Extreme conditions

While thousands of climbers have reached the summit of Qomolangma since it was first conquered in 1953, the northern route used by the Chinese survey party is regarded as one of the most difficult approaches to the top.

The team members had to contend with low oxygen levels, biting cold and extreme winds as they carried the surveying equipment, which weighed about 30 kilograms in total, and oxygen cylinders.

As technology improves, satellites, radars and gravity measuring technologies are increasingly being used to determine accurate figures. However, someone still has to carry the instruments to the summit.

"The surveyor's beacon had to be carried to the summit by hand," said Zhang Qingtao, deputy leader of the survey team.

Scaling Qomolangma is no easy task. The average air temperature at the summit is -30 C, four degrees lower than in Antarctica. The snow is often very deep, and hurricane-force winds blow constantly. The team members started training in January to cope with the extreme conditions.

Li, the head of the team, said professional surveyors are adept at using the advanced equipment, so the data they obtain are more reliable, scientific and convincing.

He added that the surveyors' experience with the equipment helped to limit the amount of time spent at the summit, reducing their exposure to the frigid temperatures and oxygenstarved environment.

On May 18, the Ministry of Natural Resources announced that a team of 12 was expected to reach the summit two days later. However, three team members had no experience of climbing at above 8,000 meters.

A day later, the National Meteorological Center warned that Cyclone Amphan was expected to make landfall in India and Bengal on May 20, bringing heavy snow to the Qinghai-Tibet Plateau, which could affect conditions on Qomolangma.

The 12-man team attempted to reach the summit twice, on May 12 and 22, but was beaten by adverse weather both times.

After concerns were expressed about the less-experienced members of the team, the number was reduced to eight, all seasoned high-altitude climbers.

When the team reached the summit, they reverted to their professional identities as surveyors.

Phurbu, an avid climber who has reached the summit 12 times, went without supplementary oxygen for more than 110 minutes so he could work more efficiently.

"When we were on the summit, my duties were to fix the beacon, check satellite signals and wire the surveying equipment," he said.

"Time was precious and the work was so important that I removed my oxygen mask so I could see the equipment more clearly."

Dang said that although technology assists the surveying and mapping work, modern instruments are not currently accurate enough to replace traditional techniques, which provide much more precise data.

Meanwhile, the unstable airflow, fierce winds and low temperatures, mean that drones and remote sensing satellites do not yet work at the summit of Qomolangma.

Dang said the remote sensing satellite can only produce measurements accurate to about 2 meters, while manual surveys can reach an accuracy level of about 1 centimeter.

However, he noted that the current development rate of surveying drones means they will be used more frequently in the future, which may result in fewer surveyors scaling the mountain to check its height.

Palden Nyima contributed to this story.