How Mount Everest got 3 feet higher, endorsed by both Nepal and China?

CONTEXT

The Foreign Ministers of Nepal and China jointly certified the elevation of Mount Everest at 8,848.86 metres above sea level 86 cm higher than what was recognised since 1954.

 About Mount Everest

• Mount Everest rises from the border between Nepal and China.
• Everest is also known as Sagarmatha in Nepal and Mount Qomolangma in China.

BACKGROUND

No other mountain has perhaps been the subject of as much debate. Over the years, there have been debates on issues like whether it should be “rock height”, or whether the snow cladding it, too, should be accounted for.

How and when was the earlier measurement of 8,848 m done?

Earlier measurement was determined by the Survey of India in 1954, using instruments like theodolites and chains, with GPS still decades away. The elevation of 8,848 m came to be accepted in all references worldwide except by China.

In 1999, a US team put the elevation at 29,035 feet (nearly 8,850 m).