Measuring England's highest peak
Using precise GNSS to measure the highest mountain in England
Many keen mountaineers in Great Britain are familiar with the famous “Three Peaks” – the highest in Scotland, Wales and England:
1. Ben Nevis
2. Snowdon
3. Scafell Pike
In recent times, the summits of Snowdon and Ben Nevis have been re-measured by Ordnance Survey Great Britain (OS) to validate their heights using the latest techniques.
With the 80th anniversary of the triangulation of Great Britain in 2016, the time was right to re-measure the height of the remaining one; Scafell Pike, Cumbria – England’s highest peak.
In early 2016, the re-measurement of Great Britain’s highest peak, Ben Nevis in Scotland revealed with great enthusiasm that Ben Nevis was indeed higher than was previously thought. The published height increased from 1,344m to 1,345m after rounding-up.
Reaching for the stars
As part of the ITV series Countrywise, Ordnance Survey was approached by the show to help presenter Ben Fogle re-measure the height of Scafell Pike. Ordnance Survey turned to Leica Geosystems’ professional equipment and solutions to help deliver this exciting project. The new self-learning Leica GS16 GNSS smart receiver was deemed the best tool for the job.
Scafell Pike’s height published on the Ordnance Survey maps was derived from measurements last taken in the 1937 during the triangulation of Great Britain. The current height is rounded to 978 metres. A re-measuring of the mountain was certainly needed.
The aim of the project was to re-measure the mountain using the latest GNSS technology. As well as giving an accurate measurement of the true height of England’s highest peak, the undertaking also validated the previous measurements taken about 79 years ago.
Fogle and outside broadcast team set off on the trail from Wasdale Head to Scafell Pike with a team of surveyors from Ordnance Survey. They carried with them the latest Leica Geosystems GNSS technology, a newly released Leica GS16 smart receiver, Leica CS35 rugged tablet computer and a level and staff. This compact lightweight solution was a far cry from the theodolites carried in the original survey all those years ago.
Due to the challenging remote location of the mountain top and the absence of a mobile phone signal, it was not possible to use Network RTK corrections. Nor was it possible to attain a base and rover RTK solution via radio due to the distance from known points in the valley and obscuring terrain. For the highest level of accuracy, post-processing of a static session would definitively calculate the new height. By using the Leica GS16 receiver, however, it was also possible to calculate the new height to within a few centimetres whilst at the summit of the mountain by using the SmartLink service provided by Leica Geosystems for a quick value.
What is SmartLink?
SmartLink is a Precise Point Positioning (PPP) service that enables the GNSS user to achieve real-time centimetre accurate positioning worldwide, ideal when working in remote areas around the globe. SmartLink does not require a local RTK base station or network RTK infrastructure to operate meaning lower capital investments in equipment and the ease of simply visiting the site with a SmartLink compatible receiver is the only requisite.
How does SmartLink work?
SmartLink is based on the tried and tested PPP technique, originally developed in the early 1990s for off-shore positioning such as in the oil and gas industries where operations are usually remote – consequently away from reference stations and communications links. SmartLink includes a global network of GNSS reference stations, communications links and diverse network control centres, all with redundancy for guaranteed availability of service for critical operations.
SmartLink corrects or models GNSS error sources such as ionospheric and tropospheric delays and satellite orbits and clocks. Correction signals are then computed and uplinked to a series of seven geostationary satellites and broadcast to the compatible GNSS receiver, such as the Leica GS16 used on Scafell Pike. By delivering GNSS corrections via satellite, the common weakness of network RTK – the mobile phone network - is overcome.
Leica Geosystems’ continual innovation has recently brought the benefits of using SmartLink to the land survey community. A positive side-effect of using SmartLink is particularly of interest to regular RTK users should the RTK Data Link be interrupted, such as due to mobile network busy or terrain obscuring radio links, then SmartLink seamlessly bridges the gap to maintain uninterrupted centimetre positioning eliminating the frustration of downtime awaiting data links to be restored.
Reaching the peak of performance
At the summit, the Leica GS16 was mounted on the summit trig pillar to observe satellite signals and calculate the height of the trig pillar. The versatility of the Leica GS16 also allowed the raw observations to be recorded for definitive static post-processing back at the office.
The Bernese scientific software was used for this ratification of SmartLink using precise ephemeris data, the OS Net v2009 coordinate reference frame and OSTN15/ OSGM15 transformation and geoid model. Interestingly, and to prove the quality of real-time SmartLink obtained height, the post-processing calculated height differed by only 2-4 cm.
OS Head of Multi-Media, Andy Steggall, who took part in the expedition says, “it was remarkable that the OS surveyors of yesteryear were so accurate. What was such a major physical effort then has been replaced with the precision accuracy provided by state of the art equipment now. But it shows that the tradition of dedication and commitment to getting it right burns brightly. We’re sure the story will make compelling TV. We can reveal that the new height calculated for the highest point in England is 978.072 metres Above Ordnance Datum (AOD) in accordance with the current geoid model, OSGM15.”
