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. [cascade flags: wikipedia_dropped_other_sources_exist | tier=bronze indep_score=0.525 clusters=4 claim_tier=notable | framework convergence=partial on Historical records of the Marseille tide gauge observations and Charles Lallemand's 1897 determination of the NGF zero, as documented and republished by IGN, SHOM, and academic historians (upstreams: Service du Nivellement Général de la France, Charles Lallemand's original 1897 determination, IGN historical records) — publishers independent, evidence may not be | kind=empirical eligibility=high]
Sources
- Les réseaux de nivellement français - Géodésie (seed:geography_and_travel)
- https://tourisme-marseille.com/fiche/les-plaques-reperes-de-nivellement-ngf-de-marseille/ (corroboration)
- http://bdl.ahp-numerique.fr/focus-instruments-ac-maregraphe-marseille (corroboration)
- https://journals.openedition.org/ephaistos/8716?lang=en (corroboration)
- https://refmar.shom.fr/sites/default/files/fiches_data/GT_TSH_Fiche_referentiels_verticaux_Version_fevrier2024.pdf (corroboration)