It was exactly 150 years ago that James Nicolson measured the highest mountain in the world, then known as Peak "B", later Peak XV and then Mt. Everest. Subsequent analysis of the data by the British India Survey determined its height as 29,002 feet (8840m) above sea level. This measurement was carried out in the years 1847-49 by vertical triangulation from six points at a distance of more than 150 km, without taking into account the deflection of the vertical produced by the Himalayan chain, the discrepancy between geoid and ellipsoid, and roughly estimating the refraction of the atmosphere.
About ten years later the topographers of the Survey of India discovered another group of very high mountains in the western part of the Himalayas. The highest of them turned out to be the one indicated as K2: 8611 metres.
Bradford Washburn’s excellent 1988 National Geographic map of the Mt. Everest region, produced in Switzerland using Leica photogrammetry equipment to analyse images made by Leica aerial photography systems, shows the peak of Mt. Everest with the decades-old official height of 8848m.
In 1987 the rumor that K2 might have been higher than Everest led to the creation of the EV-K2-CNR Committee by Professor Ardito Desio, the leader of the Italian mountaineering expedition who first climbed K2. The purpose was to remeasure both mountains and find out the truth.
The recent development of very accurate electronic distance meters and of satellite positioning systems has brought about a great increase in the accuracy of the measurements of the elevations and of the coordinates of the summits in the WGS84 system.
The 1992 remeasuring campaign of Mt. Everest
To verify the height of Mt. Everest with the most modern techniques, the Chinese National Bureau of Surveying and Mapping (NBSM) in Beijing offered to carry out the measurement from the Tibetan side, while the Italian National Research Council (CNR) agreed to organise the climbing expedition to carry the instruments necessary for the survey to the top of the mountain, with Italian topographers performing the measurement from the Nepali side.
The climbing expedition was arranged for September 1992. Since it was the first time that a GPS system would work at –40°C, a measurement from the ground was also arranged with distance meters and theodolites. A set of reflecting prisms was needed at the top of the mountain for the distance meters and as a target for the theodolites. The Leica ME5000 distance meter, the most precise instrument available on the market (0.2mm+0.2ppm) was used together with a Leica T3000 precision theodolite to match it. It was also decided to bring along a Leica DI3000 distance meter, slightly less accurate, but easier to use.
At a distance of 10-12 km three prisms were sufficient for a good measurement with both instruments. However two prism sets were needed, one facing the Nepali side and the other facing the Rongbuk Monastery in Tibet, where the Chinese surveyors led by Professor Jun-Yong Chen planned to make the measurement with distance meters and a Wild T2 theodolite. The horizontal angle was calculated on the map at 76° while both sets were tilted at 12° in the vertical plane. Leica Geosystems, Switzerland, built the tripod on which the prisms were set, meeting very strict specifications. The total weight had to be less than 10 kg and it had to be divisible into two parts. The structure was built of aluminium, except for the lowest part which was an ice piton made of stainless steel. Another piton was made to hold the GPS antenna.
The beacon at the peak of Mt. Everest was targeted from six sites in Tibet and Nepal.
The data recorded during the Mt. Everest campaign
Two climbers, Benoit Chamoux and Oswald Santin, reached the summit on the 29th of September 1992 at 10.30, read the temperature (-15°C) and activated the Leica GPS 200 system, which had already been positioned close to the peak the previous day, and had spent the night at -30°C on the roof of the world. At the same time, four other Leica GPS 200 sets were started: two in the valley of the Khumbu in Nepal and two at the end of the Rongbuk glacier in Tibet. They then assembled the tripod and oriented the prisms. When we saw that the red spot of the laser beam reflected from the summit, we realised that our measurement had a chance of success after all. The climbers remained on the summit for two hours, until we were sure that our two stations on the Nepali side and the Chinese on the northern slopes had received a good signal with the distance meters. This surveying campaign integrated all the most modern techniques of our decade:
a) Distance and angle measurements
The summit was observed from three points in Nepal and three in Tibet. Within each group of points all possible angles and distances were measured, yielding two trigonometric networks with one common target – the top of Everest. The geodetic network on the Nepali side was measured as a base triangle (KNL) from the corners of which the top of Everest (E) has been aimed at by the Italian team. On the Tibetan side the Chinese surveyors observed the summit from the three points R, III7 and W1. Both geodetic networks had one common target point: the top of Everest.
