Building a trig

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Building a trig

Postby ted » Thu Jun 25, 2020 11:43 am

This is an extract transcribed from page 122 of
'Ordnance Survey. Map Makers to Britain since 1791'
by Tim Owen and Elaine Pilbeam, published by the OS
in 1992.

The trig. on Cadair Idris

In 1936 the first of what would eventually
number over 6500 triangulation pillars began to
appear on high and exposed places around
Britain. One pillar with a chequered past is that
on Pen y gadair, the highest point on the ridge
known as Cadair (or Cader) Idris. The pillar, 6
miles south of Dolgellau, sits astride a cairn on
a big bluff mountain, typical of this area in the
Snowdonia National Park.

The site was originally selected in 1804, the
triangulation point being marked with a chisel-
cut arrow on a free-stone block. A cairn was
erected over it, both to pinpoint the mark and to
support a wooden pole used for the survey

In 1936 enquiries, to find the owner of the
land on which the cairn stood, were made in
preparation for the retriangulation of Great
Britain. A trig. pillar was to be built on the
summit and eventually four people gave their
permission, all of them claiming ownership.

Work commenced on Monday 10 August
1936 and the tale is best told in the the pillar
builder's own plaintive words:

Report on Construction of Triangulation pillar
at Cader Idris Aug 22nd, 1936

The above pillar was completed on Aug 22nd

Mon 10th I travelled here from
Treorchy, leaving Treorchy at 10.30am
arriving here at 10 pm. Nothing done to day
owning to late arrival.

Tues 11th Went to station to see about
O.S. stores, which did not arrive until late
afternoon, made arrangements with contractor
re material and removal of O.S. stores from

I inspected trig point and found only pack
horses could do the job.

Wed 12th Waiting for arrival of
materials and stores and interviewing farmers re
pack horses.

Nothing done to day on construction.

Thurs 13th I have now secured horses.
To day we managed to get one load to trig point
and then had to give up owing to rain and heavy
mists on top of Cader Idris. Before starting to
day we have had to make a road up the side of
the rocks. Today we got a horse stuck in a bog.
The mists on Cader are terrible every day, until
the afternoon's, and cannot attempt getting up
until the afternoon's.

Fri 14th This afternoon we managed
to get another load to the top of the mountain
3/4 of the way to trig point, but had to leave same
there owing to heavy mists and rain, we were
wet to the skin and returned to digs.

Sat 15th Weather here to day is terrible
and nothing can be done. One cannot see the

Mon 17th This morning we managed to
get another load to the trig point. It was a
terrible day, heavy rain and mists. Then we had
to give up owing to being wet to the skin. I
thought it would clear up but it kept on all day.

Tues 18th I went down 5 ft 6 ins and
found O.S.Box, I made a wall of stones 3 ft
square and covered O.S. Box with usual 11 x 2
wooden box. About 4 ft down I found a silver
coin, George the third 1804, a five shilling
piece. I filled in base to about 3 parts of the way
up and left same owing to nasty mists and rain
setting in.

Wed 19th Weather to day is terrible,
raining all day. Nothing can be done to day, fed
up with it.

Thurs 20th I left digs at 8 am for the
trig point. I have now constructed base 5 ft 8 ins
deep, 3 ft square. Have inserted O.S. point on
base in wooden box and also four supporting
irons. I returned to digs at 8 pm, tired and ready
for bed.

Fri 21st To day I inserted flush
bracket, put up shutterings, also centre pipe and
beacon pipe, also four sighting tubes and outer
tubes, inserted spider and three brass loops. I
am staying tonight at the top of Cader Idris in
an old hut for the night.

Sat 22nd 6 am. We took down
shuttering and cut wire off flush bracket,
removed centre pipe, filled sighting tubes and
faced pillar, cleared site and had O.S. stores
removed to farm. The above pillar had the new
centre cap. This has been a tough job, but
thank God it is up.

Aug 22 - 1936 PS: I have made a path
around the pillar with spare materials. This is a
bad time of year to get pack horses, owing to
hay making and they know it. The person who
did the haulage with the pack horses was a coal
merchant from Dolgelly.

