France's zero level for altitudes was fixed using tide-gauge observations at Marseille taken between February 1885 and December 1896.
Notes on verification
Multiple independent sources (French Wikipedia, INSA/SHOM thesis, Bureau des longitudes, IGN, press article) confirm the Marseille tide-gauge observations from Feb 1885 to late 1896/early 1897 were used to establish France's altitude datum (Nivellement Général de la France).
Sources
- Le nivellement - Géodésie (geodesie.ign.fr)
- https://refmar.shom.fr/sites/default/files/2024-10/Philippe_2003.pdf (refmar.shom.fr)
- http://bdl.ahp-numerique.fr/focus-instruments-ac-maregraphe-marseille (bdl.ahp-numerique.fr)
- https://laclepublique.fr/bouches-du-rhone/articles/2026/2026-09-20-maregraphe-acces-public/ (laclepublique.fr)