Mount Everest, Measurement of
Mount Everest, Measurement of
A process began around 1800 that would ultimately establish Mount Everest as the world's tallest mountain. Started by Englishman William Lambton, this process was referred to as "The Great Trigonometrical Survey," and led to a rigorous mapping of India.
In 1796, Lambton was posted to India as a British lieutenant. Lambton's arrival coincided with increasing subcontinent colonization, and maps and surveys of these British-conquered territories were of great interest. Lambton proposed a more exacting survey than any attempted before in Asia. The resulting measurements would yield, for example, more accurate values for India's width. Lambton believed that the most important outcome would be a better understanding of Earth's geodetic shape.
Early Mapping of the Earth
Fifty years before Lambton's proposed survey, French scientists determined that Earth is better described as an oblate spheroid instead of a sphere. This meant that the distance from Earth's center to the equator is greater than the distance from its center to either pole. Lambton's survey, renamed "The Great Trigonometrical Survey of India," would help calculate the amount of this equatorial bulge , and thereby result in a better model of Earth's shape. As an unintended by-product, a precise height of the Himalayan Mountains would also be sought.
Points on Earth are given a measurement of latitude and longitude . The longitudinal line passing through Greenwich, England, is called the Prime Meridian , and all lines of longitude are measured from it. Lambton's survey centered on the longitudinal line 78 degrees east from the prime meridian and running from India's southern tip to the Himalayan foothills. This arc-of-longitude became known as "The Great Arc." From the central hub of the Great Arc, an accurate survey of all of India could be performed.
Trigonometry in Mapping
The survey Lambton headed from around 1800 until his death in 1823 ran more than 1,000 miles northwards from India's southern coast. The survey consisted of a web of triangles whose vertices were located precisely.
The basic idea behind traditional surveying is to use trigonometry (the mathematics of triangles) and measuring devices to locate exactly "new" points on Earth from points whose locations are already accurately known. A right triangle is used to determine the location of new point C relative to a known point A, as shown in the above illustration. There are six measurements for any triangle: three angles and three lengths of the sides. Given the lengths of two of three sides and one angular measurement, or given two angles and one side, any triangle is uniquely determined. Since the triangle is a right triangle, angle ABC is 90 degrees. Using a device called a transit , angle CAB can be measured. The length of side AC is now measured by using, for example, a chain. The length of sides AB and BC can now be computed. AB is the lateral distance from A to C, while length BC is the elevation of point C above point A. Point C can now be used as the starting point for a new triangle, and the procedure is repeated.
This method is a much-simplified version of what took place in the Great Survey. For example, the orientation and length of a baseline had to be determined laboriously. Lambton's first baseline near the eastern Indian coast measured approximately 7.5 miles, and it was measured using a specially constructed 100-foot iron chain. In terrible subtropical heat and humidity, the first baseline took 57 days to complete. Lambton then turned west and plunged into the Indian jungle. Because of the jungle canopy, large numbers of trees had to be cut down so that towers could be built; people standing on the towers could then make the necessary angular measurements.
Into this scene, British Lieutenant George Everest* arrived in 1819. Everest joined the Great Survey as one of many engineers reporting to Lambton. For the next several years, Everest worked in terrible jungle conditions of heat, humidity, and monsoon. Eventually, he collapsed from malaria and fever, and he left India in 1822, but he returned to his duties to become head of the survey after Lambton's death. Everest developed many new and innovative techniques, and despite poor health and the terrible climate, he pushed the Great Arc to the Himalayan foothills in northern India by the early 1840s. In 1843, Everest retired from his duties and returned to England.
*Unlike the pronunciation used today, George Everest pronounced his name "Eve-rest," like "evening."
Measuring Mountain Heights
Everest never attempted to measure any of the heights of the Himalayan range. However, two of his subordinates, Andrew Waugh and John Armstrong, made measurements from the Himalayan foothills. Many of the apparently loftiest mountains lay north of India within Nepal or Tibet. Nepal had closed its borders to foreigners, so the team could only estimate mountain distances. Nevertheless, with estimated distances and several angular measurements, the surveyors computed heights for various mountain peaks. From the present-day town of Darjeeling, Waugh measured a height for "Kangchenjunga," which is now known to be the world's third tallest mountain.* Waugh's measured height of 28,176 feet is within seven feet of today's accepted value. In 1847 both Waugh and Armstrong, from different locations, took measurements of a mountain suspected to be even taller than Kangchenjunga. Since no local name could be determined, it was simply listed in survey records as "Himalaya Peak XV."
*The most common translation of Kangchenjunga is "Five Treasuries of the Great Snow," from the five high peaks that rise from its surrounding glaciers.
Since Peak XV measurements were taken from a great distance away, terrestrial refraction (the bending of light in Earth's atmosphere) could have had a profound effect on any angles measured. Mathematical constants called "coefficients of refraction " had to be included in the elevation computations to correct for this phenomenon. By 1856, armed with better coefficients of refraction and with more accurate angular measurements, Waugh communicated his finding that Peak XV was computed at 29,002 feet above sea level. Moreover, Waugh recommended that this mountain be officially named Mount Everest to honor Everest's important role in the Great Survey.
In 1953, about a century after Mount Everest's height was first clearly ascertained, Edmund Hillary and Tenzing Norgay became the first to reach its summit. Since that time, mountaineers have placed various devices on Mount Everest to more accurately determine its height. For example, in 1992, an American expedition placed a reflector atop the mountain to bounce laser light off its surface. This device led to a measurement of 29,031 feet. However, like earlier measurements, this one included the snowcap's depth. Since the snowcap can vary, it would be advantageous to determine Everest's height minus this layer, estimated at between 30 and 60 feet. It has been proposed that a future expedition use ground penetrating radar to find the snow pack depth and thereby determine the height of Everest's rocky apex.
The latest surveying method used to measure Mount Everest's elevation makes use of the Global Positioning System (GPS). GPS uses satellite signals to determine the coordinates of points on Earth's surface. The National Geographic Society announced in November 1999 a revised height of 29,035 feet (using GPS) for Mount Everest, but that measurement, as others before it, includes the ice and snow layers.
see also Angles of Elevation and Depression; Angles, Measurement of; Global Positioning System.
Philip Edward Koth (with
William Arthur Atkins)
Keay, John. "The Great Arc: The Dramatic Tale of How India Was Mapped and Everest Was Named." New York: Harper Collins Publishers, 2000.
"8,848 m, The Height of Mt Everest." Chiba University web site (pictures, history and firsthand account from climber of Mt. Everest). <http://www.m.chiba-u.ac.jp/class/respir/hyoko_e.htm>.
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