Generic selectors
Exact matches only
Search in title
Search in content
Post Type Selectors
Filter by Categories
ArcGIS Pro

Above the Ordnance Datum

What is the Ordnance Datum

Ordnance Datum (OD) is a critical concept in topographical mapping and surveying, serving as a reference point for measuring altitudes and heights. Here’s a detailed look at its various implementations in the United Kingdom and Ireland:

Ordnance Datum Newlyn (ODN) in Great Britain:

  • Established as the mean sea level (MSL) recorded at the Newlyn Tidal Observatory in Cornwall between 1915 and 1921.
  • This replaced the previous OD, which was based on MSL recorded in Liverpool’s Victoria Dock in 1844 (ODL).
  • The Newlyn Tidal Observatory was chosen for its stability and location, being set in granite bedrock and far from major rivers, providing a more accurate representation of deep ocean sea levels.
  • The switch from Liverpool (ODL) to Newlyn (ODN) was gradual, with about 40% of secondary and tertiary levellings still using the Liverpool datum as late as 1950. However, since the Third Geodetic Levelling and the publication of Ordnance Survey maps from March 1956, the Newlyn datum has been the standard.

Ordnance Datum Liverpool (ODL):

  • Prior to 1921, the OD was defined based on MSL recorded at Victoria Dock, Liverpool, during a brief period in 1844.
  • Initially, OD was set as a horizontal plane 100 feet below a benchmark on St John’s Church, Liverpool, but later redefined to mean sea level.

Belfast Ordnance Datum in Northern Ireland:

  • Defined as the MSL at Clarendon Dock, Belfast, between 1951 and 1956.
  • This serves as the OD for the Ordnance Survey of Northern Ireland.

Malin Ordnance Datum in Ireland:

  • Established as the MSL at Portmoor Pier, Malin Head, County Donegal, between 1960 and 1969.
  • This replaced the previous Poolbeg Ordnance Datum, which was based on the low water of the spring tide at Poolbeg Lighthouse, Dublin, on April 8, 1837. The Poolbeg OD was approximately 2.7 meters (9 feet) lower than the Malin OD.

Changes and Developments:

  • The First Geodetic Levelling of England and Wales (1840–1860) required a reliable datum plane. Initially based on a benchmark in Liverpool, it was later redefined to MSL.
  • By the Second Geodetic Levelling (1912–1921), the need for stability led to the use of Fundamental Bench Marks in solid rock. Significant differences in sea level measurements at different locations led to the selection of Newlyn as the standard reference point.
  • The Newlyn Tidal Observatory, integral to this system, was given grade II listing on December 11, 2018, recognizing its historical and scientific importance.

These various datums reflect the evolving understanding and technological capabilities in geodetic surveying, ensuring more accurate and reliable mapping across the UK and Ireland.

Above the Ordnance Datum (AOD)

“Above the Ordnance Datum” (AOD) is a term used in mapping and surveying to describe the height of a location relative to a specified Ordnance Datum (OD). The Ordnance Datum is a reference point used to measure altitudes and heights in a particular region. Here’s how it works:

Definition of AOD: When a spot height or the elevation of a feature on a map is described as being “above the Ordnance Datum,” it means that the height of that feature is measured from the baseline established by the Ordnance Datum. This baseline is typically mean sea level at a specific location.

Importance in Mapping: Using an Ordnance Datum as a reference point ensures uniformity and accuracy in the measurement of heights and depths across a region. This is crucial for a variety of applications, including topographic mapping, construction, and flood risk assessment.

Examples in the UK and Ireland:

  • In Great Britain, the Ordnance Datum is the Ordnance Datum Newlyn (ODN), which is based on the mean sea level recorded at the Newlyn Tidal Observatory between 1915 and 1921.
  • In Northern Ireland, the reference is the Belfast Ordnance Datum.
  • In the Republic of Ireland, it’s the Malin Ordnance Datum.

Practical Use: On topographical maps, spot heights and contours are often expressed in terms of AOD. For instance, if a hill’s summit is listed as being 250 meters AOD, it means that the summit is 250 meters above the mean sea level as defined by the region’s Ordnance Datum.

Variations Over Time: The Ordnance Datum has changed over time as more accurate measurements became possible. For example, in the UK, the shift from the Liverpool Ordnance Datum to the Newlyn Ordnance Datum was a significant change in the early 20th century.

Understanding “above the Ordnance Datum” is essential for interpreting geographical and topographical information accurately, particularly in contexts where precise elevation data is critical.

Frequently asked about the application of Ordnance Datum

How is Ordnance Datum important for flood risk assessment and planning?

