[GRASS-user] WGS84 (epsg:4326) - OSBG36 British National Grid (epsg:27700) offset problem - SOLVED

[John A Stevenson]

Hi,

I had a problem importing GPS data and features digitised from Google 
Earth (both WGS84 datum) into locations with a British National Grid 
(OSBG36) projection.  I was working in a region in northwest Scotland, 
and I found that everything that I imported was plotting about 100 ENE 
of where it should do.

I have finally worked out why, but after a lot of searching, so I am 
posting a summary of what I found here so that google will pick it up in 
the archives if anyone else has the same problem in future.

The short answer is that the projection parameters that proj4 was using 
were wrong, but they are fixed now.  Noone using an up-to-date version 
of proj4 should have this problem, but the Dag rpm repository still has 
an old one.

The problem was that going to OSGB36 requires both a projection and a 
datum conversion {1}.  Prior to October 2007 {2}, proj was only carrying 
out the projection, thus resulting in the large offset.  You can check 
if you have the new version by running 'proj -v' or by looking at your 
epsg file:

cat /usr/share/proj/epsg | grep -A 1 "British National Grid"

# OSGB 1936 / British National Grid
<27700> +proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 
+y_0=-100000 +ellps=airy +datum=OSGB36 +units=m +no_defs  <>

The new versions have +datum=OSGB36.

If you have an old version, you can correct it by replacing the line with:

+proj=tmerc +lat_0=49 +lon_0=-2 +k=0.999601 +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 <>

A complication is that OSGB36 is slightly distorted with respect to GPS 
projections (such as WGS84 and ETRS89) {3}.  This offset is small, and 
is only important for higher precision surveying.  Many searches about 
OSGB36 offsets bring up pages relating to this.  If you want to 
compensate for this too,
you can download a nadgrid file and use it {4}.  For my data, this moved 
the points by about 1 m.

I hope that this helps anyone with the same problem,

Cheers

John

{1} - http://lists.maptools.org/pipermail/proj/2005-November/001854.html
{2} - http://trac.osgeo.org/proj/changeset/1416
{3} - 
http://www.ordnancesurvey.co.uk/oswebsite/gps/osnetfreeservices/about/surveying_osnet.html#6
{4} - http://osdir.com/ml/gis.proj-4.devel/2006-11/msg00056.html

-- 


Dr John Stevenson
Postdoctoral Research Associate
School of Earth, Atmospheric and Environmental Sciences
Williamson Building (Room 2.42)
University of Manchester
Manchester M13 9PL, UK
tel. +44(0)161 306 6585; fax. +44(0)161 306 9361;
john.stevenson at manchester.ac.uk