[Qgis-user] OSGB coordinates to WGS84 lat/lon problem

[M.E.Dodd]

7m error often associated with the approx. transformation that consumer grade gps units (or many of the coordinate conversion websites) carry out compared to the full OSTN02 definitive transformation.  If possible should always use the OSTN02 transformation.

Below are a few examples of points in wgs84 and osgb format transformed with OSTN02 if you want to check some others

Point ID	Latitude	Longitude	Ellipsoid Height	Northing	Easting	Orth. height	Time
RTCM-Ref 0000	52.0242012	-0.7717783	139.488		236908.8906	484368.9928	92.26152	31/05/2011 14:44
RTCM-Ref 0014	51.9015014	-0.1784953	158.652		224117.4848	525415.3853	112.47864	31/05/2011 14:38
RTCM-Ref 0019	52.25181	-2.19794	87.084		261532.2794	386583.8284	37.4539		14/06/2010 09:30
1		52.0652051	-2.6857082	99.02443	240980.03	353090		48.15		13/04/2012 15:16


-----Original Message-----
From: Nick Hopton [mailto:nhopton at gmail.com] 
Sent: 10 May 2012 11:13
To: qgis-user at lists.osgeo.org
Subject: [Qgis-user] OSGB coordinates to WGS84 lat/lon problem

A chap on QGIS at Stack Exchange is having problems with accuracy when transforming from Ordnance Survey grid coordinates to WGS84 lat/lon. He's finding discrepancies in the order of seven metres.

Using QGIS 1.7.4, OSGEO4W under Vista I was able to reproduce the problem, but using QGIS Lisboa RC1 under Ubuntu 11.10 produced transformations good to better than a metre. My first reaction was that there must problems with the QGIS 1.7.4 Proj4 parameter string for EPSG:27700, but on checking I found that 1.7.4 and Lisboa use identical strings for this CRS.

I'm now at a bit of a loss to understand how following an identical procedure on both systems can produce differing results.

If anyone would care to check this, below is the csv file I used for my tests.

Station,x,y
Inkpen,437346.0,161624.8
Hodmore,468296.3,178184.9
Hindhead,489984.6,135909.7

Under Ubuntu these coordinates transformed to:
Inkpen	437346	161624.8	-1.465102	51.352478
Hodmore	468296.3	178184.9	-1.017527	51.498487
Hindhead	489984.6	135909.7	-0.715814	51.115423

Under Vista they transformed to:
Inkpen	437346	161624.8	-1.46511	51.352424
Hodmore	468296.3	178184.9	-1.017525	51.498435
Hindhead	489984.6	135909.7	-0.715808	51.115362

In my part of the world the differences amount to about six metres on the ground. The Inkpen test point is in fact an old triangulation pillar and the point as transformed by Ubuntu sits exactly on this when viewed in Goorgle Earth. 

Any ideas?

Nick.


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