Many experienced hill walkers will inform you (and any one else who’ll listen) that “Trig Points” are always sited at the highest point on a hill. This is not true.
Triangulation Pillars are positioned so that they can be seen from other “Trig Points” (normally the highest point but not always, as an example the Ben Nevis Trig Point is not at the highest point on the mountain), the angle between them can be used to form a part of a triangle, or other geometric shape. This shape in turn is used by map makers and others to enable them to work out their exact location.
The graphics below show the summit Triangulation Points on each of the Four Peaks.

Ben Nevis,  highest mountain in Scotland (& Britain)

1,344m (4,408ft)

The route that we have chosen (there are many) is to follow the footpath up from the bridge by the Achintee car park, towards John’s Wall Corner, then over the 8 zig zags to the summit. We will return down the same path. The average time spent on Ben Nevis is 7 hours.

Scafell Pike, highest mountain in England 978m (3,209ft)

The walk up Scafell Pike starts from just outside (not in) the hamlet of Seatoller and follows the stream (Grains Gill) over Stockley Bridge, then up to the Great End massif. We follow the track up and through Calf Cove and then along the ridge to Scafell Pike. We return down the same track. The average time walking on this mountain is 7 hours. 

highest mountain in Wales 1,085m (3,560ft)

We will be taking the Llanberris footpath up past The Halfway House and the Monolith to the summit. We will return down the same path. Over the last few years the average time each walker has spent on Snowdon is 5 hours.