.maptools.org>
>I've updated the "transformations" table within the srs.db which ships with proj4 for QGIS (Mapserver, Grass et al) so that it now correctly uses OSTN15 (EPSG 7709).
>This has been tested, I even showed it working to Ordnance Survey on the 24th Sept.
>My question is, how do I move this forward so that it is implemented within the Proj4 in the future?
>I can provide links to the updated srs.db and the OSTN15 gsb file if necessary.
>Nicholas Duggan FRGS Cgeog(GIS) MRi
I'm just double-checking. Is it the original, published NTv2 file, OSTN15_NTv2.gsb? Because the original one gives offsets from ETRS89 to OSGB36. That's opposite what EPSG::7709 says which has the source as OSGB36 and the target as ETRS89. I believe OS plans to publish 2 NTv2 files, one in each 'direction.' EPSG is waiting for OS before they decide on whether the current coordinate operations need to be modified or replaced.
One reason for publishing both is for software that assumes all transformations are from the local/older GeoCRS to the more general/newer GeoCRS.
So, you might want to hold off for a little while to see what happens.
(Esri employee and IOGP (EPSG) geodesy subcommittee member)
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