Triangulation Pillar

Trig Point

Triangulation Stations or Triangulation Pillars or ‘trig points’ consist of either a concrete or natural stone pillar, 1.2 metres, high tapering towards the top. On the top a brass plate with three arms and a central depression is fixed to accept a survey instrument.  They were used by Ordnance Survey to accurately map the U.K. They were usually placed on top of the highest piece of ground available in order to give an unobstructed view from one pillar to another. A theodolite was placed on the top of the pillar and this was used to accurately calculate the distance to the next pillar using triangulation, hence the name of the pillar. The mapping project began in 1936 and gave the UK the accurate set of O.S. maps that we still use today. Modern satellites have made the pillars obsolete and their only use today is in the hobby of Trig Hunting. The standard trig point design is credited to Brigadier Martin Hotine (1898–1968), the then head of the Trigonometrical and Levelling Division of the Ordnance Survey.

The only accessible pillar at Stokes Bay is on top of the earth bank of Fort Gilkicker at the north east end with views overlooking the Solent and Fort Monckton.
It is designated as :
OSGB36 Station :SU31/T071
T:UK waypoint :TP3234

Trig Point
Triangulation Pillar SU31/T071 Gilkicker Fort
Pillar bench mark
Pillar bench mark S2709
Trig point
The mounting bracket for a theodolite on top of the Trig Point

The Trig point at Fort Gilkicker is a third order one. The next nearest ones are:
TP21083 Alverstoke Church Tower
TP21046 Browndown Battery
TP21020 On the spire of Holy Trinity Church.
TP3236 The keep of Fort Rowner
TP21021 The water tower of Haslar Hospital

Primary Trig Points
consist of the main set of around 378 pillars
Secondary Trig Points
were positioned to fill in the gaps between the primary ones.
3rd order Trig Points
filled in the gaps between the second order stations.
4th order
Fourth order stations filled in the gaps between the 3rd order stations.
13th Order (GPS)
The marks for GPS Stations, which were marks surveyed by GPS before the Passive Station Database was created, are sometimes referred to as 13th order.

O.S. Data on the Gilkicker Triangulation Pillar
O.S. Data on the Gilkicker Triangulation Pillar

A benchmark on the base of the Gilkicker pillar identifies itself as:
OS BM  S2709.
A bench mark was used to mark a point as an elevation reference.
The height of a benchmark is calculated relative to the heights of nearby benchmarks in a network extending from a fundamental benchmark. A fundamental benchmark is a point with a precisely known relationship to the level datum of the area, typically mean sea level. The position and height of each benchmark is shown on large-scale maps.
There are three much earlier bench marks, placed at various points along Stokes Bay by the Royal Engineers in  1860.
One is in the brickwork of the tunnel between Number 1 Battery and Number 2 Battery.
The second is on the brickwork of the bridge leading to Alverbank Hotel in Stanley Park.
The third is in the brickwork of an expense magazine in Number 5  Battery. There were probably more at Number three and four batteries, which are now lost.

Bench Mark 1
Bench Mark at No.1 Battery
Bench Mark at No.1 Battery
Bench Mark at No.1 Battery
Bench Mark at Alverbank
Bench Mark on the bridge over the Alver at Alverbank.
Bench Mark at Number 5 Battery
Bench Mark at Number 5 Battery
Bench Mark at Number 5 Battery
Bench Mark at Number 5 Battery

For a more detailed list with photo albums visit:
For more details of Triangulation Stations visit


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