For data 1889-1936 add 10.414 m to all values to refer to RLR (1964)
For data 1937-2008 add 5.005 m to all values to refer to RLR (1964)
For data 2009 onwards SUBTRACT 0.1 m from all values to refer to RLR (1964)
RLR (1964) is 9.9m below BM a
