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,
André 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 always
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 everything
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["Transver
se_Mercator"],PARAMETER["latitude_of_origin",49],PARAMETER["central_meridian
",-2],PARAMETER["scale_factor",0.9996012717],PARAMETER["false_easting",40000
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 the
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 machines
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.roua...@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.chap...@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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