The origin of the NGF/IGN69 altitude system for metropolitan France was established from tide-gauge observations made at the Marseille Tide Gauge (Marégraphe de Marseille) between February 1, 1885 and January 1, 1897.
Notes on verification
Confirmed by multiple independent authoritative sources (SHOM, IGN, academic journal, heritage sites) citing the same exact dates, though minor variant (1896) appears in one source.
Sources
- Les réseaux de nivellement français - Géodésie (geodesie.ign.fr)
- https://tourisme-marseille.com/fiche/les-plaques-reperes-de-nivellement-ngf-de-marseille/ (tourisme-marseille.com)
- http://bdl.ahp-numerique.fr/focus-instruments-ac-maregraphe-marseille (bdl.ahp-numerique.fr)
- https://journals.openedition.org/ephaistos/8716?lang=en (journals.openedition.org)
- https://refmar.shom.fr/sites/default/files/fiches_data/GT_TSH_Fiche_referentiels_verticaux_Version_fevrier2024.pdf (refmar.shom.fr)