Ever think about the problem of measuring sea level? Turns out it’s quite the horrendous problem – but that may be changing, according to Laura Spinney in NewScientist (11 February 2017, paywall):
Earth’s geoid from DeepEarthScience (2013)
If Earth were a perfect sphere, we might use GPS measurements: these calculate the user’s distance from the centre of the GPS satellites’ orbits. But Earth looks more like a rugby ball, with a radius 21 kilometres longer at the equator than at the poles. It’s a lumpy rugby ball too, with a depression of about 100 metres to the south of India, for example, and a peak of about 100 metres over Indonesia.
These lumps are in Earth’s geoid, or gravitational surface – a plane that you would move across if you did no work in the vertical dimension, like a marble rolling over a table. They occur because gravity is stronger where mass accumulates, as in a mountain or denser rocks. The geoid largely determines where the surface of the sea lies. If you were to swim from India to Indonesia, you would move 200 metres away from Earth’s centre.
Agreeing on a vertical standard, therefore, boils down to agreeing on a model of the geoid – and with the latest satellite measurements, we’re getting close to doing that. In 2002, NASA and the German Aerospace Center launched the Gravity Recovery and Climate Experiment (GRACE) satellite, and seven years later, the European Space Agency launched its Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) mission. GOCE orbited until 2013, while GRACE is still in orbit, and the two now have enough data to make a geoid model accurate to within a few centimetres. “The gravity field is smoothed because the satellite is far from the Earth’s masses,” says Rummel, who led the GOCE mission, “But it can be complemented by terrestrial gravity measurements.” Together, the two provide the millimetre accuracy required for, say, building bridges.
I hadn’t thought about this so much. So are there any plans?
The technical capability that underpins a geoid-based global standard of height is there – but is there the political will to agree on it? Perhaps. The US, Canada and Mexico have announced that they will switch to a unified geoid-based height system in 2022, and a meeting in Prague of the International Union of Geodesy and Geophysics in 2015 passed a resolution to support the adoption of a single global reference frame. “We agreed,” says Ihde. “Now we have to put it into practice.”
And …
[Ecuadoran peak] Chimborazo beats Everest by a whopping 2 kilometres.
I sense outrage on the horizon.
