Lowest astronomical tide

According to the recommendation of the IHO (International Hydrographic Organization), the chart datum must match the level of the lowest astronomical tide as closely as possible. The term "astronomical" means that it is not directly observed but calculated from the generative force of the tide due to the gravitational action of the Moon and the Sun. However, the astronomical tide is generally calculated using the harmonic formula of harmonic constants obtained from the analysis of previous observations.



There are two difficulties:

  • First the accuracy of the calculation is highly variable depending on the quality and duration of prior observations available, the correction for meteorological and oceanographic effects, the methods and means of calculation and the type and amplitude of the tide.
  • Second, during the observation periods, level changes also occurred caused by other phenomena (not astronomical, but meteorological and oceanographic) as well as a secular trend of sea level variation. As a result, calculations made on the basis of observations made at different times yield different results and are marred by uncertainty.

Does the lowest astronomical tide match the chart datum?


The notion of lowest tide is:

  • inherently approximate;
  • closely related to notions of mean sea level.

As it does not meet the criteria of accuracy and stability, it cannot, strictly speaking, constitute an exact height reference for hydrography, but once calculated, it can be used to situate the chart datum, which will be "approximately" the same as the lowest astronomical tide.


To find out more:


  • Simon B. (2007). La Marée - La marée océanique et côtière. Edition Institut océanographique, 434pp.



Last updated: 12/12/2012