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A spider is a fixture set into the top of triangulation pillars, for mounting theodolites. It is described as follows:

Into the top of the pillar is set a brass fitting called a 'spider' incorporating three grooves 120° apart. The spider ensures that instruments can be automatically centred over the intersection of the three grooves when the feet of the tribrach of the instruments are placed in the grooves [1]. There is no deliberate orientation to the spider, apart from making it symmetrical to the top (i.e. one arm points to the centre of one side). [2]

In the middle of the spider is a removable central plug.

Overgate Croft Farm - a standard spider and pillar

A list is available on T:UK

A spider may occur as part of a non-standard pillar, for example: Louven Howe Aux 1

Louven Howe Aux 1 - non-pillar base

Occasionally, spiders can also be found set into other structures such as water towers, church towers and sea walls.

A spider set in a sea wall

They were also used on Bilby Towers.

Newcastle University - non-OS pillar and spider

15 instances of non-pillar spiders are recorded in the OSGB36 list [3].

Of these, 7 have been destroyed:

North Anston Water Tower
Worksop Water Tower
Cement Mixer
East Cowes
Sea Wall
Four Ashes Wireless Mast East

2 are possibly missing:


3 are unknown and/or inaccessible:

Berrington Church Tower
Honington Aerodrome,

3 are known to exist:

Lincoln Minster (end of spider just visible from the roof)
Pevensey Azimuth Mark
Sea Wall


  1. History of the Retriangulation of Great Britain
  2. email from Ian Wilson to the Trigonomy mailing list, 22/08/2009
  3. https://www.ordnancesurvey.co.uk/legacy/docs/gps/CompleteTrigArchive.zip