Did you know that if you swam from a point south of India to Indonesia you would be swimming 200 metres uphill?
Did you know that the Ecuadorean peak Chimborazo is two kilometres higher that Mount Everest if measured from the centre of the earth?
Those are not the only issues with sea level. When the Swiss and the Germans built a bridge at the border at Laufenberg, the Germans calibrated the height to a benchmark in Amsterdam, while the Swiss used one in Marseilles. They knew there was 27cm difference, but they added where they should have subtracted and met with a yawning gap of 54cm!
Thing is, the earth is not round, the surface isn’t even, freshwater is less dense the saltwater, water expands with temperature, and countries by and large have their own idea of zero. Sea level is gauged to 10 different locations in Denmark. At least the Germans and the Swedes take their cue from Amsterdam. The Brits have had three goes at straightening out their measurements. The second attempt started in 1912, and was completes in 1952. By that time I’m sure the whole country would have tilted.
This conundrum is addressed in an article in the New Scientist (paywalled).
This link explains that the earth’s rotational velocity (1,674.4 km/h) causes the planet to bulge at the equator. The Earth is what is known as an “oblate spheroid”. This is from the Earth2014 global relief model, with distances in distance from the geocentre denoted by color:
Earth has an equatorial diameter of 12,756 km, and a polar diameter of 12713.6 km, nearly 43km less. Mt Chiborazo is closer to the equator than Mt Everest. While it is 6,263.47 meters above the local sea level compared to Mt Everests’s 8,848 meters, depending on who you ask, Mt Chiborazo is the highest protrusion on the earth’s surface.
The difference between India and Indonesia is due to gravitational differences. Mountains and heavier rocks get pulled towards the earth’s centre.
Apparently around 100 different standards are used around the world, but help is at hand:
- 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…Together, the two provide the millimetre accuracy required for, say, building bridges.
Within about five years everything will be standardised relative to a point at the centre of the earth. Thing is, on the surface everything changes all the time.
For perspective, have a think about this:
- In short, objects located along the equator are about 21 km further away from the center of the Earth (geocenter) than objects located at the poles. Naturally, there are some deviations in the local topography where objects located away from the equator are closer or father away from the center of the Earth than others in the same region.
The most notable exceptions are the Mariana Trench – the deepest place on Earth, at 10,911 m (35,797 ft) below local sea level – and Mt. Everest, which is 8,848 meters (29,029 ft) above local sea level. However, these two geological features represent a very minor variation when compared to Earth’s overall shape – 0.17% and 0.14% respectively.
The mean diameter of the earth is given as 12,742km. I understand the average ocean depth is 3.6km, or about 0.056% of the radius. When you go back 15 million years or more to compare sea levels, the continents were in different places, and the shape of the ocean basins no doubt influenced how much land protruded above the waves.