Add 6.133m to data values up to 1959 to refer to RLR (1968)
Add 5.98m to data values 1960-1978 to refer to RLR (1968)
Add 5.909m to data values1979-1997 to refer to RLR (1968)
Add 5.834m to data values 1998-2002 to refer to RLR (1968)
Add 5.984m to data 2003 onwards to refer to RLR (1968)
RLR (1968) is 10.4m below BM 1
LLD = Land Levelling Datum
