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About This Site

Created in November 2006 to provide a service to Trigpointers in Ireland. Regestired Users can upload condition reports and photographs.
In February 2008, support was added by request of a number of users, for OS items other than TPs.

OS Item Types

PTPPrimary Triangulation Pillar
TPTriangulation Pillar
NPFBNon PillarFlush Bracket
BMBench Mark other than a FB
PGPSPassive GPS Station
BTBase Tower

What is a Trigpoint?

It is usually a hand-cast concrete pillar, 4 feet high and 2 feet square at the base, tapering towards a flat top. It is part of a network of surveying stations that were used by Ordance Survey for mapping.
The formal term is triagulation pillar, but this is usually shortened to trig pillar or trig point. On this site this is abreviated even further to TP.

TPs usually have a 'spider' or 'top plate' used to fix a theodolite or other ordnance surveying device to. Many have a flush Bracket fixed to the side that has an identifying number on it. There are different designs, as you will discover as you read condition reports and see the photographs that these contain.

Below left is a spider and at the right, one type of flush bracket. It is attached to the Triangulation Pillar at Ballycultra, located at J 4199 7932.


What is Trigpointing?

Some walkers 'bag' summits over a chosen height, some seek 'treasure' using GPS equipment (Geocaching), others are content to simply roam the countryside, savouring its sights, scents and sounds, but I fell in love with Triangulation Pillars.

Trigpoints are found dotted about the countryside in a variety of landscapes. Many are on remote summits and are eminently suitable as destinations for the ardent hill walker. Some are placed on less lofty perches and could easily be included in a leisurely ramble. By their very nature, being originally used by Ordnance Survey for map making, they usually command a view of the landscape over which they preside.

I say usually, because OS no longer maintains or uses them and they are slowly falling into disrepair. Some have been overtaken by forestry plantations, transmitting mast installations or have disappeared all together. On the other hand, many are still as good as new and overlook magnificent scenery. That, perhaps, is why I visit them. As I approach their reported location I become excited, eager to discover how this one has stood the test of time.

Trigpointing is actually quite a popular hobby, perhaps more so in GB, but there is now a growing group of Irish Trigpointers. It requires no special equipment and can easily be combined with the sort of walking you u sually do. Trigpointing Ireland is a community site, founded just over a year ago. Registered users contribute to the content by logging condition reports on the TPs they find. Why not add a small notebook to your pack and join in. It is great fun and adds purpose to your walks.

Take a look at some reports on this site to see what is required. Photographs are optional and an accurate GPS location is a luxury. However you should be aware that quoted grid references are only to the nearest 100 meters and some TPs are now well hidden. See the report for Berk Hill.

The History of Ordnance Survey in Ireland

The Irish Ordnance Survey, under Lt-Col Thomas Colby, completed the world’s first large-scale mapping of an entire country by 1846. The survey, at a scale of 6 inches to one mile, was primarily a townland survey and not detailed enough for a proper valuation, so a more comprehensive survey was completed. By 1867, from Fair Head to Mizen Head every road, track, hedge, fence and stone wall, every river and stream, every house and barn had been surveyed so that Sir Richard Griffith and his team could plot their valuations.

Surveying an entire country was a long and complicated task. Firstly Colby and his team had to build a framework of trigonometrical stations upon which the mapping could be based. They are usually hand-cast in concrete, 4 feet high and 2 feet square at the base, tapering towards the top. The formal term for these columns is triagulation pillar, but this is usually shortened to trig pillar or trig point. Trig points are situated so that at least two other points can be seen from each station.

The mapping was done by triangulation and by creating a series of primary triangles. Sightings were taken between stations using theodolites. Much of this work was done at night to reduce the distortion caused by heated air. With some of the sightings over 150kms, Colby needed more powerful light than that provided by the Argand Lamps he was using. Colby’s assistant, Thomas Drummond, who displayed unique talents as an inventor and mathematician, invented the Limelight. Drummond was also instrumental in improving the heliostat reflector which was used for daylight observations.

In order to complete the network of triangles, the length of one leg of one triangle had to be measured. The leg chosen was along the shores of Lough Foyle and this measured distance is known as the Baseline. The accuracy of the measurement of the Baseline was paramount as it set the scale for the whole survey. Colby, not satisfied with previous methods of measuring baselines, asked Drummond to develop a method based on the compensation principle, using parallel bars of two different metals (iron and brass). Drummond’s design allowed the bars to be connected in the middle with steel tongues fixed across the ends, leaving them free to shift according to the expansion or contraction of each bar and allow the coefficient of expansion of metal to be used in the calculation of distance.

Measurement of the baseline began in 1827 and was carried out under tenting to avoid fluctuations in temperature. The bars were mounted on tripods and the total distance of 7.89 miles, which included crossing the River Roe, was completed in November 1828 after 60 days of measurement by 70 men. The accuracy achieved is still marvelled at today.

Based on an item at http://www.osni.gov.uk/index/aboutus/history.htm
with additional information from various sources.