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List:       qgis-user
Subject:    Re: [Qgis-user] EPSG:27700 - OSGB 1936 / British National Grid shapefiles are incorrectly interprete
From:       Andre Joost <andre+joost () nurfuerspam ! de>
Date:       2013-01-31 16:35:43
Message-ID: kee7h1$m3g$3 () ger ! gmane ! org
[Download RAW message or body]

As long as *all* your project and data is in EPSG:27700, you will not =

see any difference. But as soon as you add Google or openstreetmap tiles =

with openlayersplugin, or data from other sources in other CRS, the =

+towgs84 datum shift parameters comes into effect. The values used up to =

QGIS 1.7.4 were quiet good, QGIS 1.8.0 sadly returned to some =

low-accuracy parameters. For best accuracy, OS has issued a ntv2 grid file.

The .prj file by ESRI does not have any information on datum shift =

values incorporated. Mainly because there are several choices to do the =

transformation with different accuracy and complexity, as stated above.

HTH,
Andr=E9 Joost


Am 31.01.2013 14:25, schrieb Andrew Chapman:
> Pardon my ignorance here - I'm not (nor do I wish to become) a GIS expert
> with detailed knowledge of all the ins and outs of CRSs. While I can
> understand that to some users it could be critical to know with ultimate
> accuracy the location of any specified point as a world coordinate, what
> ultimately matters to me is to be able to agree with other users that we
> consistently interpret our mapping data in the same way - point A is alwa=
ys
> in the same place on each of our systems, subject obviously to specifying
> the CRS.
>
> Using shapefiles, as I understand it, the .prj file specifies how everyth=
ing
> should line up. For OSGB 1936 this has historically been
>
> PROJCS["OSGB_1936_British_National_Grid",GEOGCS["GCS_OSGB
> 1936",DATUM["D_OSGB_1936",SPHEROID["Airy_1830",6377563.396,299.3249646]],=
PRI
> MEM["Greenwich",0],UNIT["Degree",0.017453292519943295]],PROJECTION["Trans=
ver
> se_Mercator"],PARAMETER["latitude_of_origin",49],PARAMETER["central_merid=
ian
> ",-2],PARAMETER["scale_factor",0.9996012717],PARAMETER["false_easting",40=
000
> 0],PARAMETER["false_northing",-100000],UNIT["Meter",1]]
>
> If I get layers from Ordnance Survey, my county council GIS department or
> other town and parish councils, I would expect my example point A to stay=
 in
> the same place whatever GIS software I use.
>
> I'm not sure what is being suggested here as a fix. Is it going back to t=
he
> pre 1.8.0 solution or providing an alternative CRS that the user will need
> to select for all affected layers in every project? If the latter it will
> potentially cause problems for those of us sharing projects between machi=
nes
> using different version of QGIS.
>
> Sorry - I'm just having trouble getting my head around this!
>
> Andrew Chapman
>
> -----Original Message-----
> From: Even Rouault [mailto:even.rouault@mines-paris.org]
> Sent: 31 January 2013 12:24
> To: Andrew Chapman
> Cc: qgis-user@lists.osgeo.org
> Subject: Re: [Qgis-user] EPSG:27700 - OSGB 1936 / British National Grid
> shapefiles are incorrectly interpreted by QGIS 1.8.0 and later
>
> Selon Andrew Chapman<andrew.chapman@donkagen.co.uk>:
>
>
> There's a GDAL ticket about that : http://trac.osgeo.org/gdal/ticket/4597
>
> Frank, I've attached in it a libgeotiff patch that should solve this by
> defining an override.


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