I'd have to go with the existing Chinese measurement at present ...a measurement on the rock itself and not the top of an ice pack.
I agree that the rock is the highest point-not the snow cap. But, as they sort of alluded to in the article, we measure mountains as heights above "sea level", not above the ellipsoid. They say it can be measured to ±1 m, I sort of disagree-it can be measured to centimeter level accuracy ABOVE THE ELLIPSOID. The trick is converting that to a height above "sea level". Because the mountains are so massive, there is a likely a significant gravity anomaly present there. Good gravity is needed to develop an accurate geoid model, which is needed to convert to height above "sea level" (whatever that is). Yes, there are global geoid models (EGM08, for example), but these have an accuracy of probably around 1 m in that area, maybe 0.5 m, so perhaps that is what they meant by the 1 m accuracy.
We are extremely fortunate to have a very good geoid model for use in the USA. Imagine the difficulties involved in getting good orthometric heights before geoid models. Well, that is what we had to do in the early days of GPS. Fortunately, we also have a relatively dense network of benchmarks in he USA, which is what we used back then. We would use 3,4, or even 10 or more benchmarks in a netowrk depending on the size of the area and if the geoid was smooth or not. I
recently was asked about GPS control for Lidar in a country that has neither a good geoid model nor a good leveling network. If you want water to flow, you need to somehow model the geoid slope at least, if not the absolute height above a geoid reference surface. This can be done by leveling across the area, or by taking astronomic measurements to determine deflection of the vertical.
I assume that the Chinese have a lot of gravity data around the Himalayas, although that may only be on one side.
If we would classify mountain heigths as above the ellipsoid, this would solve the problem!
It's a moving target.
Mount Everest Moving to China
Date 2000/11/25 22:46:02 | Topic: Hindu Press International
Source: India Today News Service, November 17, 2000
BEIJING, CHINA: According to a survey done by Chinese scientists, the world's tallest peak, Mt. Everest, is moving into China at a speed of six to seven centimeters per year from its position on the Nepal-China border. This is nothing new, of course, as the entire India subcontinent--once separated from Asia by ocean--first crashed into China 50 million years ago. These researchers also found that the snow cover on the top of the Mt. Everest has also been descending over the past three decades and added that "it had a connection with global warming."
The trick is converting that to a height above "sea level".
Amen!
After spending 15 years or so trying to get geoid models to match, even crudely, to the actual geoid contours I have always been amused when someone claims that they set a GPS on a mountain top and came up with an "accurate" elevation. One actually claimed that a western US mountain top was 4" higher or lower than published. I thought-really 4"! Of course that may have been geoid 96 or 99. By running the same observation again and applying Geoid 09 that 4" may have come back to the old number, or it might be 1'.
There is a way to get a good number. While they carry the GPS up there run a bench loop up and back! 😉
I vote bench loop too! This is the perfect opportunity to try out my new ice axe/level rod. 😉
Bench loop?
Surely you jest.
Trig. maybe.
:coffee:
Trig levels would probably be the best way. But where would you find a good bench mark to start from? Then the crew should, of course, tie into a bench on the opposite side of the mountain. And keep the points as close as possible to pick up gravity slope. And shoot both directions! Sounds like a lot of fun!;-)
Trig levels are useless because of problems with defining the deflection of the geoid. It all reduces to knowledge of the geoid in that area of the world which is practically nil.
Dr. Muneendra Kumar was chief of triangulation for a year in Nepal,(while he was an Officer in the Survey of India),and he still thinks the idea of "accurately" determining the height above msl for Everest to be silly with today's technology and knowledge of the geoid. Dr. Kumar later went on to become Chief Geodesist of the U.S. Department of Defense, and is known world-wide as "Dr. WGS84."
There is insufficient knowledge of the geoid in the Himalayas to determine such an elevation within meters (plural).
Yeah, I was just joking. Running a survey up the mountain would be close to if not impossible.
As an example of geoid slope: I have a control point that has a geoid height of
-40.0, on a mountain top 21 miles west the calculated geoid height in Geoid 03 is
-26.7. I know the 03 model is working pretty well in the foothills, but what is the chance that the mountain top geoid is as accurate?
