[Proj] towgs84 values for EPSG:27700

[Mikael Rittri]

> Is this expected behaviour?

I can say that I, for one, didn't expect it. Because as far as I know,
the default datum shifts in both Proj.4 and GDAL/OGR come from the
gcs.csv file that is part of libgeotiff. 

However, the gcs.csv file is changed now and then. The default datum
shift of OSGB 1936 was changed from the 7-parameter (epsg:1314) to the
3-parameter (epsg:1195) in the libgeotiff changeset 2023, of May 23, 2011.
The changeset message was just "upgrade to EPSG 7.6 database".

http://trac.osgeo.org/geotiff/changeset/2023/trunk/libgeotiff/csv/gcs.csv

(And I just happened to notice that in the same changeset, the default
 datum shift for Qornoq 1927 was changed from the 7-parameter epsg:1798,
 accuracy 1 m, to the 3-parameter epsg:1797, accuracy 48 m.)

I don't know in what libgeotiff release this changeset was (or will be)
included. But it seems that when you used GDAL/OGR from trunk, this changeset was included, whereas when you used Proj version 4.7.1,
it wasn't.

See also Frank's blog: 
http://fwarmerdam.blogspot.com/2010/03/in-last-few-weeks-i-believe-i-have-made.html

and 

http://trac.osgeo.org/geotiff/ticket/33

Best regards,

Mikael Rittri
Carmenta
Sweden
http://www.carmenta.com

-----Original Message-----
From: proj-bounces at lists.maptools.org [mailto:proj-bounces at lists.maptools.org] On Behalf Of Hermann Peifer
Sent: Monday, February 20, 2012 6:56 AM
To: PROJ.4 and general Projections Discussions
Subject: Re: [Proj] towgs84 values for EPSG:27700

On 19/02/2012 21:06, support.mn at elisanet.fi wrote:
>
> So the reason for the difference is that the OSGB36
> datum is not any standard value.
>

I am aware that there always might be many values for a single EPSG 
code. I also don't want to debate in how far the values of the "ETRS89 
(WGS84) to OSGB36/ODN Helmert transformation", on p. 33 of [1], can be 
considered as "standard values". My simple (slightly rephrased) question 
was:

ogr2ogr -s_srs epsg:27700 # expands to something
cs2cs -v +init=epsg:27700 # expands to something else

Is this expected behaviour?

Hermann

[1]
http://www.ordnancesurvey.co.uk/oswebsite/gps/docs/A_Guide_to_Coordinate_Systems_in_Great_Britain.pdf 

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