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

[Andy Harfoot]

Hi Nick,

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?

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

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.

The OSTN02 gsb can be downloaded here:
http://www.ordnancesurvey.co.uk/oswebsite/gps/osnetfreeservices/furtherinfo/ostn02_ntv2.html
Referencing it in Proj4 is described here:
http://trac.osgeo.org/proj/wiki/GenParms#nadgrids-GridBasedDatumAdjustments

BUT I haven't tried this!

Andy

Nick Hopton wrote:
> Micha Silver wrote
>   
>> I;m getting numbers identical to your Ubuntu results in three tests: 
>>     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. 
>>     Maybe that will help to smoke out the problem?
>>
>>     
>
> Thanks for this Micha, though it's left me flummoxed for want of an
> explanation. The person on SE and I both have the same problem, but if you
> can't reproduce it then it's obviously not universal.
>
> For miked, thanks for the comment, I know about this problem which is why I
> used as my test points stations from the OS passive stations database whose
> positions are known accurately.
>
> Nick.
>
>
> --
> View this message in context: http://osgeo-org.1560.n6.nabble.com/OSGB-coordinates-to-WGS84-lat-lon-problem-tp4965339p4965857.html
> Sent from the Quantum GIS - User mailing list archive at Nabble.com.
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