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 08-Feb-2011 :
After receiving levelling values for nap up to 2005 and the revised values 2005 onwards from Douwe Dillingh Hoek van Holland was made RLR. RLR factors up to 2005 6.845m. From 2005 onwards 6.873m
=========================== 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.

