Plug

A plug is the removable centre of a spider. The circumference is normally inscribed 'ORDNANCE SURVEY TRIANGULATION STATION' but the lettering may be absent for 'Special Projects' trigs (for example, the Project Emily pillar at Driffield East. The style of the lettering varies; in some cases it is raised from the plug surface whilst in others, it is indented.

The plug is made of brass and consists of two parts. The whole plug can be removed with a special tool to allow a theodolite to be accurately plumbed over the upper centre mark at the base of the pillar. Actual observations were taken from the centre of the spider. A fairly straightforward calculation called 'reduction to centre' was used to correct the observations so that they were true as if taken over the mark.

The centre portion of the plug could be inverted, revealing grub screws and a centrally drilled hole. It could be removed, disclosing a screw thread about 1.5 inches in diameter. This allowed a beacon to be mounted over the approximate location of the mark.

It could also used to mount a target called a 'Broomfield Pole'. This screwed into the centre hole in the plug while mounted on the pillar. Broomfield Poles were named after the OS surveyor who first used them. They had brass ends and were made of metal and were heavy. They were often guyed in place and could be used in multiples if extra height was needed. An example of a brass end may be seen at Beinn Dearg.

Broomfield poles were replaced by 'Fenner Poles' in the 1970s. These were very similar except they were made of plastic and very light. They were designed by Bob Fenner, Chief Surveyor Geodetic Surveys at that time.

Occasionally, a pillar may be encountered where the plug has been reinstated with the centre portion still inverted, for example at Dimlington.

The brass centre plugs were frequently stolen so OS used light plastic plugs instead. These do not have the centre screw thread. They were sacrificial – if removed they were replaced with new. Some had a rough surface to mimic concrete – some were smooth. ^[1] In other cases, the original plug has been replaced by material variously described as tarmac, asphalt, black concrete or resin. Elsewhere, the plug may be missing with the centre tube exposed to the weather or filled with debris.