Is 12 inches really so important?

Is Yr Wyddfa (Snowdon) 3,560 ft or only 3,559 ft?

Most sources will tell you that the height of Yr Wyddfa (Snowdon) is 1,085 m, or 3,560 ft.

But just how correct is the figure of 3,560 ft?  Let’s go back a few hundred years …

In 1682, the height was first calculated at 3,720 ft  (1,130 m), a figure arrived at using basic trigonometry. Then in 1697, Edmund Halley (of comet fame) used barometric pressure to calculate a height of 3,580ft, a remarkably close figure.

In July 1777, Major-General William Roy, a Scottish military engineer and surveyor, whose work led to the creation of the O.S., conducted both a trigonometrical measurement and a barometrical measurement of Yr Wyddfa. The former gave a height of 3,555.4 ft, the latter a height of 3,548.9 ft – both remarkably close to the true figure.

An official survey of 1842 by a team from the Royal Engineers established a height of 3,571 ft, then a later survey in 1886, again by the military and using the traditional method of triangulation, established the height at 3,560 ft. This figure was adopted and published on the O.S. First Edition map of 6 inches to the mile, published in 1889, and for most of the 20th century the O.S. map showed this height.

A photo of the 1886 survey, which was carried out over several weeks. The wooden frame, which was tied down with rope, held a lamp and light-reflecting plates.

However, the last 1:25,000 O.S. map to be published in imperial measurements (i.e. showing contours and heights in feet) was issued in 1982, and by now showed Yr Wyddfa‘s height as 3,559 ft. The following edition, revised in 1983 and published in 1984, was metric and showed Yr Wyddfa‘s height as 1,085m, the figure we are familiar with today. (There was no re-surveying at this point; both 3,559 ft and 3,560 ft convert to 1,085 m to the nearest metre.)

In September 2014, Yr Wyddfa‘s height was surveyed using satellite technology, as part the TV programme The Mountain. The team streamed 3 hours’ worth of data one evening, with another 3 hours’ worth the following morning (two hours is the O.S. minimum standard) and it confirmed that the actual height of the mountain (calculated to the highest point of bedrock) was 1,084.87 m (1,085 m to the nearest metre)  and 3,559.3 ft (3,559 ft to the nearest foot).

So why do so many sources cite 3,560 ft as Yr Wyddfa‘s height when the last imperial O.S. map clearly showed it as only 3,559 ft, and subsequent re-measuring has confirmed this?

If you’re ever in a pub quiz, and a question relates to Yr Wyddfa‘s height in feet, the correct answer is almost certain to be given as 3,560ft … but we know differently, don’t we?

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