ordnance datum
Very LowTechnical, Geographic, Cartographic, Surveying
Definition
Meaning
A vertical reference point, or mean sea level, established by a national mapping agency as the basis for measuring heights and altitudes on maps and surveys.
The standardized zero elevation level used as the starting point for calculating all other heights within a country's geographic survey system.
Linguistics
Semantic Notes
Always used as a singular, capitalized proper noun referring to a specific, defined reference point (e.g., Ordnance Datum Newlyn). It is a cornerstone concept in geodesy and cartography.
Dialectal Variation
British vs American Usage
Differences
In British English, 'Ordnance Datum' (capitalised) is the term of art, specifically linked to Ordnance Survey (the national mapping agency). In American English, the equivalent concept is typically referred to as 'North American Vertical Datum (NAVD)' or a 'geodetic datum'.
Connotations
In the UK, the term carries connotations of national mapping heritage and the authority of the Ordnance Survey. In the US, it is a purely technical engineering/surveying term.
Frequency
Exclusively used in professional surveying, cartography, and civil engineering contexts in the UK. Almost never used in American English, where 'datum' or 'vertical datum' are the generic terms.
Vocabulary
Collocations
Grammar
Valency Patterns
height above [Ordnance Datum Newlyn]referenced to [the Ordnance Datum][location] is [number] metres above Ordnance DatumVocabulary
Synonyms
Strong
Neutral
Weak
Vocabulary
Antonyms
Phrases
Idioms & Phrases
- “[not applicable for this technical term]”
Usage
Context Usage
Business
[almost never used]
Academic
Used in geography, earth sciences, and civil engineering papers discussing elevation data.
Everyday
[virtually never used]
Technical
Essential term in surveying, cartography, flood risk assessment, construction planning, and geographic information systems (GIS).
Examples
By Part of Speech
verb
British English
- [The height was] datummed to Ordnance Datum Newlyn.
American English
- [The survey] referenced all elevations to the North American Vertical Datum.
adverb
British English
- [Not applicable]
American English
- [Not applicable]
adjective
British English
- The ordnance-datum level is crucial for planning.
Examples
By CEFR Level
- [Too technical for A2]
- [Too technical for B1]
- The mountain's peak is 978 metres above Ordnance Datum.
- Engineers must check the site's height relative to the national ordnance datum.
- Flood defence walls are constructed to a level of 5.2 metres above Ordnance Datum Newlyn to account for storm surges.
- All UK topographic maps published by the Ordnance Survey use Ordnance Datum as the vertical reference for contour lines.
Learning
Memory Aids
Mnemonic
Think 'ORDNance Survey sets the stanDARD (datum) for measuring lanD'.
Conceptual Metaphor
THE FOUNDATION OF HEIGHT (It is the conceptual 'floor' or 'baseline' upon which all other vertical measurements are built.)
Watch out
Common Pitfalls
Translation Traps (for Russian speakers)
- Avoid translating 'ordnance' as 'артиллерия' (artillery). Here it's part of the proper name 'Ordnance Survey' ('Топографическая служба'). 'Datum' is singular, its plural is 'data', but here it's a fixed singular term; do not translate it as 'данные' (data). The closest equivalent is 'отметка нуля' or 'геодезический репер'.
Common Mistakes
- Using 'ordnance data' (plural) incorrectly.
- Confusing it with 'horizontal datum' which deals with latitude/longitude.
- Not capitalising it when referring to the official UK system.
- Pronouncing 'ordnance' as 'ordinance'.
Practice
Quiz
What is 'Ordnance Datum' primarily used for?
FAQ
Frequently Asked Questions
It is a precise, fixed definition of mean sea level averaged over a specific period at a specific location (like Newlyn, Cornwall), adopted as the national standard. It is not the instantaneous local sea level.
It originates from the 'Ordnance Survey', the UK's national mapping agency, founded in 1791. The name 'Ordnance' reflects its original military purpose under the Board of Ordnance.
Yes. For example, the US uses the North American Vertical Datum of 1988 (NAVD 88). France uses the 'Niveau général de la France' (NGF). Each country defines its own official vertical reference.
The definition is fixed. The current British standard, Ordnance Datum Newlyn (ODN), was established from 1915 to 1921. While the fixed point doesn't change, more precise realisations of it or new datums (like those based on GPS) may be developed, but ODN remains the legal basis for most maps.