Opened 12 years ago

Closed 11 years ago

#4597 closed defect (fixed)

[PATCH - to upstream in libgeotiff] Consider better OSGB 1936 Default Datum Shift

Reported by: warmerdam Owned by: warmerdam
Priority: normal Milestone: 1.10.0
Component: OGR_SRS Version: svn-trunk
Severity: normal Keywords: OSGB36 datum shift
Cc:

Description

currently OSGB 1936 (GCS 4277) is using this datum shift by default:

490,1195,4277,4326,Derived at 38 stations.,"For military purposes only. Accuracy 10m, 10m and 15m in X, Y and Z axes.",1264,49.96,60.84,-7.56,1.78,1,0,9603,375,-111,431,1

But some uses feel the following would be more appropriate. Review and consider.

495,1314,4277,4326,"For a more accurate transformation see OSGB 1936 / British National Grid to ETRS89 (2) (code 1039): contact the Ordnance Survey of Great Britain (http://www.gps.gov.uk/gpssurveying.asp) for details.",Oil exploration. Accuracy better than 4m and generally better than 2m.,1264,49.96,60.84,-7.56,1.78,1,0,9606,446.448,-125.157,542.06,0.15,0.247,0.842,-20.489,0

Attachments (1)

ticket4597.patch (579 bytes ) - added by Even Rouault 11 years ago.
Better datum shift for OSGB 1936

Download all attachments as: .zip

Change History (5)

comment:1 by mikrit, 11 years ago

Aye.

comment:2 by 45136, 11 years ago

Version: unspecifiedsvn-trunk

I would favour that. The second values are identical with the +datum values used until GDAL 1.7.3.

There are many complaints since the abandonning of the +datum, leading to a regression in accuracy for OSGB1936. See http://gis.stackexchange.com/questions/40461/raster-incorrectly-reprojected-to-osgb27700 and the linked references.

comment:3 by Even Rouault, 11 years ago

Summary: Consider better OSGB 1936 Default Datum Shift[PATCH - to upstream in libgeotiff] Consider better OSGB 1936 Default Datum Shift

I've attached a patch that should fix it by overriding the definition of EPSG:4277 by selecting the EPSG:1314 datum shift. However this should be applied in libgeotiff first.

by Even Rouault, 11 years ago

Attachment: ticket4597.patch added

Better datum shift for OSGB 1936

comment:4 by warmerdam, 11 years ago

Keywords: OSGB36 datum shift added
Milestone: 1.10.0
Resolution: fixed
Status: newclosed

I have applied a variation on this patch in libgeotiff - adding the preferred shift in the datum_shift_prefs.csv - and regenerated files in libgeotiff.

http://trac.osgeo.org/geotiff/changeset/2312

Pulled changes down into GDAL (r25589).

I checked and there is no need to refresh PROJ.4 since it uses +datum=OSGB36 which already expands to the right values.

The GDAL changes will be part of 1.10.

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