Re Triangulation pillar at Cader Idris
Aug 22nd 1936

£ s. d.
pay 3 at 10/-
4 at 9/-
6 at 6/- 5 2 0
allowance at 7/6 4 2 6
Two Labourers
50 hours each 5 0 0
Pack Horse from farm to
trig point and men (3) 26 0 0
Haulage of material and
stores to farm etc 2 10 0
25 Cwt Chippings - 10 0
10 Cwt Sand - 5 6
7 Cwt Cement 1 8 0
Bus fares seeing about
stores and materials
cycle had not arrived,
also taxi on arrival at
station 10pm 4 6
£43 2 6

Despite the pillar builder's efforts, Captain
Hotine showed his displeasure with the work,
making the following comments on the report

CS [Chief Surveyor]
We should show (on the bill?) the number of
journeys and the distance or time. It is not
enough simply to put in a bill for £26 with
no details. This pillar costs more than Ben
Lochmond where horses had to be
transported by lorry.

The pillar was repaired at intervals but in
1983 it was found to have fallen over. In 1985
the remains of the old pillar and the cairn were
demolished to find the original 1804 mark.
Materials for a new pillar, weighing around 1 1/2
tonnes, were transported to the summit, by
helicopter this time, and the trig pillar was
finally restored to full operation on 21 June
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Re: Building a trig

Postby agentmancuso » Thu Jun 25, 2020 2:55 pm

Fabulous :)
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Re: Building a trig

Postby MAC.HAWK » Sat Jun 27, 2020 8:54 pm

Yes indeed, great stuff.
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Re: Building a trig

Postby Axe Edge » Sat Jun 27, 2020 9:14 pm

Excellent, a great insight to the work and conditions that had to be encountered. Thank you so much for sharing.
Axe Edge
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Re: Building a trig

Postby ted » Fri Jul 03, 2020 2:27 pm

It is thanks to the integrity of the pillar builders that
most of the pillars they constructed stand firm to this day,
having withstood the unwelcome attention of frost, subsidence,
lightning strike or the occasional energetic vandal.

Endless speculation surrounded the pillar's intended
purpose and the surveyors employed recorded some of the
more fanciful suggestions put forward by local people:

A neon light at the summit for the guidance of aircraft;
A sundial;
A searchlight or the mounting of an anti-aircraft gun;
An oil well;
In commemoration of the Jubilee.

In some places, such as the remote points in the
Highlands and Islands of Scotland, the pillars were
constructed as round columns, concrete being poured
into a large cardboard tube, for some reason called a 'Vanessa
tube'. These tubes were allowed to rot away or were stripped
off, and the pillar painted at the next visit when initial
observations would take place. There was an obvious saving in
time and effort here, as normal pillars were constructed with
wooden shuttering, which had to be retrieved by the pillar
builder before the observation party arrived.

Some of the original stations had been marked by the sappers
with a cairn, so that they could be easily located in the
future. Over the years, these sometimes acquired the status of
an 'ancient monument' and became an object of local veneration.
In one such case, Ordnance Survey was accused of
vandalism and removing cairns' which were 'relics of the past
ages' with a plea to 'leave the ancient monuments in their
original state'. There was a sudden close of the correspondence
when the history became clear.

Where new trig. pillars were needed at or on recognised
archaeological sites, Ordnance Survey was careful to seek
advice from antiquarian authorities and eminent archaeologists,
to ensure that any ancient remains were not disturbed. At
prominent beauty spots, all attempts were again made to
satisfy local interests. These including incorporating various
devices on the sides of the pillars, such as waymarkers,
plaques and collecting boxes, and incorporating topographs
(plates with directional information) on the pillar tops.

