Trig Points, more correctly known as triangulation pillars, or trigonometrical stations (and also by other similar terms) were erected by the Ordnance Survey between 1936 and early 1960s as part of a hugely ambitious plan to re-triangulate the whole of the British Isles in fine detail - a process which means that today we as UK geocachers benefit from arguably the best mapping in the world. A fuller description is beyond the scope of this page, but this excellent website goes into much more detail.
Many cachers are aware of the excellent Ye Ole Survey Monuments cache, based on Trig Points throughout the British Isles. This is an extremely rare moveable Virtual cache, and has a reputation as a bit of a marmite cache – some cachers love logging YSMs, others don’t approve at all! The cache is loggable multiple times, on the same GC number, but individual logs count towards the finder’s statistics as each YSM has its own individual number.
UPDATE Sadly as of May 2017 GroundSpeak have decided to archive the GC45CC YOSM Virtual cache, though the above link still works, and this cache is still viable.
This cache is based on YSMs, some of which have in the past visited various local Trig Points, including Beacon Hill for Geolympix 2016 - why not climb to the trig point and log GC45CC Beacon Hill while you're doing this cache?1. But you don't need to have visited the Trig Points, or logged the YSMs, to solve this cache. Calculate the equation in the attached image and get a hit in the checker below for further details:
1 The Beacon Hill version will not be available until Geolympix 2016 week.