ALL DUTCH METRIC DATA ARE MEASURED TO NAP (NORMAAL AMSTERDAMSCH PEIL) PRIOR TO 1936 VALUES BASED ON HIGH AND LOW WATERS OR ON 4 OR 8 READINGS/DAY =========================== FOLLOWING PSMSL DOCUMENTATION ADDED 22-Jan-2016 : The note above stating that periods of the data are MTL is misleading. Sections of the original data were MTL. However, Pub. Sci. 5 states “For these periods a shallow water correction has been added to the mean heights of high and low water in order to give a mean height of sea-level comparable with those otherwise calculated for other periods.” Thus, a correction to MSL has already been applied.
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