"cannon and great guns collectively, artillery," 1540s, an old, clipped form of ordinance (q.v.) which word was attested from late 14c. in the sense of "military materials, provisions of war;" a sense now obsolete but which led to the specialized meanings "engines for discharging missiles" (early 15c.) and "branch of the military concerned with stores and materials" (late 15c.). The shorter word was established in these distinct senses by 17c.
The Ordnance survey (1833), an official geographical survey of Great Britain and Ireland, was undertaken by the government under the direction of the Master-General of the Ordnance (the natural choice, gunners being thoroughly trained in surveying ranges and distances).