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Documentation
Converts an Ordnance Survey of Great Britain pair of grid letters to a northing value, measured in 100km units. This will be a single digit on mainland Britain. It may be 10, 11, or 12 for the Orkneys or further north.

Example:  gives. This value should be prepended to the second half of the grid reference digits (right-padded with zeroes to 5 digits). So TQ234678 has a northing of 167800.