BETWEEN 1847 and 2005, the height of Mt Qomolangma (Mt Everest) was measured no less than 10 times - each time with different results. Why are we so obsessed with this mountain? Zhang Jiangqi reports.

The year was 1719 - the 58th year of the reign of the Emperor Kangxi during the Qing Dynasty (1644-1911). Placed on a floor of green-gray bricks was a copperplate map of Mt Qomolangma, the mountain also known as Mt Everest.

This map, painstakingly produced based on field research, accurately indicated the location of the world's highest mountain, with its name labeled in Manchu to signify it as part of the empire.

In 1721, a woodcut version of this map was produced, this time with the features labeled in Chinese characters. In 1933, more than two centuries later, the same map was reproduced in Europe, with its features labeled in French.

From then on, the name "Qomolangma" (in Tibetan meaning Saint Mother) appeared in many different languages. Lin Chao, a geographer from Peking University who wrote a 1958 essay titled "The Discovery and Naming of Mount Qomolangma," believed that it was in China that the mountain was first identified and named.

In the 19th century, the colonial powers of Britain, France and Germany embarked on a series of trigonometric surveys to map and consolidate their overseas territories. These efforts, which were later known as the Great Trigonometric Surveys, were to last for half a century.

This work was primarily concerned with determining the positions and distances between surface features and territorial boundaries, and to represent all this information accurately on a map. It was around this time that George Everest, the surveyor-general of India, carried out the Great Trigonometric Survey of India from the Indian plains to the Himalayas, where every peak in the range was measured.

In 1847, Mt Qomolangma was verified as the world's highest peak. Back then, the observation station was sited 322 kilometers from the mountain, and the height obtained was 8,778 meters. The mountain was later named Everest by the Royal Geographical Society in honor of George Everest.

Calculating the height of a mountain on Earth is tricky business. The ellipsoidal shape of the planet means that trigonometric calculations get increasingly difficult the further the observer is from the mountain.

Complex mathematical and physical analyses are needed to pinpoint the precise height and location of the mountain on a curved plane. Subsequent surveys were able to take this into account and provide a more accurate height.

From 1847 to the present, the height of Mt Qomolangma was measured more than 10 times. Some of the heights obtained were: 8,778 meters (1847), 8,839.8 meters (1849-50), 8,882 meters (1880-83), 8,863.6 meters (1921), 8,847.6 meters (1952-54), 8,848.98 meters (1966-68), 8,848.13 meters (1975), 8,846.27 meters (1992), 8,850 meters (1998-99), 8,848.82 meters (2004) and 8,844.43 meters (2005).

Put a chopstick in a glass of water, and you'll find it looks bent. This is the result of light being refracted as it passes through materials of different densities (or refractive indices). Likewise, light from space is refracted as it passes through the dense atmosphere. On Mt Qomolangma, an observation station 3,000 meters below the summit faces the same problem.

As air gets progressively denser toward sea level, light rays from the summit are curved so that an observer will perceive the peak as being higher than it really is.

This also explains why researchers constantly release weather balloons while measuring the peak - they need to take into account the temperature, humidity and pressure at different heights to ascertain just how much light is bent by the time it reaches the observation station. From this, researchers can compensate for the refraction to arrive at a more accurate figure for the mountain's height and location.

It can be seen that the error involved gets smaller the closer the observation station is to the summit. In fact, it would be ideal if one could take measurements directly on the summit itself.

Prior to 1975, surveys were done at a distance from two or more peaks with known heights. From these reference points researchers used trigonometry to obtain the height of Mt Qomolangma.

However, the summit is shaped like a pyramid, with an area of around 27 square meters right on its top. Thus, observers from afar would invariably aim at different points on the summit, meaning a substantial error would be inherent in the calculated height. Pre-1975 measurements were off by as much as 13 meters.

In 1975, a Chinese team planted the first survey marker on the summit. By enabling observation stations from various locations to converge on that one reference point, the accuracy was greatly increased to within 0.36 meters.

Measuring woes

Formed around 60 million years ago when the Indian subcontinent slammed into the Eurasian crustal plate, Mt Qomolangma lies on the border between China and Nepal. Unlike the more gradual slopes on its southeastern side, which are covered in snow all year round, the steepness of its northern and northwestern slopes ensures the rock faces remain relatively clear of ice and snow.

