[Proj] Proj4 transform from WGS84 to UK Northings and Eastings help needed ...

[Clifford J Mugnier]

Dear Ms. Bamborough,

The parameters you  have used appear to be correct for WGS84 Datum
coordinates with respect to the ellipsoid dimensions, but WGS84 does NOT
reference the Airy ellipsoid.  However, the British National Grid is NOT on
the WGS84 Datum; it is on the Ordnance Survey of Great Britain Datum of
1936 (OSGB36), which indeed IS on the Airy ellipsoid.  You will need to
FIRST perform a "datum shift" of the WGS84 coordinates such that they will
then match (and fit) the datum that the remainder of your data is on, which
I presume is OSGB36.

Consider a bit of reading with some of my past columns on this topic, you
can obtain free pdf downloads from the "ASPRS" url listed below my
signature block.  Note my October 2003 column; it's on Great Britain.

Good luck,

Clifford J. Mugnier C.P.,C.M.S.
National Director (2006-2008)
Photogrammetric Applications Division
AMERICAN SOCIETY FOR PHOTOGRAMMETRY AND REMOTE SENSING
and
Chief of Geodesy
Center for GeoInformatics
LOUISIANA STATE UNIVERSITY
Dept. of Civil Engineering
CEBA 3223A
Baton Rouge, LA  70810
Facsimile   (225) 578-8652
Voice       (225) 578-8536
------------------------------------------------------------
http://www.ASPRS.org/resources/GRIDS
http://www.cee.lsu.edu/facultyStaff/mugnier/index.html
------------------------------------------------------------


Hiya all

I have some Navteq data, which I believe is in WGS84 decimal degrees.
I am using an ESRI product (which uses proj4) to project the data
into british national grid northings and eastings. However, when I
project the data, the coordinates appear to be between 100 to 200m
out from where I would expect. I use the following settings for the
proj4 translation:

proj=tmerc
a=6378137.0
b=6356752.3141
k_0=0.9996012717
ellps=airy
lat_0=49d0n
lon_0=2d0w
x_0=400000
y_0=-100000

Would anyone have any suggestions as to why we receive this position
offset error ?

Penny