Measurement
Mount Everest north face from Rongbuk in Tibet
Another aerial view of Mount Everest from the south, with Lhotse in front and
Nuptse on the left In 1856, Andrew Waugh announced Everest (then known as Peak
XV) as 29,002 feet (8,840 m) high, after several years of calculations based
on observations made by the Great Trigonometric Grid Survey.
More recently, the mountain has been found to be 8,848 metres (29,029 ft) high, although there is some variation in the measurements. On May 22, 2005, the People's Republic of China's Everest Expedition Team ascended to the top of the mountain. After several months' measurement and calculation, on October 9, 2005, the PRC's State Bureau of Surveying and Mapping officially announced the height of Everest as 8,844.43 m ± 0.21 m (29,017.16 ± 0.69 ft). They claimed it was the most accurate measurement to date. This height is based on the actual highest point of rock and not on the snow and ice covering it. The Chinese team also measured a snow/ice depth of 3.5 m,which is in agreement with a net elevation of 8,848 m. The snow and ice thickness varies over time, making a definitive height of the snow cap impossible to determine.
The elevation of 8,848 m (29,029 ft) was first determined by an Indian survey in 1955, made closer to the mountain, also using theodolites. It was subsequently reaffirmed by a 1975 Chinese measurement. In both cases the snow cap, not the rock head, was measured. In May 1999 an American Everest Expedition, directed by Bradford Washburn, anchored a GPS unit into the highest bedrock. A rock head elevation of 8,850 m (29,035 ft), and a snow/ice elevation 1 m (3 ft) higher, were obtained via this device. Although it has not been officially recognized by Nepal, this figure is widely quoted. Geoid uncertainty casts doubt upon the accuracy claimed by both the 1999 and 2005 surveys.
A detailed photogrammetric map (at a scale of 1:50,000) of the Khumbu region, including the south side of Mount Everest, was made by Erwin Schneider as part of the 1955 International Himalayan Expedition, which also attempted Lhotse. An even more detailed topographic map of the Everest area was made in the late 1980s under the direction of Bradford Washburn, using extensive aerial photography.
It is thought that the plate tectonics of the area are adding to the height and moving the summit north-eastwards. Two accounts suggest the rates of change are 4 mm (0.16 in) per year (upwards) and 3-6 mm (0.12-0.25 in) per year (northeastwards), but another account mentions more lateral movement (27 mm/1.1 in), and even shrinkage has been suggested.
The Mount Everest region, and the Himalayas in general, are thought to be experiencing ice-melt due to global warming.