Add 6.304m to data values up to 1969 to refer to RLR (1963)
Add 6.292m to data values 1970-2002 to refer to RLR (1963)
Add 5.999m to data values 2003 onwards  to refer to RLR (1963)
RLR (1963) is 10.4m below BM UD40
