Add 6.971m to data values 1956-1962  to refer to RLR (1962)
Add 6.904m to data values 1978-1980 to refer to RLR (1962)
Add 6.504m to data values 2001 onwards to refer to RLR (1962)
RLR (1962) is 9.644m below BM RN 7
