Sea level values are defined relative to a reference map which is arbitrary and local. These two attributes should be borne in mind when tide gauge measurements are analysed.
In France, the hydrographic zero or nautical chart zero is a choice that is very common, especially if the tide gauge was installed for hydrographic purposes. The hydrographic zero is defined as the level of the lowest astronomical tides. However, this is still a convention. If you are interested in this topic you might like to read the article published in issue 79 of the review XYZ by Wöppelmann et al. (1999).
Another option which is less common today is a mean sea level, calculated over a given period. For example, the zero of the historical tide gauge of Marseille is the mean sea level observed between 1885 and1896. This zero is also known as the Zero of the General Levelling of France, or NGF-IGN69 Zero today.
To alert users to this conventional aspect of tide gauge zeros, the Permanent Service for Mean Sea Level (PSMSL) deliberately takes the tide gauge zeros and redefines them based on a tide gauge bench mark by using an arbitrary level under this benchmark of about 7000 mm, without any link whatsoever to the sea level (mean or lowest tide) and calls this reference ’RLR’ (Revised Local Reference). They therefore insist on the relative nature of the tide gauge measurements : a local measurement of sea level in relation to the base on which the tide gauge lies and which is represented by a physical bench mark (a tide bench mark).