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

[Nick Hopton]

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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