On the southeastern slopes, the influx of warm, moist air from the Indian Ocean rises high up into the atmosphere, creating spectacular cloud formations that resemble flags fluttering in the wind. The resulting snowfall that accumulates on the gradual southeastern slopes form the source of the glaciers on Mt Qomolangma.

In 1975, when the Chinese team planted the first survey marker on the summit, they noted that the snow on the peak was 0.92 meters deep. Their reported height of 8,848.13 meters was obtained after subtracting the depth of the snow (0.92 meter) from the absolute height of the mountain (8,849.05 meters).

However, in 1992 when an Italian team planted their survey marker on the summit, they found that the depth of snow was 2.55 meters - an increase of 1.63 meters from 1975. However, the absolute height of the mountain (with the snow cover included) was 0.01 meter shorter than the 1975 figure.

The complexities involved became even more apparent when later teams arrived at the summit with more sophisticated instruments. In 1997, an American team saw the survey marker planted by the Chinese in 1975, but it was lying more than 30 meters below the summit. Thirty years of accumulated snow had caused the survey marker to slide down the southeastern slope under the force of gravity.

The snow that had subsided is continually replenished by falling snow, but the depth of this snow cover is not a constant. Seven separate surveys between 1953 and 2005 yielded values that differed by as much as 2.7 meters, and no discernible trends could be observed.

And it's not just the depth of the snow on the summit. There's also the question of where the highest point really is. Many surveyors consider the true height of a mountain to be the height of the rock beneath the snow. This means that the highest part of the mountain - with snow cover and all as it appears to the eye - may not correspond to the highest point of rock below the snow.

This is indeed so for Mt Qomolangma, where its eastern side receives far more snow than the rest. This was verified in 2005 by a surveying team, whose radar readings indicated that the spot with the thickest snow did not lie over the highest point of rock.

Even then, not all surveyors agree that the true height of a mountain should be obtained by subtracting the depth of snow above it. Some believe the actual height should include the snow cover. Meanwhile, as the debate continues, the mountain is still on the move.

GPS data collected over several years indicates that Mt Qomolangma is moving northeast at a rate of 48 millimeters a year. It has been estimated that the region around the summit could rise by as much as 12.7 millimeters a year. Some experts believe that in the next hundred years, the mountain would rise around 2 meters, and shift its location 5 meters to the northeast.

So it seems like the mountain is on the rise. Why, then, is this not reflected in the measurements of Mt Qomolangma over the years?

The Chinese survey of 2005 obtained a height that was actually 3.7 meters lower than the 1975 figure, but the State Bureau of Surveying and Mapping clarified their findings, saying: "Based on past and current research, it is still inconclusive if Mt Qomolangma is indeed getting lower.

This is because there are many factors that influence the figures obtained on previous surveys, such as the depth of ice cover, rate of tectonic activity and advances in surveying technology ... But what we are convinced of is that the latest figure [8,844.43 meters] is the most accurate and reliable one to date."

Lofty ambitions

Why are we so obsessed with measuring Mt Qomolangma? What does it matter to us if the mountain is 8,848 meters - or not?

Simply because it is the world's tallest peak. This unique quality has made it an icon of superlatives: the best, the toughest, the holy grail in the pursuit of perfection. The early attempts to explore Qomolangma represent more than just individual ambition.

National pride was very much at stake, and the ability to survey and reach this lofty goal was a display of the technological prowess of a country.

It was the Indian surveyor B. L. Gulatee who calculated the height of Mt Qomolangma as 8,848 meters in 1954 - the figure accepted by most of the world today. Gulatee's measurement was 8,847.6 meters, which was rounded off to 8,848 meters. However, this figure was obtained without a survey marker on the summit, and the nearest observation station was 50 kilometers from the summit, which meant the calculated height could be off by more than 5.17 meters.

In contrast, the height obtained in 1975 by the Chinese team with a survey marker on the summit - 8,848.13 meters - had an error margin of only 0.36 meters.

As the world's highest point, Mt Qomolangma has inspired climbers for centuries.

From 1992 to 2005, the writer was heavily involved in surveying the mountain, and had reached an altitude of 7,100 meters in 2005. This mountain has a magnetic appeal that has changed the lives of countless people - from the scientist to the mountaineer to people who are simply awed by this work of nature. This is perhaps the greatest gift Mt Qomolangma has for us.