ordinance datum

ordinance datum

A surveyor uses an ordinance datum to measure the elevation of a hill.

Definition
  1. Noun:
    • A fixed reference point for measuring heights: "ordinance datum" is a specific, legally established level of the sea's surface used as the baseline (zero point) for measuring elevations and depths on maps and charts. It is often derived from tidal observations at a particular location (such as a port) and is adopted by a government or surveying authority.
Usage Examples
  • (The elevation is measured from the official sea-level baseline.)
  • (They referenced the established sea level to determine safe construction levels.)
Advanced Usage

  • "Ordnance Datum Newlyn": A specific, widely used ordinance datum in the United Kingdom, based on mean sea level at Newlyn, Cornwall, from 1915 to 1921.

    • All UK topographic maps use Ordnance Datum Newlyn for elevation references. (This particular datum is the standard for British surveying.)

  • "to be above/below ordinance datum": to be at a height or depth relative to the official sea-level baseline.

    • The lowest point of the valley is 15 meters below ordinance datum. (It is lower than the official sea-level zero.)
Variants and Related Words

  • Ordnance (n): a government department responsible for military supplies and surveying; also refers to cannon or artillery.

    • The Ordnance Survey is the national mapping agency for Great Britain. (The government agency that produces maps using ordinance datum.)

  • Datum (n): a single piece of information or a fixed starting point of a scale.

    • The surveyor established a datum for the construction project. (A reference point was set.)
Synonyms
  • Sea level datum: a baseline based on mean sea level.
  • Vertical datum: any reference surface used for measuring elevations.
Related Idioms
  • "Tied to ordinance datum": a surveying term meaning that a measurement is directly linked to the official baseline.
    • All bench marks in the region are tied to ordinance datum for consistency. (Every elevation marker is referenced to the same sea-level standard.)
Notes
  • "Ordinance datum" is often spelled as "ordnance datum" (with a missing 'i' in "ordnance") in technical contexts, but the meaning remains the same. The term is primarily used in cartography, civil engineering, and geodesy.