How to measure a mountain

The first recorded attempt to measure Everest by modern calculations came in 1856, when surveyors commissioned by the then Indian surveyor George Everest estimated its height at 29,002 feet. Without going into the sine and cosine of the craft, suffice it to say that by using accurate measurements of distance and multiple readings of angles from different positions, it is possible to use trigonometry to determine the height of any feature. A key instrument in this exercise is the theodolite, which helps measure angles. It’s similar to an instrument you may have observed that engineers use in road construction. As it turned out, the height determined by British Raj officials was quite close to that which Indian officials would compute in the 1950s with more modern instruments: 29,028 feet, or 8,848 meters. It is still the most cited figure on the height of Everest.

NOW, EXPERIENCES COME TO HIGHS BY GPS

The key tool used by Chinese and Nepali surveyors was a GPS receiver that was carried to the top of Everest. The time factor taken for signals to travel from the receiver to multiple satellites and the distance of those satellites from fixed objects provides a very accurate approximation of the location and altitude of the peak.

A member of a Chinese investigation team sets up investigation equipment on the summit of Mount Everest (Photo Xinhua by AP)

A member of a Chinese investigation team sets up investigation equipment on the summit of Mount Everest (Photo Xinhua by AP)

But although GPS and similar systems may have enabled large jumps by pinning heights, measuring a mountain depends a lot on several moving parts.

OF SEA LEVELS AND SNOW

GPS can tell you where the top is, but to find the height of a mountain, it’s just as critical to know where the bottom lies. Because mountains are measured as their elevation of sea level.

Global Navigation Satellite System receiver is seen attached to a tripod at the height of Mount Everest (Photo: AFP)

Global Navigation Satellite System receiver is seen attached to a tripod at the height of Mount Everest (Photo: AFP)

170 years ago, the Everest crew, led by mathematician Radhanath Sikdar, solved the problem of sea level by actually working up from the Bay of Bengal by employing a network of sight lines until Everest itself was visible and measurable by trigonometric formulas.

Another factor that proves controversial is whether to calculate the depth of the snow on a mountaintop as part of its height. Nepali surveyors used ground-penetrating radar to actually measure the height of the snow standing on the rocky ridge of Everest but in the past it has been debated whether the snow should be part of the calculation.

HOW THE SEA LEVELS

To accurately determine sea level by interpreting GPS data, scientists now rely on two models that conceptualize the shape of the Earth: an ellipsoid that presents the Earth’s surface as smooth and uniform and is used to measure geographic coordinates; and geoid: a model that considers gravity and how it affects sea levels and how the rotating Earth swells at the equator and is flat at the poles. The geoid model is a close substitute for average sea level. Calculations of the ellipsoidal and geoid elevations finally give the orthometric height, or the height above sea level.

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Moreover EVEREST AND GROW


Everest, like the rest of the Himalayan range, grows about half a centimeter annually as a result of flat tectonic collisions. The Indian plate pushing into the Eurasian plate over the last 40 to 50 million years is causing the peak to rise, albeit almost negligibly.

Source: Alex Copley, University of Cambridge

Source: Alex Copley, University of Cambridge

But earthquakes have the opposite effect – they can reduce the height of a peak. The 7.8-magnitude earthquake that struck Nepal in 2015 may have reduced the snowfall of Everest. Scientists have found that some other peaks near the epicenter have seen their elevations decrease by about a meter after the earthquake. An earthquake in 1934 was estimated to have cut the mountain by about two feet. Nepal’s Investigation Department said the 2015 disaster was a key reason behind measuring Everest’s altitude again.

Source: National Geographic, NOAA, Slate, media reports

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