The construction of a triangulation pillar is more complicated than might at first be supposed. The location of the station is actually defined by a Lower Centre Mark, fixed into the bedrock. This is isolated from the rest of the pillar by means of a sacrificial wooden cover. The surveyed mark is the Upper Centre Mark, which is set exactly above the Lower Centre Mark during construction (or during reconstruction of toppled pillars). This Centre Mark can be lit by torches inserted into the sighting tubes. A Theodolite mounted on the Spider can be plumbed over this Upper Centre Mark. The Centre Pipes were initially made of steel tubing, on the assumption that they would support permanent metal beacons. However such beacons were not used, so later pillars used a lighter cardboard tube, just strong enough to support the concrete while it set.