In all the current maps, Ben Nevis – Britain’s highest peak – is listed as 1,344m tall. But it’s actually 1,345m. That was the discovery of Ordnance Survey geodetic consultant Mark Greaves, who made a new, highly accurate GPS measurement in the autumn last year.
In fact, the actual difference in result was not a whole metre, but enough for the height to be rounded up from 1,344.527m to 1,345m.
Greaves explains that two hours of tracking a total of 12 navigation satellites was needed to make the measurement. In that time, half a million bits of information from the satellites were received and recorded by a GPS device on top of Ben Nevis. This information contained time data and data on the wavelength of the signals, but even though those signals travel at the speed of light, it took a few microseconds to get from the satellites to Earth.
“Because the code from the satellite has to travel 20,000km, it’s delayed in time,” explains Greaves. “So the receiver has a code which is offset.”