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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].

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

Overgate Croft Farm - a standard spider and pillar

In addition, a spider may occur as part of non-standard pillars, 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 [2].

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. https://www.ordnancesurvey.co.uk/legacy/docs/gps/CompleteTrigArchive.zip