J3125 : Bench Mark, Silent Valley

taken 4 months ago, 7 km from Glasdrumman, Northern Ireland

Bench Mark, Silent Valley
Bench Mark, Silent Valley
Large bolt bench mark on a wall in front of the Ben Crom reservoir wall. This is the last in a levelling line that runs up through the valley. The mark has been levelled to 215.35 metres above MSL.

See J3125 : Ben Crom Reservoir for a wide angle view of the location.
The Ben Crom Reservoir

The reservoir, the last part of the Belfast Water Commissioners plan for the area, was constructed in the upper Silent Valley from 1953-57 to hold and supply water into the larger Silent Valley Reservoir below. The dam wall is 700 feet across and the reservoir has a total capacity of 1.7 million gallons.

Bench Mark

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.

Creative Commons Licence [Some Rights Reserved]   © Copyright Rossographer and licensed for reuse under this Creative Commons Licence.
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J3125, 73 images   (more nearby search)
Photographer
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Date Taken
Saturday, 10 April, 2021   (more nearby)
Submitted
Sunday, 11 April, 2021
Geographical Context
Energy infrastructure  Historic sites and artefacts  Uplands  Lakes, Wetland, Bog  Water resources 
Place (from Tags)
Silent Valley  Mourne Mountains 
Primary Subject of Photo
Reservoir  Benchmark 
Subject Location
Irish: geotagged! J 31456 25509 [1m precision]
WGS84: 54:9.7075N 5:59.2939W
Camera Location
Irish: geotagged! J 31456 25509
View Direction
West-northwest (about 292 degrees)
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Other Tags
Silent Valley  Reservoir  Mountains of Mourne  Mourne  Kingdom of Mourne  Mournes  Bench Mark  Benchmark Site  OS Bench Mark Location  Bolt Bench Mark  O/S Bench Mark  OS Benchmark 

Click a tag, to view other nearby images.

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