Cirrostratus map, Watercolor by Andhré des Gachons, February 20, 1916.

he geographical coordinates given seem to use Paris as the prime meridian (2.33723° east of Greenwich) as on maps of the time. There is also an error in the latitude, which is actually 48° 50′ North and not 40° 50′. “At the exact point of the intersection of the meridian and the parallel” therefore refers to these coordinates: 2° 10′ E, 48° 50′ N, or in current coordinates 4.50390°E, 48.83333°N, leaving La Chaussée-sur-Marne in the direction of Saint-Martin-aux-Champs as indicated (Saint-Martin road).