English: Line of sight to Plynlimon A 'surveyors eye view' from the secondary trig point at Banc y Garn, looking towards the primary triangulation station on Plynlimon which is some 5½ miles away. When carrying out triangulation work, many observations were made at night with a light illuminating the target station. From 1957, the micro-wave radio distance measuring "Tellurometer" was introduced for recording distances between stations. At this location, the surrounding forestry doesn't quite obscure the view of Plynlimon but it was often the case that no line of sight was available from ground stations because of the growth of forestry or because of other obstructions. In this situation, portable steel towers were constructed over the ground station. Observations from towers had their own problems. The towers flexed in even moderate winds so observations could not be carried out in windy conditions. In hot weather, the steel expanded and this could affect the observed results so the work was done as quickly as possible to minimise errors.