To install click the Add extension button. That's it.

The source code for the WIKI 2 extension is being checked by specialists of the Mozilla Foundation, Google, and Apple. You could also do it yourself at any point in time.

4,5
Kelly Slayton
Congratulations on this excellent venture… what a great idea!
Alexander Grigorievskiy
I use WIKI 2 every day and almost forgot how the original Wikipedia looks like.
Live Statistics
English Articles
Improved in 24 Hours
Added in 24 Hours
Languages
Recent
Show all languages
What we do. Every page goes through several hundred of perfecting techniques; in live mode. Quite the same Wikipedia. Just better.
.
Leo
Newton
Brights
Milds

Ordnance datum

From Wikipedia, the free encyclopedia

Vertical references in Europe
Vertical references in Europe
View from Newlyn Harbour showing the lighthouse and Tidal Observatory to its right, both painted red and white.
View from Newlyn Harbour showing the lighthouse and Tidal Observatory to its right, both painted red and white.

In the British Isles, an ordnance datum or 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 AOD for "above ordnance datum". Usually mean sea level (MSL) is used for the datum. In particular:

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: 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 to abandon the attempt 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.[2]

Tunnel datum

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;[3] 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;[4] thus a depth of −60 m is 140 m ATD

See also

Trinity High Water

References

  1. ^ a b Irish Grid Reference System Archived 2008-06-14 at the Wayback Machine from OSI website
  2. ^ Bradshaw, et al. (2016). "A Century of Sea Level Measurements at Newlyn, Southwest England,". Marine Geodesy. 39 (2): 115–140. doi:10.1080/01490419.2015.1121175.
  3. ^ "Victoria Station Upgrade Environmental Statement: Main Report" (PDF). Transport for London. 15 November 2007. pp. 2–6, §2.5.4, fn 2. Retrieved 28 July 2008.
  4. ^ Radcliffe, Eric (1995). "Control and Construction Surveys". In Colin J Kirkland (ed.). Engineering the Channel Tunnel. Taylor & Francis. p. 53. ISBN 0-419-17920-8.

External links


This page was last edited on 2 April 2021, at 21:36
Basis of this page is in Wikipedia. Text is available under the CC BY-SA 3.0 Unported License. Non-text media are available under their specified licenses. Wikipedia® is a registered trademark of the Wikimedia Foundation, Inc. WIKI 2 is an independent company and has no affiliation with Wikimedia Foundation.