ˌOrdnance ˈSurvey ˌmap BrE AmE noun [countable] British English
[ Date: 1800-1900 ; Origin: Because the maps were originally made for the department responsible for ordnance: ordnance 'military materials' (14-20 centuries) , from Old French ordenance ]
a map which shows all the roads, paths, hills etc of an area in detail