DEFINITION OF DATUMS

Mean Sea Level (MSL)

= MSL is the natural reference level of the geoid, and is defined as the average level taken up by the sea along a long period (18.6 years if possible).
= In practice it is obtained from the mean of all tide readings or MSL is the average level taken up by the sea.
= It coincides very nearly with the geoid.
= The accuracy of MSL depends on a long period of tidal observation.
= The best result can be obtained with 18.6 years observation.
= MSL is the best vertical reference level we have and as such is the basic reference level for tides.
= Observed MSL is subject to change due;
· Polar melting or glacial activity.
· Meteorological effects (rain, wind, pressure).
· Short period effects (sea, swell, surges, etc.).
· Tectonics (crustal changes).
· Astronomical effects (changes in MSL due to the combined effect like seasonal meteorological variation).
· Mean of all tide readings over a long period (30 days to 18.6 years).
· Multiple of 25 hourly observations (preferably 3 days to 75 hours).
· Mean of 25 hourly observations (poorer value).