[From Home Office File HO 99/17]

Hobhouse to Smelt 25th Aug 1825

Sir

The Board of Ordnance having applied to Mr Sec Peel for his assistance in affording Protection to the Staves and other Objects employed in the Trignometrical Survey conducted under the Board and having reported that such objects have been erected at Snae Fell, North Barule, South Barule, and Calf of Man in the Island under your Command, I have received Mr Peel's Directions to desire that you will take such Steps as may seem to you to be necessary for the Purpose of ensuring the Preservation of the Objects on these four Trignometrical Stations

I am &c H Hobhouse

Notes

Smelt sent out circulars to the Captains of Parish and the Coroners to the same effect.

The only mention I can find about the protected objects is by Welch in his Six Day's Tour commenting on the summit of Snaefell

The point of Snafel, like that of Snowdon, is crowned with a rude pillar of stone, surmounted by as rude a column or post, probably as a mark for some trignometrical survey of the country.

The marks and the accurate measurement of the angles between a base point and two other visible marks allowed the determination of one angle of a triangle, by using the angle measured at one of the two distant marks between the base point and the other mark a unique triangle is specified; by successively adding further marks and the angles between them a mesh of known triangles is obtained - a highly accurate measurement of a single baseline is now sufficient to fully specify the location of all marks relative to the baseline. However many such angles need to be measured with high accuracy to avoid build up on significant errors.

The measurements taken using the four locations would be between them and marks set up on adjacent coasts of the surrounding countries in the Irish Sea basin - measurements taken at many sites over a long period from the 1783 onwards were used to derive the first triangulation of 1853 though by the early twentieth century the inaccuracy was becoming noticeable.

As the Board of Ordnance was resposible for these surveys and the resulting maps, the name Ordnance Survey came to be used for these maps and in the Island was recognised as the basis of legal demarcation of land.

Neely covers some of the history of the Ordnance Survey and mentions the all important Ramsden's theodolite, a huge instrument that could be split into comomponents and reassembled at the measurement site, a pair of which, were used for the measurements - he mentions that a track up to the top of South Barrule was created by the observers to reach their camp as no doubt several days would be needed to wait for weather conditions to be suitable to take measurements.

The "Trig Pillar" was devised in 1936 as a very stable base on which to set a theodolite to take the angle measurements needed. A Re-Triangulation of Great Britain was concluded in the period post WW2 for which two Trig stations on the Island were used - Snaefell and South Barrule.

Reference

Barrie Corlson OS Tales PV Publications Stevenage 1997 ISBN 0-946779-48-1 is a highly amusing series of tales of "One Man's Part in the Re-Triangulation of Great Britain" - it being appropriate that his first tale is set on the Island - these measurements were taken at night using lights as the marks as when as signalling between stations using Morse code.