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By Dennis Collin 

The Ordnance Survey is employing new GPS technology to reassess the summit height values of British mountains and hills. Many of which were measured many years ago with older and bulkier equipment from the 1940's no less.

Ben Nevis, Britain's highest mountain has 'gained a metre' giving it a height of 1345, with all new OS maps will reflect this new surveyed value.

The Ordnance Landranger series of maps at1:50,000 scale makes it popular for walkers and climbers                                                               

The official measuring point on Ben Nevis is one of those squat concrete (Trig/trigonometric) pillars familiar to many cross country walkers.

Recently the cairn and associated trig pillar was repaired and the opportunity was taken to verify the mountains exact height with newer equipment.

Since the last time was done more than 60 years ago, it is not surprising that values have changed and comparing results showed the change in height. Back in 1949 it took three weeks to complete the job after hauling heavy gear up Ben Nevis. These days doing a similar task can take just hours.

In those days, theodolites positioned on the "trig pillars" could be used to work out elevations by deriving angles from a system of national benchmarks. Often having to be strongly secured from wind with strong cord. This was quite a lengthy process, not only was the kit older and heavier, but reading the theodolite and its gauges along with horizontal and vertical correction bubbles made reading these theodolites quite a skilled job.

When re-measuring Nevis using modern GPS equipment which is now has an accuracy of better than a centimetre. A height of 1344.527m was obtained.

Whilst not being vastly different from the 1949 measurement the improvement in technology and the more reliable satellite measurement is possibly only a few measured centimetres higher. But of course the fractional metre measurement means that when rounded up .the actual height is set to the nearest metre.

The upward changing values is also a consequence of the Earth's crust rebounding after the loss of the massive ice sheet that used to sit on top of the region thousands of years ago. Known as Isostatic Rebound, it is, though, a very slow process, on the order of a millimetre per year, and not normally something the Ordnance Survey has to worry too much about, but, over time of course this slight change may influence the 'rounding' of said measurements which will result in the changes of a few heights of Britain’s peaks.

In this case the change would accrue and could account for 50 millimetres or so gain in height, which makes all the difference when accounting rounding height measurement.