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.
Vertical references in EuropeView from Newlyn Harbour showing the lighthouse and Tidal Observatory to its right, both painted red and white.
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, in 1840 used a benchmark on St. John’s Church, Liverpool.[2]
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.[3]
Ordnance Datum Newlyn and its antecedentsedit
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: 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.[4][5]
^ abIrish Grid Reference System 2008-06-14 at the Wayback Machine from OSI website
^Bradshaw, et al. (2016). "A Century of Sea Level Measurements at Newlyn, Southwest England". Marine Geodesy. 39 (2): 115–140. Bibcode:2016MarGe..39..115B. doi:10.1080/01490419.2015.1121175.
^"Ordnance Survey Benchmark locator". from the original on 27 December 2021. Retrieved 21 December 2021.
^"Newlyn tidal observatory and part of the south pier". English Heritage. Retrieved 17 October 2023.
^"Victoria Station Upgrade Environmental Statement: Main Report" (PDF). Transport for London. 15 November 2007. pp. 2–6, §2.5.4, fn 2. (PDF) from the original on 4 June 2011. Retrieved 28 July 2008.
^ Radcliffe, Eric (1995). "Control and Construction Surveys". In Colin J Kirkland (ed.). Engineering the Channel Tunnel. Taylor & Francis. p. 53. ISBN0-419-17920-8.
External linksedit
Ordnance Survey webpage on datum height differences
January 01, 1970
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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 Vertical references in Europe View from Newlyn Harbour showing the lighthouse and Tidal Observatory to its right both painted red and white Contents 1 British Isles 1 1 Ordnance Datum Newlyn and its antecedents 1 2 Tunnel datum 2 See also 3 References 4 External linksBritish Isles editIn Great Britain OD for the Ordnance Survey is Ordnance Datum Newlyn ODN defined as the MSL as recorded by the Newlyn Tidal Observatory between 1915 and 1921 1 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 in 1840 used a benchmark on St John s Church Liverpool 2 In Northern Ireland OD for the Ordnance Survey of Northern Ireland is Belfast Ordnance Datum the MSL at Clarendon Dock Belfast 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 3 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 3 Ordnance Datum Newlyn and its antecedents edit 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 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 4 5 Newlyn Tidal Observatory was given grade II listing on 11 December 2018 6 Tunnel datum edit 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 7 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 8 thus a depth of 60 m is 140 m ATDSee also editOrdnance 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 Official authority for lighthouses in EnglandPages displaying short descriptions of redirect targets Vertical Offshore Reference Frames UK and Irish hydrographic vertical datumReferences edit Ordnance Survey Benchmark locator Archived from the original on 27 December 2021 Retrieved 21 December 2021 https maritimearchaeologytrust org newlyn ordnance and other datums a b Irish Grid Reference System Archived 2008 06 14 at the Wayback Machine from OSI website Bradshaw et al 2016 A Century of Sea Level Measurements at Newlyn Southwest England Marine Geodesy 39 2 115 140 Bibcode 2016MarGe 39 115B doi 10 1080 01490419 2015 1121175 Ordnance Survey Benchmark locator Archived from the original on 27 December 2021 Retrieved 21 December 2021 Newlyn tidal observatory and part of the south pier English Heritage Retrieved 17 October 2023 Victoria Station Upgrade Environmental Statement Main Report PDF Transport for London 15 November 2007 pp 2 6 2 5 4 fn 2 Archived PDF from the original on 4 June 2011 Retrieved 28 July 2008 Radcliffe Eric 1995 Control and Construction Surveys In Colin J Kirkland ed Engineering the Channel Tunnel Taylor amp Francis p 53 ISBN 0 419 17920 8 External links editOrdnance Survey webpage on datum height differences Retrieved from https en wikipedia org w index php title Ordnance datum amp oldid 1215878126, wikipedia, wiki, book, books, library,
