Spider

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Spider

A spider

A spider, or tribrach, is the brass mount, set into the top of triangulation pillars, for mounting theodolites. It consists of three grooved arms, set at 120°, in the middle of which is a removable central plug. As well as being used on pillars, spiders can also be found set into man-made structures such as sea walls. They were also used on Bilby Towers.