Pronunciation: [ˈɔːdnəns dˈe͡ɪtəm] (IPA) 
Ordnance datum is a technical term used in cartography and surveying. The correct spelling of this word is [ˈɔːdnəns ˈdeɪtəm], with the stress on the second syllable of each word. Ordnance datum refers to a vertical reference point used for measuring elevation or altitude above sea level. It is important to use the correct spelling of ordnance datum in order to ensure accurate and reliable surveying and mapping data.
Ordnance Datum is a geodetic vertical reference point used in cartography and surveying, particularly in the United Kingdom. It is a fixed elevation used as a base reference for measuring heights and depths in relation to sea level. The Ordnance Datum is often referred to as OD and is crucial in maintaining consistency and accuracy throughout geographic mapping.
The Ordnance Datum is typically determined by measuring the average level of the sea at a specific location over a fixed period of time. This measurement is then used to establish a baseline elevation for all subsequent surveys and map productions. By using a fixed reference point, mapping agencies and professionals can ensure that measurements and elevations are consistent across different areas and projects.
Additionally, Ordnance Datum is fundamental in determining the vertical distance between various features on a map, such as hills, valleys, buildings, or water bodies, which greatly assists in construction, navigation, urban planning, and engineering projects. By providing a standardized reference point, Ordnance Datum simplifies the process of calculating height differentials and assessing the impact of topographical features on a given area.
Ordnance Datum is generally measured in meters or feet, depending on the country or region. It serves as a crucial factor in accurately representing the three-dimensional layout of the Earth's surface on maps, allowing for efficient and reliable analysis and decision-making in various fields.
The term "ordnance datum" has its etymology rooted in the field of cartography and geodesy.
The word "ordnance" refers to military equipment, especially artillery, ammunition, and other supplies. In this context, it is derived from the Latin word "ordō" meaning "order" or "arrangement".
The word "datum" comes from the Latin word "datus", which means "given". In the context of surveying and mapping, it refers to a reference point or line from which heights, depths, and other measurements are calculated or measured.
When combined, "ordnance datum" refers to a specific reference point or level used as a baseline for measuring heights and depths in cartography and geodesy. It is typically denoted by a benchmark or a fixed survey point representing mean sea level or another established reference point.
