Explained: Why did Mount Everest’s height change?

Context: China and Nepal agreed this week on a new standard height for Mount Everest, the rugged Himalayan peak that straddles their border.

• They jointly presented a new official figure of 8,848.86 meters (29,031.69 feet) above sea level.

Geological ups and downs

• The mountain’s height changes. The movement of tectonic plates can lift it up ever so gradually, while earthquakes can bring it down.
• The countervailing forces may help maintain a degree of stability over time, said Dang Yamin, a member of a Chinese team that surveyed Everest’s height earlier this year.

Measuring mountains

• There’s more than one way to measure a mountain. Nepalese team set up a satellite navigation marker on Everest’s peak to gauge its exact position via GPS satellites. It took measurements with modern, laser-equipped versions of instruments called theodolites, first used to gauge the mountain’s height in 1856 by measuring angles using trigonometry.
• A Chinese team undertook a similar mission this spring, though it used the Chinese-made Beidou constellation of navigation satellites, along with other equipment.
• As measured from the Earth’s core, Ecuador’s Mount Chimborazo is the world’s highest, standing more than 2,072 meters (6,800 feet) above Everest. Because the Earth bulges in the middle, mountains along the equator are farther from the core.
• Measuring from the foot of the mountain to the peak, Hawaii’s Mauna Kea is the tallest. Most of it, however, is under the sea.

https://www.thehindu.com/sci-tech/science/explained-why-did-mount-everests-height-change/article33296283.ece?homepage=true