H6733 : Bench Marks, Monaghan

taken 5 months ago, near to Monaghan, Milltown, Cortolym Bridge and Rossmore, Monaghan, Ireland

Bench Marks, Monaghan
Bench Marks, Monaghan
Two bench marks on the base of a pillar at the front of Monaghan Courthouse H6733 : The Court House.

The top mark dates from the First Geodetic Levelling of Ireland, 1839-43. This mark was in a line both from Netwonbutler to Belfast (the 48th mark in that line) and from Monaghan to Carrowkeel and Buncrana (the 1st mark in that line).
The original remarks for the mark describe it as "Copper bolt driven into pillar in front of Monaghan Court House; 0.2 feet above sill of door".
The mark was originally levelled to 208.921 feet above sea level; an arrow pointing downwards usually denotes that the mark is unreliable but on this occasion I'm not sure that applies as there was no room for the stonemason to cut the arrow below the mark as it was so close to the base of the pillar.

The large OSBM marked bolt was placed here c1960 and is on a primary line between the fundamental bench marks at Armagh and Newtownbutler; I can't find a height for this mark currently.
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.
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H6733, 421 images   (more nearby )
Photographer
  (find more nearby)
Date Taken
Sunday, 28 May, 2017   (more nearby)
Submitted
Monday, 29 May, 2017
Geographical Context
Historic sites and artefacts  City, Town centre 
Place (from Tags)
Monaghan 
Primary Subject of Photo
Benchmark 
Subject Location
Irish: geotagged! H 671 337 [100m precision]
WGS84: 54:14.8672N 6:58.2194W
Camera Location
Irish: geotagged! H 671 337
View Direction
Southeast (about 135 degrees)
Clickable map
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Other Tags
Bench Mark  Bench-Mark  Bench Marks  Benchmark Site  Cut Bench Mark  OS Bench Mark Location  Bolt Bench Mark  OS Benchmark  Monaghan 

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