[GIS] Ordnance Survey data to WGS-84

coordinate systemparameters

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?

Best 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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