There is no reliable fixed point anywhere on earth. Everything is in flux. The sea moves back and forth in a regular ebb and flow. The moon’s gravitational pull also influences the earth’s crust, which rises and falls daily by as much as 60 centimeters. Even the North and the South Pole move a couple of centimeters every year.
Despite this, technicians working for the Nepalese government have been charged with determining the exact height — to within a few centimeters — of Mount Everest. Helping them are mountain climbers and some very high-tech equipment.
Using very simple means, James Nicolson was the first to measure the altitude of the world’s highest mountain 150 years ago. He recorded 8,840 meters – just over 29,000 feet. That was astonishingly close to the present widely-accepted figure, which is 8,848.40 meters, but which is, however, disputed by some. The technicians presently measuring Everest are due to present their results to the Nepalese government in two years’ time.
Their most important tool is the Global Positioning System (GPS), which involves two dozen satellites that orbit the Earth and send radio signals. The satellites are placed in such a way that four of them cover every part of the planet at any given time. Since their exact positions are known, GPS receivers like the Leica Geosystems SmartRover can calculate their own positions from the data and exact time.
Since these receivers only weigh a few kilograms, they can be brought up to Everest’s peaks with relative ease. Measurements are made every two seconds for about an hour. As the GPS signals are not disturbed on their way through the ionosphere and the troposphere, measurement technicians can combine the data measured on the mountain with data from the GPS receivers on the ground.
Beams and prisms
Also in use are tachymeters that measure both horizontal and vertical angles as well as distance, and beam infrared rays that reflect off prisms placed on the mountain peaks. Distance can be measured by the amount of time it takes the beam to reach the prism and return.
Another tool used by the technicians are planes equipped with an airborne scanner that scans the mountain. Some have a sensor that takes pictures of the mountain, operating like a camera – but with a lot higher resolution. Points on the pictures can only be exactly defined if the flying altitude at the time the picture was taken can be precisely determined.
The geodatists have to take numerous influences and conditions into account, starting with base level — the usual reference point being sea level. This can, however, vary. For example, around Sri Lanka the Indian Ocean level measures 105 meters lower; to the northeast of Australia, it’s nearly 80 meters higher, which is why technicians calculate an average theoretical sea level for every point on earth.
The earth’s gravitational field isn‘t the same everywhere either, and that also has to be taken into account in the measurements. This is not a reading taken on the mountain. “To do that, you’d have to use a gravimeter and they are very heavy. The National Geographic Institute (IGN) in Paris tried twice to get one up a mountain and failed both times,” says Farouk Kadded, a product manager at Leica Geosystems who also leads expeditions on France’s Mont Blanc.
When used in the mountains, GPS receivers, tachymeters and sensors are extremely sensitive to weather conditions. Temperatures on Mount Everest are around minus 40 degrees Celsius, and atmospheric pressure around 326 millibars – about a third of normal pressure on the ground. This too has to be taken into account in calculating measurements.
There are myriad of other areas that could lead to miscalculation. For example, how high the snow cover is on the mountain has to be subtracted from the result: the point is to calculate the elevation of the naked rock. If a geodatist on the mountain were to jam his GPS device mounted on a pole seven centimeters into the snow, measurement results would be false. Radars and spectrometers can measure the thickness of snow by emitting waves of different lengths.
In view of all this high tech effort, the question poses itself: What for? For one thing, spectacular projects of this type enable the manufacturers of various devices to get a lot of publicity, says Kadded. But also “because we can test the reliability of our Leica Geosystems GPS devices.”
“On Mont Blanc, every two years we measure not only the height at the top but a lot of other points to determine the form and volume of the ice cap and to be able to trace the changes over time,” says Kadded.
Such data is crucial, he says, to climate researchers, meteorologists, glaciologists — and nivologists: scientists who study snow.
Read the original article in German
Photo – Steve Hicks