Ordnance datum
Anordnance datum(OD) is avertical datumused by an ordnance survey as the basis for deriving altitudes onmaps.Aspot heightmay be expressed asabove ordnance datum(AOD). Usuallymean sea level(MSL) at a particular place is used for the datum.
British Isles
[edit]- In Great Britain, OD for theOrdnance SurveyisOrdnance Datum Newlyn(ODN), defined as the MSL as recorded by theNewlyn Tidal Observatorybetween 1915 and 1921.[1]
- Prior to 1921, OD wasOrdnance Datum Liverpool(ODL) defined as MSL as recorded in theVictoria Dock, Liverpool,during a short period in 1844. The first datum, in 1840 used a benchmark onSt. John’s Church,Liverpool.[2]
- InNorthern Ireland,OD for theOrdnance Survey of Northern IrelandisBelfast Ordnance Datum:the MSL at Clarendon Dock,Belfast,between 1951 and 1956.
- InIreland,OD for theOrdnance Survey of IrelandisMalin Ordnance Datum:the MSL at Portmoor Pier,Malin Head,County Donegal,between 1960 and 1969.[3]
- Prior to 1970,Poolbeg Ordnance Datumwas used: the low water ofspring tideatPoolbeg 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 theVictoria 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: atDunbar,NewlynandFelixstowe.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 givengrade II listingon 11 December 2018.[6]
Tunnel datum
[edit]Tunnel datumis a datum based on an ordnance datum and used in designing tunnels which pass below sea level.
- for theLondon 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 theChannel Tunnel,a tunnel datum of ODN −200 m is used;[8]thus a depth of −60 m is 140 m ATD
See also
[edit]- 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– Official authority for lighthouses in England
- Vertical Offshore Reference Frames– UK and Irish hydrographic vertical datum
References
[edit]- ^"Ordnance Survey Benchmark locator".Archivedfrom the original on 27 December 2021.Retrieved21 December2021.
- ^"Newlyn Ordnance and Other Datums".8 June 2021.
- ^abIrish Grid Reference SystemArchived2008-06-14 at theWayback Machinefrom 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".Archivedfrom the original on 27 December 2021.Retrieved21 December2021.
- ^"Newlyn tidal observatory and part of the south pier".English Heritage.Retrieved17 October2023.
- ^"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.Retrieved28 July2008.
- ^ Radcliffe, Eric (1995). "Control and Construction Surveys". In Colin J Kirkland (ed.).Engineering the Channel Tunnel.Taylor & Francis. p. 53.ISBN0-419-17920-8.