6

In ArcGIS 10 a new transformation has been added for converting between WGS84 and British National Grid.

What is OSGB_1936_To_WGS_1984_NGA_7PAR?

How should it be used when compared with the other available transformations?

Is it better?

5

The OSGB_1936_To_WGS_1984_Petroleum has a listed accuracy of around 2m. We have a significantly larger accuracy value for OSGB_1936_To_WGS_1984_NGA_7PAR. It was published by NGA (was NIMA was DMA). The transformation parameters can be found in the Geotrans software, but there's no accuracy information. Esri added it for interoperability because it's in Geotrans.

3

Rotation & Translation has been changed (corrected?)

GEOGTRAN["OSGB_1936_To_WGS_1984_NGA_7PAR",GEOGCS["GCS_OSGB_1936",DATUM["D_OSGB_1936",SPHEROID["Airy_1830",6377563.396,299.3249646]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]],GEOGCS["GCS_WGS_1984",DATUM["D_WGS_1984",SPHEROID["WGS_1984",6378137.0,298.257223563]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]],METHOD["Coordinate_Frame"],PARAMETER["X_Axis_Translation",446.0],PARAMETER["Y_Axis_Translation",-99.0],PARAMETER["Z_Axis_Translation",544.0],**PARAMETER["X_Axis_Rotation",-0.945],PARAMETER["Y_Axis_Rotation",-0.261],PARAMETER["Z_Axis_Rotation",-0.435]**,PARAMETER["Scale_Difference",-20.8927]]


GEOGTRAN["OSGB_1936_To_WGS_1984_Petroleum",GEOGCS["GCS_OSGB_1936",DATUM["D_OSGB_1936",SPHEROID["Airy_1830",6377563.396,299.3249646]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]],GEOGCS["GCS_WGS_1984",DATUM["D_WGS_1984",SPHEROID["WGS_1984",6378137.0,298.257223563]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]],METHOD["Position_Vector"],PARAMETER["X_Axis_Translation",446.448],PARAMETER["Y_Axis_Translation",-125.157],PARAMETER["Z_Axis_Translation",542.06],PARAMETER["X_Axis_Rotation",0.15],PARAMETER["Y_Axis_Rotation",0.247],PARAMETER["Z_Axis_Rotation",0.842],PARAMETER["Scale_Difference",-20.489]]

Is it better? Depends on the Scale of the final mapping data. Sub 1:1250 maybe. above 1:1250 very little. You will have to do a compare and see with your data.

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