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Recently I have reprojected Ordnance Survey data from British National Grid (ORD SURV GB) to WGS-84 using ArcGIS 9.2. After re-projecting I have found some shift in the data. Can any body guide me what i am doing wrong?

Thanks for you reply.

Here is my observations. I have download the boundary files from here: http://parlvid.mysociety.org:81/os/

When I re-project boundaries there no shift. But when i used any layer from the tile, it is shifted.

After this I have just compare the projection information of both layers.

Boundary file have this projection info:

Projection: Transverse_Mercator false_easting: 400000,000000 false_northing: -100000,000000 central_meridian: -2,000000 scale_factor: 0,999601 latitude_of_origin: 49,000000 Linear Unit: METER (1,000000)

Geographic Coordinate System: unnamed Angular Unit: degree (0,017453292519943299) Prime Meridian: Greenwich (0,000000000000000000) Datum: D_OSGB_1936 Spheroid: Airy - 1848 Semimajor Axis: 6377563,000000000000000000 Semiminor Axis: 6356256,161000000300000000 Inverse Flattening: 299,319997677743000000

and in tile layer for example NaturalFeature_Area have following projection info:

Projection: Transverse_Mercator False_Easting: 400000,000000 False_Northing: -100000,000000 Central_Meridian: -2,000000 Scale_Factor: 0,999601 Latitude_Of_Origin: 49,000000 Linear Unit: Meter (1,000000)

Geographic Coordinate System: GCS_OSGB_1936 Angular Unit: Degree (0,017453292519943299) Prime Meridian: Greenwich (0,000000000000000000) Datum: D_OSGB_1936 Spheroid: Airy_1830 Semimajor Axis: 6377563,395999999700000000 Semiminor Axis: 6356256,909237285100000000 Inverse Flattening: 299,324964599999990000

The bold lines are changed in both layers.

Any idea which one is correct?

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I've never seen an ellipsoid with a semimajor axis of 6377563.0 m nor with an inverse flattening of 299.31999767. EPSG (epsg-registry.org) has the values of the second one: 6377563.396, 299.3249646 for Airy 1830. There's a modified Airy 1849, but it doesn't have those values. –  mkennedy Dec 2 '10 at 2:43
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1 Answer

Use the Petroleum Transformation in your project parameters for more accurate results:

OSGB36_To_WGS84_Petroleum - this seems to be a 7 parameter Helmert transformation and seems to use the parameters suggested by the UK Ordnance Survey. As such, it's accuracy should be 5m at worse.


Have had no issues with the default transformation but rarely need to create maps under 1:2000

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