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I;m getting numbers identical to your Ubuntu results in three tests:<br>
QGIS 1.7.4 on Win7, QGIS1.7.4 on Scientific Linux, and also using
cs2cs. All the same, and all = to your Ubuntu values.<br>
Maybe that will help to smoke out the problem?<br>
<br>
On 10/05/2012 13:13, Nick Hopton wrote:
<blockquote cite="mid:1336644801740-4965339.post@n6.nabble.com"
type="cite">
<pre wrap="">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.
--
View this message in context: <a class="moz-txt-link-freetext" href="http://osgeo-org.1560.n6.nabble.com/OSGB-coordinates-to-WGS84-lat-lon-problem-tp4965339.html">http://osgeo-org.1560.n6.nabble.com/OSGB-coordinates-to-WGS84-lat-lon-problem-tp4965339.html</a>
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</pre>
</blockquote>
<p><br>
</p>
<br>
<pre class="moz-signature" cols="72">--
Micha Silver
052-3665918
</pre>
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