science

Ordnance datum

Ordnance datum is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Ordnance datum rather than just read about it. In short: An ordnance datum (OD) is a vertical datum used by an Ordnance Survey as the basis for deriving altitudes on maps. A spot height may be expressed as above ordnance datum (AOD).

Key takeaways

Ordnance datum belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Ordnance datum to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Ordnance datum from memory before moving on to harder problems.

Reference excerpt

An ordnance datum (OD) is a vertical datum used by an Ordnance Survey as the basis for deriving altitudes on maps. A spot height may be expressed as above ordnance datum (AOD). Usually mean sea level (MSL) at a particular place is used for the datum.

British Isles In Great Britain, OD for the Ordnance Survey is Ordnance Datum Newlyn (ODN), defined as the mean sea level (MSL) as recorded by the Newlyn Tidal Observatory between 1915 and 1921. Prior to 1921, OD was Ordnance Datum Liverpool (ODL) defined as MSL as recorded in the Victoria Dock, Liverpool, during a short period in 1844. The first datum – established in 1840 – used a benchmark on St. John’s Church in central Liverpool. In Northern Ireland, OD for the Ordnance Survey of Northern Ireland is Belfast Ordnance Datum: the MSL at Clarendon Dock, Belfast Harbour, between 1951 and 1956. In Ireland, OD for the Ordnance Survey of Ireland is Malin Ordnance Datum: the MSL at Portmoor Pier, Malin Head, County Donegal, between 1960 and 1969. Prior to 1970, Poolbeg Ordnance Datum was used: the low water of spring tide at Poolbeg Lighthouse, Dublin, on 8 April 1837. Poolbeg OD was about 2.7 metres (9 feet) lower than Malin OD.

Ordnance Datum Newlyn and its antecedents The First Geodetic Levelling of England and Wales (1840–1860) needed to define a datum plane from which to specify spot heights. At first it was specified as a horizontal plane 100 feet below an arbitrary benchmark on St John's Church, Liverpool. Subsequently, however, it was redefined as mean sea level (MSL). To establish MSL, tidal observations were taken at the Victoria Dock, Liverpool, over a short period in 1844. By the time of the Second Geodetic Levelling (1912–1921) the importance of stability was better appreciated and so it was decided to use Fundamental Bench Marks (FBMs) installed in solid rock, rather than on buildings as before. To measure average MSL around Great Britain three tide gauges were employed; once each at Dunbar, Newlyn, and Felixstowe. However, it was found that the measured difference between the Dunbar and Newlyn stations was 0.81 feet (0.247m), far larger than could be accounted for by error. The difference was real. Accordingly, it was decided not to use average MSL and fix on one site: MSL Newlyn. Newlyn has certain practical advantages: it is set in granite bedrock, is far from major rivers, and it better represents deep ocean sea levels. The difference between ODL (Liverpool) and ODN (Newlyn) was found to be 0.13 feet. It took some time for the changed definition — Liverpool to Newlyn — to work through the system: by 1950, some 40% of the lower secondary and tertiary levellings were still using the Liverpool datum. But following the Third Geodetic Levelling, Ordnance Survey maps published since March 1956 give spot heights above the Newlyn datum. Newlyn Tidal Observatory was given grade II listing on 11 December 2018.

Tunnel datum A tunnel datum is a datum based on an ordnance datum and used in designing tunnels which pass below sea level.

for the London Underground, a tunnel datum of ODN −100 m is used; thus a depth of −60 m AOD is 40 m ATD (above tunnel datum) for the Channel Tunnel, a tunnel datum of ODN −200 m is used; thus a depth of −60 m is 140 m ATD

See also Ordnance Survey National Grid – System of geographic grid references used in Great Britain Principal Triangulation of Great Britain – 1791–1853 geodetic survey of Britain Chart datum – Level of water from which depths displayed on a nautical chart are measured Trinity High Water Vertical Offshore Reference Frames – UK and Irish hydrographic vertical datum

References

External links Ordnance Survey webpage on datum height differences

Illustrations

Worked examples

Example 1 — a first encounter with Ordnance datum

Start with the simplest possible case. Write down what Ordnance datum claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Ordnance datum before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Ordnance datum ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Ordnance datum

In research: Ordnance datum appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Ordnance datum in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Ordnance datum is common in secondary-school and first-year university syllabi. It links to neighbouring topics Geodetic surveys, Historical geography of the United Kingdom, Maps of the United Kingdom, so understanding it makes those chapters shorter.
In everyday life: Look for Ordnance datum outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Ordnance datum in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Ordnance datum means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Ordnance datum out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Ordnance datum in simple terms?

An ordnance datum (OD) is a vertical datum used by an Ordnance Survey as the basis for deriving altitudes on maps. A spot height may be expressed as above ordnance datum (AOD).

Why does Ordnance datum matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Ordnance datum?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Ordnance datum.