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

[Nick Hopton]

Andy Harfoot wrote
> 
> Have you tried the convertor here: 
> http://gps.ordnancesurvey.co.uk/convert.asp
> to see whether the higher accuracy coordinates obtained from QGIS match 
> those obtained using OSTN02?
> 

For the record, the Survey gives the following (ETRS89) lat/lon coordinates
for the points:

Inkpen: 51.3524757 -1.4650872
Hodmore: 51.4984893 -1.0170283
Hindhead: 51.1154179 -0.7157938

I think ETRS89 has drifted about half a metre away from WGS84 at the present
time, but even so the transformations done by Lisboa under Ubuntu look very
good.



> It looks as though Proj4 can use grid based transformations like OSTN02 
> through the use of gsb files. Could it be that this is the case in the 
> higher accuracy situation? I find gsb files at the following location on 
> my Win7 QGIS 1.7.4 install:
> C:\OSGeo4W\share\proj
> 

At present by default QGIS doesn't use a grid-shift file for EPSG:27700. The
datum-shift is carried out by the +towgs84 part of the Proj parameter
string. 



> I can't see any files in this directory suggesting they are the OSTN02 
> grid though, and I can reproduce your 'Vista' results as follows, 
> suggesting that my QGIS is using the lower accuracy seven parameter 
> transform.
> 

It's not using a grid-shift file. But it's interesting that you get the same
poor transformations that I get with 1.7.4 under Visa. Lisboa RC1 under
Ubuntu produces much better transformations (for me) but I don't know why
this is the case.

Nick.


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