Triangulation Pillars
From ManxWiki
Triangulation Pillars also known as "Trig Points" are concrete pillars, about 4' tall, which were used by the Ordnance Survey in order to determine the exact shape of Britain in a project known as the "Retriangulation of Great Britain", which was carried out from 1936 to 1962.
The network of trig points was built and measured starting with a set of several hundred primary trig points, most of which were placed on high hills so as to be able to link to one another across long distances. In addition, a larger set of roughly six thousand secondary trig points were added to allow the construction of a finer mesh which would extend the reference frame of the primary mesh over shorter distances.
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Triangulation
Trig Points are generally located on the highest bit of ground in the area so that there is a direct line of sight from one to the next. A graved metal plate is on the top of a pillar for securing a theodolite or reflector.
By setting a theodolite on the top of the pillar, accurate bearings to two or more nearby trig points could be taken. This process is called "triangulation".
Such triangulation methods dominated accurate large-scale land surveying until the rise of Global navigation satellite systems in the 1980s.
Current Usage
There are 6550 such pillars listed in the T:UK database. While most of them have fallen into disuse, about 184 of them are currently used in the Passive Station network. These are maintained so they can be used as accurately located anchor points for differential GPS comparisons, being re-surveyed every five years to calibrate for any geological movements of the ground.
Isle of Man Triangulation Points
The Isle of Man has 14 triangulation pillars at the following locations.
Ballacregga Farm
Ballawhannel
Ballig
Cyblane Farm
Corrins Hill
Cronk ny Arrey
Cronk ny Arrey Laa
Howstrake Golf Course
North Barrule
Sartfield
Slieau Freoaghane
Slieau Ruy
Snaefell
South Barrule
In addition the island has two triangulation aerials (they're a an aerial shaped with a mushroom type top) located on the Ronaldsway main airport building and at Ramsey Cottage Hospital.
Trigpointing
Trigpointing is a recreational activity by walkers who use the pillars as waypoints, often recording the points visited to create a personal database.
Access
The following traingulation pillars are on hills in areas of public ramblage and access to such is not a problem other than having to make the effort on foot to reach them.
Corrins Hill
Cronk ny Arrey Laa
North Barrule
Slieau Freoaghane
Slieau Ruy
Snaefell
South Barrule
The remaining are on private land and permission should be sought from the landowner if a visit is required.
For extended details of triangulation in the Isle of Man & Great Britain (Ireland, north & south is excluded) click on the external link to Trigpointing UK