Ordnance Datum (OD) plays a crucial role in flood risk assessment and planning. Here’s how it’s important:

  1. Baseline for Elevation Measurements: OD provides a standardized baseline for measuring elevations. In flood risk assessment, it’s vital to know the exact elevation of land relative to sea level. Knowing how high an area is above the OD helps in determining how susceptible it is to flooding, especially in coastal and low-lying regions.
  2. Flood Modeling and Mapping: Accurate elevation data based on OD is essential for creating reliable flood models and maps. These models predict how high water could rise and which areas might be inundated during different types of flood events. This information is crucial for urban planning and development.
  3. Risk Assessment: Planners and engineers use OD-based elevation data to assess the risk of flooding to specific areas. This assessment is key in making informed decisions about where to build, what flood defenses are needed, and how to manage land use effectively to minimize flood risks.
  4. Building Regulations and Zoning: Local governments often use OD and associated flood risk assessments to set building regulations and zoning laws. For example, they might restrict construction in areas that are below or just above the OD level to reduce the risk of flood damage.
  5. Insurance and Property Values: Information about how high an area or property is above OD can influence flood insurance rates and property values. Areas with higher elevations above the OD generally have lower flood risks and, consequently, lower insurance premiums.
  6. Emergency Preparedness and Response: During flood events, knowing the elevation of areas above OD helps emergency services in planning rescue and relief operations. It aids in identifying which areas are most at risk and prioritizing resources and efforts.
  7. Climate Change Adaptation: As sea levels rise due to climate change, the role of OD in flood risk assessment becomes even more critical. Understanding how much higher sea levels are in relation to OD over time helps in planning long-term strategies for coastal defense and adapting urban infrastructures.
  8. Public Awareness and Education: Public knowledge of how their local area relates to the OD can enhance awareness of flood risks. This is important for community-level planning and individual decision-making about property purchase and protection measures.

What is the difference between Ordnance Datum and sea level?

The relationship between Ordnance Datum (OD) and sea level is a key aspect of understanding geographical and surveying concepts. Here’s a detailed comparison:

Ordnance Datum (OD):

  • Definition: OD is a fixed reference point used as the standard for measuring elevations in a particular country or region. It’s essentially a vertical datum.
  • Establishment: OD is often established based on a carefully measured and averaged sea level at a specific location over a certain period. For instance, in the UK, the Ordnance Datum Newlyn (ODN) is based on the mean sea level at Newlyn in Cornwall, observed between 1915 and 1921.
  • Purpose: The primary purpose of OD is to provide a consistent and stable reference for elevation measurements across a region. It is crucial for accurate mapping, surveying, construction, and environmental planning.

Sea Level:

  • Definition: Sea level refers to the average height of the ocean’s surface, a concept known as mean sea level (MSL). It’s a dynamic and fluctuating measurement, influenced by factors like tides, weather, and climatic changes.
  • Variability: Sea level is not consistent across the globe due to gravitational variations, wind patterns, ocean currents, and other factors. It also changes over time due to phenomena like tectonic movements, melting of polar ice, and thermal expansion of water under global warming.
  • Measurement: Modern techniques for measuring sea level include satellite altimetry and tide gauges. These methods provide data that can be used for various purposes, including studying climate change and aiding in coastal management.

Key Differences:

  1. Stability vs. Variability: OD is a fixed, stable reference point, whereas sea level is variable both spatially and temporally.
  2. Purpose and Application: OD is used for surveying, mapping, and construction, providing a stable baseline for elevation measurements. Sea level is a broader concept used in oceanography, climatology, and global environmental monitoring.
  3. Regional Specificity vs. Global Concept: OD is specific to a region and based on local sea level measurements. Mean sea level, on the other hand, is a global concept, though it varies regionally due to various geographical and environmental factors.

Can Ordnance Datum be used in GPS and modern digital mapping technologies?

Ordnance Datum (OD) can indeed be integrated with GPS and modern digital mapping technologies, though the relationship and usage are somewhat complex due to the different nature of the systems involved. Here’s an overview:

GPS and Vertical Datums:

  • GPS (Global Positioning System) provides geographic coordinates in terms of latitude, longitude, and altitude. The altitude given by GPS is typically referenced to a global ellipsoidal model of the Earth, such as the WGS 84 ellipsoid.
  • This ellipsoidal model is different from the local mean sea level measurements used to establish Ordnance Datum. Therefore, the altitudes from GPS don’t directly correspond to heights above the OD.

Integrating OD with GPS:

  • To use GPS data in a system based on Ordnance Datum (or any local mean sea level datum), a conversion is necessary. This involves using a geoid model, which represents the mean sea level across the Earth’s surface.
  • The difference between the ellipsoid (used by GPS) and the geoid (which approximates mean sea level) is known as the geoid height. By applying this correction, GPS altitude can be converted to a height above the OD.

Digital Mapping Technologies:

  • Modern digital mapping and Geographic Information Systems (GIS) can handle multiple datums and coordinate systems. These systems can convert between global coordinates (like those from GPS) and local height systems (like OD).
  • When mapping and surveying within regions that use OD (like the UK or Ireland), GIS tools often include functionality to work with these specific datums, ensuring that elevations are correctly referenced to the local standard.

Practical Applications:

  • In practical applications like construction, land surveying, and environmental management, professionals often use tools that can bridge the gap between GPS data and local datum references. This ensures that the elevations used in planning and analysis are both globally accurate and locally relevant.


  • One challenge in integrating these systems is the need for accurate geoid models, which can vary in precision. As geoid models are refined, the accuracy of converting GPS altitudes to OD heights improves.
About the Author
I'm Daniel O'Donohue, the voice and creator behind The MapScaping Podcast ( A podcast for the geospatial community ). With a professional background as a geospatial specialist, I've spent years harnessing the power of spatial to unravel the complexities of our world, one layer at a time.