Add 6.169m to data values 1948 to Aug. 1950 to refer to RLR (1969)
Add 6.154m to data values Sept. 1950 to 1987 to refer to RLR (1969)
Add 6.157m to data values 1988 onwards to refer to RLR (1969)
RLR (1969) is 8.6m below BM P.M. 84
