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

[M.E.Dodd]

I have tried with plain .csv files of points or shapefiles and both give the error in matching up OSGB and WGS84 coordinates for the same point. Basically the points are shown several metres apart when you allow on the fly transformation, error on all the same amount and in same direction.

Qgis says it has the correct CRS for the shapefiles.


-----Original Message-----
From: Etienne Tourigny [mailto:etourigny.dev at gmail.com] 
Sent: 13 July 2012 12:46
To: M.E.Dodd
Cc: Nick Hopton
Subject: Re: [Qgis-user] Re: OSGB coordinates to WGS84 lat/lon problem

you should probably send this to the mailing list.

You should neot be seing this problem with shapefiles in qgis-1.8.
Have you tried it?  What CRS does QGis set for the files?

cheers
Etienne

On Fri, Jul 13, 2012 at 6:53 AM, M.E.Dodd <m.e.dodd at open.ac.uk> wrote:
> Has this issue been resolved as I still have difference between where points are shown using the wgs84 and osgb coordinates.  The coordinates for the points have been converted between the two systems using the ordnance survey converter with full accuracy but they appear about 4m away from each other when shown in qgis 1.9 (version from a couple of days ago) on windowsXP.  This is having two layers one in EPSG:4326 - WGS 84 and the other in EPSG:27700 - OSGB 1936 / British National Grid and allowing on the fly CRS transformation.
> I am sure in the past this was not an issue but is potentially causing significant problems now, not least when trying to argue that qgis should be taught instead of other gis systems and people say that its useless because it has problems like this (this is currently a debate in my department where we are trying to decide which system to go with).
>
> -----Original Message-----
> From: Etienne Tourigny [mailto:etourigny.dev at gmail.com]
> Sent: 10 May 2012 23:59
> To: Nick Hopton
> Cc: qgis-user at lists.osgeo.org
> Subject: Re: [Qgis-user] Re: OSGB coordinates to WGS84 lat/lon problem
>
> datum shift transformations have been fixed in qgis-1.8, which may 
> explain this.  This has been addressed in this list (and also
> qgis-dev) in the last months
>
> Etienne
>
> On Thu, May 10, 2012 at 4:04 PM, Nick Hopton <nhopton at gmail.com> wrote:
>>
>> 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.
>>
>>
>> --
>> View this message in context:
>> http://osgeo-org.1560.n6.nabble.com/OSGB-coordinates-to-WGS84-lat-lon
>> - problem-tp4965339p4966416.html Sent from the Quantum GIS - User 
>> mailing list archive at Nabble.com.
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