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I have received a couple of GIS layers with WGS84(DD) and GCS_WGS_1984 Geographical Coordinate System (GCS). I used Arcmap to convert GCS_WGS_1984 to British National Grids using Petroleum transformation to get an accurate conversion and no problem. But when I tried to convert WGS84(DD) to British National Grids, I could not select the Petroleum transformation as the parameter list menu was empty and I got a warning massage said “A datum transformation is expected, but the parameter is empty”. “No transformation was found based on the input and output coordinate system and the extent of data”. Any idea how to solve this issue and get accurate transformation between WGS84(DD) and the British National Grids? enter image description here

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The Esri software (actually Esri projection engine) doesn't have "WGS83(DD)" as a alias for our GCS_WGS_1984/WGS 1984/GCS WGS 1984 definition. Transformations include the full geographic coordinate reference systems of the source and target so they have to match.

One solution is to redefine coordinate reference system of the data that's using WGS84(DD) to the Esri definition.

Another solution would be to create a new transformation with the Create Custom Geographic Transformation tool (Data Management toolbox, Projections and Transformations toolset) that converts between WGS84(DD) and OSGB36. You would need the method and parameter values of the Petroleum transformation that you're using. You can find that information in the geographic_transformations.pdf file.

Note: If you define a custom transformation make sure the signs of the parameters match the direction of the transformation. That is, if you get the parameters from the pdf file and the transformation is defined as from OSGB36 to WGS84, the new transformation should also be defined as from OSGB36 to WGS84. If you switch the order (from WGS84 to OSGB36), you will need to change the signs of all parameters.

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