[Proj] OSGB36

[Oscar van Vlijmen]

> From: Paul Kelly <paul-grass-stjohnspoint.co.uk>
> Subject: Re: [Proj] OSGB36
> On Thu, 12 Jan 2006, Roger Oberholtzer wrote:

>> This time, I am testing OSGB36 (UK). My current method uses geotrans. I
>> am using a 7 parameter transform (in geotrans-speak).
>> A sample set of points:
>> WGS84 (somewhere near Norwich):
>> lat=52.63820000
>> lng=1.25423000
>> I should get (UK TRL say):
>> N=620282.88  E=309469.17

> You need to do a datum transformation (WGS84 to OSGB36)
> With the following command line
> cs2cs +proj=latlong +ellps=WGS84 +towgs84=0,0,0 +nodefs +to +proj=tmerc
> +lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 +y_0=-100000 +ellps=airy
> +towgs84=446.448,-125.157,542.060,0.1502,0.2470,0.8421,-20.4894 +units=m
> +no_defs
> I get these results
> 620286.26       309466.38
> for your test point. Much closer.

I can confirm the test results from mr. Kelly.
A more verbose description:
Transformation from WGS84 to OSGB36:
lat=52.63820; lon=1.25423; h=0;
This gives:
lat=52.637798, lon=1.256045, h=-45.3413
Transformation parameters:
dx=-446.448; dy=125.157; dz=-542.060;
rx=-0.1502; ry=-0.2470; rz=-0.8421; ds=20.4894;
Position vector rotation, approximate transform (like PROJ.4 does).
Transverse Mercator on Airy 1830 ellipsoid:
with: lat0=49.0; lon0=-2.0; x0=400000; y0=-100000; k0=0.9996012717;
gives: x=620286.2515, y=309466.3864

Source of the parameters:
"A guide to coordinate systems in Great Britain", Ordnance Survey, v1.4 May
2002
<http://www.gps.gov.uk/additionalInfo/images/A_guide_to_coord.pdf>

Differences with Ordance Survey results must be attributed to lack of
gridinterpolation.

Geotrans has inaccurate transformation parameters in the file 7_param.dat
Note that these are coordinate frame parameters.
With these parameters I arrive with my code at
x = 620287.2473, y = 309465.2385
(mr. Oberholtzer mentioned N=620287.247 E=309465.242)



By the way, in my previous email with subject "Re: [Proj] proj and Dutch RD"
I forgot to mention that the datum transformation parameters are coordinate
frame parameters, so the signs of the r's should be reversed if you want to
use the parameters for a position vector transform like PROJ.4's.