borrowed from French, "rod with graduated measurements," borrowed from Italian, probably borrowed from plural of New Latin stadium
"stage, gradation," going back to Latin, "unit of length" — more at stadium
According to several sources the word stadia in reference to a graduated rod was introduced by Piedmontese army engineers mapping the border between France and Savoy in 1816 (see M. Goulier, "Mémoire sur la stadia, et sur les instruments servant, conjointement avec elle, au mesurage des distances," Mémorial de l'Officier du Génie, no. 16, 2. série, tome 1 , p. 156; A. Laussedat, Recherches sur les instruments, les méthodes et le dessin topographiques, tome 1 [Paris, 1898], pp. 166-67, citing Nicodemo Jadanza, Per la storia della celerimensura). The word appeared in the title of a report on this survey, "Rapport sur la stadia," written in 1822 by a staff officer, M. de Lostende, and published in the Mémorial du Dépôt général de la Guerre, tome 4 (1828). The Piedmontese army engineer and inventor of optical instruments Ignazio Porro (1801-75) traced the word directly to the ancient unit of length (see Le tachéometrie, ou lȧart de lever des plans et faireles nivellements, Paris, 1858, p. 265), though this seems somewhat improbable semantically.