How did they measure the height of Mt. Everest?

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Methods to measure the height of Everest have changed over time, from simple geometry in the past, to GPS more recently. A story in the NY Times, “How Tall Is Mount Everest? For Nepal, It’s a Touchy Question.” discusses the controversy over different results for the height of Everest, and a planned exepdition to measure it from the summit.

Estimates of the height of Everest vary:

“Today, Everest’s height is widely recognized as 29,029 feet. But teams from around the world, including China, Denmark, Italy, India and the United States, have come up with other calculations, which have sometimes strayed a little bit higher, or a little bit lower, than that figure. Italy, in 1992, lopped seven feet off the standard height, measuring it at 29,022 feet. In 1999, a measurement by American scientists pushed the peak a little higher, saying the mountain reached 29,035 feet.”

And the height can change:

“Roger Bilham, a geologist at the University of Colorado Boulder, said Everest’s location in the zone of compression between southern Tibet and India means it sinks during earthquakes and rises in the period between them. A major earthquake in 1934 lowered the mountain by 63 centimeters, or about two feet, according to data provided by Mr. Bilham.

In the 19th century, the height of Everest was calculated by measuring the angles between the top of the mountain and points on the ground whose positions relative to the average height of the sea were already known.

Now, surveyors place a global positioning system receiver on the summit ice for an hour, and mathematically calculate the height of the sea from satellites and measurements of gravity at the base.

To prepare for the country’s own expedition, Nepali surveyors will collect measurements this month along the country’s southern plains, where they plan to calculate sea level. A team of Sherpas are also being trained to bring a GPS receiver to the summit. The cost to measure the mountain is estimated at $250,000.”