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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.

http://forums.esri.com/Thread.asp?c=93&f=984&t=132296

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

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