b) GPS measurements
The GPS Leica System 200 installed on the summit recorded every 2 seconds for 54 minutes. On the Nepali side the GPS installed at the Kala Pattar and at the point G near the Pyramid recorded at the same 2-second time interval. On the Tibetan side the recordings were performed every 15 seconds. All surveying data (triangulation/distance measurement and GPS) have been separately processed in China and in Italy. The results were presented at a meeting at the beginning of April 1993, taking into consideration also the results of the following measurements and technologies:
c) Meteorological data
For an accurate determination of the index and coefficient of refraction affecting the measurements of the theodolites and of the distance meters a special sensor was built capable of measuring the temperature and pressure of the air at the summit and sending this information to a receiver and recorder in the valley. During the measurements performed towards the summit the vertical temperature gradient was provided by sounding balloons launched from two positions. Temperature, humidity and pressure were recorded every 15 seconds.
d) Deviations of the vertical
Prof. Alessandro Caporali (Padua University) measured the deflections at four points between Lukla and Everest Base Camp. The astronomical coordinates were determined with a Leica T1600 theodolite connected to a Time Digitizing Unit made by the ETH (Zurich, Switzerland) and the geodetic coordinates with a GPS receiver (Caporali 1992).
e) Doris station
There is a station of the DORIS (Doppler Orbitography and Radiopositioning Integrated by Satellite) system to which the G point has been linked by surveying.
f) Depth of snow
On the 30th September 1992 two more climbers reached the summit. Their task was to determine the depth of the snow. This was accomplished by piercing the snow cap in several places near the tripod and referring the measurement to the main pole of the tripod where a millimetre scale had been drawn. The depth of the snow was determined as 2.55 metres.
Great importance has been given to the comparison between ground and GPS data by careful determination of the relative position between the prisms and the GPS antenna with a centimetre accuracy. Therefore not only a tape measure, a level and a compass were included in the GPS container and the alpinists were trained to use them, but also the photogrammetric method was applied. Two of the pictures taken by the mountaineers on the summit allowed the 3-dimensional reconstruction of the summit of the mountain. All data resulted in the elevation of the summit of Mt. Everest above the WGS84 ellipsoid to be 8823.51 metres with reference to the snow surface. The depth of the snow has been measured as 2.55 m and should be subtracted to achieve the value of the elevation at the actual rock top.frames.ioMain.
The Mt.Everest elevation
From the Chinese side a levelling network was brought from the Yellow Sea to the R and III7 points. In 1974 the geoid undulations were also computed with gravity observations performed up to 7900 metres (J. Y. Chen and D. S. Gum, 1980). The amount of the separation between ellipsoid and geoid was computed to be 25.14 metres. The value of the elevation of Mt. Everest referred to the Chinese High Datum has been obtained by averaging GPS and terrestrial data.
For the first time in history a GPS receiver has been operating on the summit of Mt. Everest giving a new determination of the coordinates of the summit and the value of its elevation above the WGS84 ellipsoid. This survey of the century, performed simultaneously from the Tibetan and Nepalese sides by Chinese, Italian, Nepalese and French researchers and mountaineers was also the first to determine the height of Mt. Everest geodetically using laser distance meters and theodolites in tandem.
Results for Mt. Everest from GPS and terrestrial data in 1992 measuring campaign
Everest ellipsoidal height 8823.51 (at snow level)
Ellipsoid-Geoid separation 25.14
Depth of snow -2.55
Geoidal Height 8846.10 ± 0.35m a.s.l.
Giorgio Poretti, University of Trieste