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20 Factsheet 9: Datums – Mean Sea Level and Chart Datum

Ohiwa Split
Relationship between chart datum, MSL
(Wellington Vertical Datum-1953) and
MLOS in the year 2000 for Wellington. 
The difference between MSL and MLOS is
due to the effects of long-term fluctuations
and sea-level rise over the period between
1953 and 2000.

Several different vertical datums that are used for navigational purposes and on land for surveying and engineering purposes.  Two are most common:

  • Chart datum is used for navigation and for hydrographic charts.  It typically refers to a level below which tides do not fall (often defined by the lowest astronomical tide).  Standard port tide gauges are usually set to read zero at chart datum.
  • Mean Sea Level (MSL) is a land-based vertical survey datum.  The regional vertical or MSL datums were based on sea level data collected over over several years (mostly the 1910s to 1940s, but sometimes later, depending on the region).  For example, the local vertical datum in Auckland is defined as ‘Auckland Vertical Datum-1946’, which means that its definition in 1946 was based on the mean level of the sea over a period of time prior to that year (1909 to 1923).

Caution: MSL is not to be confused with Mean Level of the Sea: MLOS is the average actual level of the sea measured over a defined period of time (eg, one year or several years).  MLOS also includes sea-level rise.  Hence it is MLOS, not MSL that equals the present level of the sea.

Plot of annual mean level of the sea (MLOS) comparing the sea level above the relevant LINZ vertical datum at Auckland (AVD-46) and Mt Maunganui (Moturiki MVD-1953) from 1972 to 2006.  Zero on the vertical axis is the MSL datum.  Present-day MLOS is higher than the MSL datum owing to the effects of sea-level rise over the last 50–80 years since the measurements used to define MSL datum.  MLOS relative to MSL datum is also higher at Auckland than at Mt.  Maunganui because MSL was defined independently and approximately seven years earlier at Auckland.  Note that although sea levels can be physically higher between locations (especially east versus west coasts), the difference is mostly due to the datums being established at different times. Plot of annual mean level of the sea (MLOS)

Tide marks for cadastral and engineering design purposes

www.linz.govt.nz/core/surveysystem/geodeticinfo/datums-projections/verticaldatums/tidalinfo/index.html

LINZ provides tide marks at standard ports for navigational purposes and for engineering and surveying.  Marks for the latter are based on tide predictions for the next 19 years and should be used for all relevant surveying and engineering work, whereas the nautical tide marks are based on predictions for the coming year only.

 

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