Add 5.759m to all data values up to 1991 to refer to RLR (1994)
SUBTRACT 4.558m to data values 1992-1997 to refer to RLR (1994)
Add 5.956m to all data values 1998 onwards to refer to RLR (1994)
RLR (1994) is 10.301m below BM 6
