J3527 : Flush Bracket, Slieve Donard Triangulation Pillar
taken 5 years ago, 3 km from Widows Row, Northern Ireland
OSNI Triangulation Stations
The re-triangulation of Northern Ireland by the Ordnance Survey Northern Ireland began in 1950. This was the first complete survey of Northern Ireland which included observations with the new primary triangulation of the country, its connection with the Republic of Ireland and the cross-channel connection between Ireland and Great Britain. This began by OSNI establishing a series of triangulation stations throughout the country. Almost all of these stations were topped by trig pillars and 80, mainly primary and secondary pillars, had been constructed by October 1949. Measurements between primary stations began in 1950 and measurements for these and the secondary stations were completed by July 1956. The construction and measurements for tertiary stations were completed later (probably no later than the mid 1960s). Only two stations are not topped by pillars - Lighthouse Island, marked by a brass rivet, and Ardglass, which utilised the top of a high stone folly. The older triangulation stations on the Lough Foyle Base Line were also re-surveyed as part of this process. A majority of the stations are still extant today but a few have been removed or destroyed.
Bench marks were historically used to record the height above sea level of a location as surveyed against the Mean Sea Level data (taken at Clarendon Dock, Belfast, for Northern Ireland data, Newlyn in Cornwall for data in Great Britain and Portmoor Pier, Malin Head, for data relating to the Republic of Ireland). They were used as part of a greater surveying network by the UK Ordnance Survey, Ordnance Survey Northern Ireland (OSNI) and the Ordnance Survey Ireland (OSI). If the exact height of one bench mark is known then the exact height of the next can be found by measuring the difference in heights, through a process of spirit levelling. In this way hundreds of thousands of bench marks were sited all around the UK & Ireland from the mid 19th to late 20th centuries. There are several distinct types of bench mark:
- Fundamental bench marks have been constructed at selected sites where foundations can be set on stable strata such as bedrock. Each FBM consists of a buried chamber with a brass bolt set in the top of a granite pillar. See NG8825 : Dornie fundamental bench mark for an example. FBMs were used in Ireland as well as GB but those in Ireland do not have any surface markers, nor are they marked on standard maps.
- Flush brackets consist of metal plates about 90 mm wide and 175 mm long. Each bracket has a unique serial number. They are most commonly found on most Triangulation Pillars, some churches or on other important civic buildings. See J3270 : Flush Bracket, Belfast for an example.
- Cut bench marks are the commonest form of mark. They consist of a horizontal bar cut into a wall or brickwork and are found just about anywhere. A broad arrow is cut immediately below the centre of the horizontal bar. See J3372 : Bench Mark, Belfast for an example. The horizontal mark may be replaced by or contain a bolt - see J1486 : Bench Mark, Antrim.
Other marks include:
- Projecting bench marks such as SD8072 : Projecting Bracket Benchmark on St Oswald's Tower
- Bolt bench marks such as SJ1888 : OSBM bolt on Hilbre Island
- Rivet bench marks such as J3978 : Bench Mark, Holywood
- Pivot bench marks such as SJ2661 : Pivot bench mark on Leeswood Bridge
Bench marks are commonly found on older buildings or other semi-permanent features such as stone bridges or walls. Due to updated mapping techniques and technological advances such as GPS, bench marks are no longer maintained. Many are still in existence and the markers will probably remain until they are eventually destroyed by redevelopment or erosion.
The Mourne Wall
The Mourne Wall is a 22 mile long wall in the Mourne Mountains. It was built between 1904 and 1922 by the Belfast Water Commissioners to enclose their water catchment areas in the Mournes and protect the area from the effects of cattle and sheep on the water course. The wall is predominately constructed from local granite stone using traditional dry stone walling techniques; on average the wall is about 1.5 metres high and 0.8 to 0.9 metres thick. It is not uniform in construction along the entire length - the 'classic' granite wall is only to be found north of Carn mountain and Long Seefin with particularly impressive sections on Slieve Commedagh and Slieve Donard; elsewhere the wall largely resembles dry stone walls found elsewhere in the Mournes and south County Down. In places, such as Slieve Muck, the wall is not constructed of granite at all.
- Grid Square
- J3527, 87 images (more nearby )
- Rossographer (find more nearby)
- Image classification?
- Supplemental image
- Date Taken
- Saturday, 11 August, 2012 (more nearby)
- Monday, 13 August, 2012
- Geographical Context
- Place (from Tags)
- Subject Location
Irish: J 35787 27685 [1m precision]
WGS84: 54:10.8121N 5:55.2596W
- Camera Location
- Irish: J 35787 27685
- View Direction
- Northeast (about 45 degrees)
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