RLR(1975) is 11.2m below BM Rifleman's Bolt 
For all data add 5.619 to refer to RLR(1975) 
Principal Datum (formerly Ordnance Datum) is approx. 1.23m below MSL(1965-83).
North Point (611/011) was replaced by Quarry Bay (611/010) and they can be 
considered to be the same site.