The ingenuity of the pillar constructors and surveyors
overcame most problems encountered and led to some
interesting innovations, such as the 'Curry stool'. Designed by
a surveyor, Mr Ernie Curry, this provided a stable triangulation
station and theodolite platform in soft peak bogs. Three
stout poles were driven at an angle, 120o apart, into the peat
until they hit firm ground (sometimes nearly 20 feet down),
and their protruding tops were covered and bound by a collar
of concrete. The result was a triangulation station that, in
diagrammatic form, looked like a vast milking stool thrust into
the peat. In the middle of the concrete collar, which was large
enough to take the outspread legs of the theodolite tripod, was
the trig bolt. The observer stood on boards, just above the
concrete, arranged so as not to disturb the theodolite.
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Re: Building a trig

Postby ted » Fri Jul 03, 2020 2:31 pm

In the first 1936 'hill season' the men were divided into
three parties. Each was led by an observer who had two
assistants, a booker (who could also observe) and a
lightkeeper-cum-general organizer. Detached from each party,
at the stations being observed to, were nine lightkeepers.
The lightkeepers were kept in touch with the observer's
progress by prearranged morse code signals, demanding
constant attention to the observer's 'leading light'.
Lightkeepers had charge of observing beacons which were
powered by 6 V car batteries; these had to be kept charged
if the lights were not to fail at a crucial moment. With one,
or even two, of these heavy batteries strapped to their backs
and often with only bicycles as transport, the lightkeepers would
travel miles to a battery-charging rendezvous or garage.

The parties were deliberately given areas well apart so
as not to interrupt each other's schemes, which also meant
that delays due to bad weather in one part of the country
might not affect progress in another. The men were indefatigable,
and achieved more than had been hoped for in the first
season. Altogether, 56 primary stations were occupied between
April and October, giving a broad framework of
triangles, most of them fully observed, extending up the
backbone of the country from the Isle of Wight to the Tyne.
Eleven of the stations observed duplicated those of the
Principal Triangulation of the 1850s, and effectively provided
continuity, as far as was possible, of geographical coordinates
on old and new maps.

The theodolites used throughout the retriangulation
were manufactured by Cooke, Troughton and Simms.
Known as Geodetic Tavistocks (or Tavis), they were named
after a conference held of 1926, at Tavistock in Devon. The
theodolites with their 5 1/2 inch horizontal circle (later
reduced to 5 inches) and weight of 32 pounds (14.5 kg) were
minuscule compared to the Ramsden 3-foot, the original
geodetic theodolite, in regular use 100 years earlier. The
Tavis proved very popular with the observers. They found
that, provided they applied what at first appeared trivial
precautions to their observing procedures, they received very
few requests for observations to be repeated.

The start of the 1937 season saw a change in the
working pattern, the plan being for three observing parties
to work their way up through Scotland, line abreast. The
lightkeepers, now in parties of one to three men, set up the
beacon lights so that one light was actually on the pillar, while
up to two others were placed eccentric from the pillar and on
direct line to their respective observing stations. This way one
station could, where necessary, be observed from three directions.

The 350-mile journey from Southampton and the
climbs to the first observing and lighting stations, including
setting up camp, took place in miserably wet conditions. But
the staff made light of the deprivations. As the observer at
Whitelyne Hill, in Northumbria, wrote 'beyond a wet bed, wet
clothes, wet everything, there is nothing to report'.

To the west, the lightkeeper at Criffel, a few miles south
of Dumfries, penned the following lament:

I'm fair foreichan speilan' up aul' Criffel ilka day,
Whilst puir aul' John S. Harrison near conked out on the way;
For luggin' batteries up a hill is no' exactly fun;
John swore the hale clanjamferie wad wey gey near a ton.

Ae nicht I spent upon that hill I got an unco fricht,
When I was gettin' spreauchled doon, the aul' mune dimmed her licht,
I waded through the heather an' tummelt owre a rock,
Then waunnert in the brecken an' got hame at fowre o'clock.

And this was one of the lower hills to climb! (John S. Harrison
was a local farmer.)

The work progressed well, except for the observations
at Ben Macdhui in the heart of the Cairngorms, which
stubbornly kept its top in the clouds, through the lower
mountains all around were clear. In June, a fourth observing
party was temporarily formed to occupy Ben Macdhui and,
with a lightkeeping party to bring food and fuel to them, they
camped on the snowy summit, until, after ten days, the clouds
had cleared long enough to allow them to complete the observations.
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