Topographic mapping.
Uses the RGS value for the Paris meridian. In Alsace, data suspected to be transformation of German network into ATF. Replaced by Nouvelle Triangulation Francaise (Paris) (code 6807) which uses the 1936 IGN value for the Paris meridian.
Definition: OGC Well Known Text 2 (2019)
DATUM["Ancienne Triangulation Francaise (Paris)", ELLIPSOID["Plessis 1817",6376523,308.64, LENGTHUNIT["metre",1]], ID["EPSG",6901]], PRIMEM["Paris RGS",2.33720833333333, ANGLEUNIT["degree",0.0174532925199433], ID["EPSG",8914]]
Definition: JSON
{ "$schema": "https://proj.org/schemas/v0.7/projjson.schema.json", "type": "GeodeticReferenceFrame", "name": "Ancienne Triangulation Francaise (Paris)", "ellipsoid": { "name": "Plessis 1817", "semi_major_axis": 6376523, "inverse_flattening": 308.64 }, "prime_meridian": { "name": "Paris RGS", "longitude": 2.33720833333333 }, "scope": "Topographic mapping.", "area": "France - mainland onshore.", "bbox": { "south_latitude": 42.33, "west_longitude": -4.87, "north_latitude": 51.14, "east_longitude": 8.23 }, "id": { "authority": "EPSG", "code": 6901 } }