1. In the beginning there was a great mathematician who invented the sphere - a massless figment of his imagination.
2. He soon grew bored with the sphere and wanted a more challenging shape for his deliberations. By making one axis shorter than the other two he created the ellipsoid. The points at which the axis crossed the surface he named the N pole and the S poles. The line on the surface which was an equal distance from the poles he called the equator.
3. Along came a physicist who had been thinking heavily about mass. He filled the ellipsoid with a perfect fluid. However, in order to maintain the mathematician's ellipsoid shape he had to rotate the body at a uniform speed.
4. Now the physicist soon discovered that if he rolled an apple on the surface from the N pole to the equator, he did no work at all. In other words he needed no energy, not even a bite from the apple!
5. He weighed the apple on a spring balance and found it weighed more at the pole than at the equator even though it had not left the surface of the ellipsoid - a very curious result. He explained that the centrifugal force created by the body's rotation had helped to move the apple further from the center of the body and against the force of gravity.
6. The physicist needed a new word to describe this surface of different gravities but which had no 'uphill'. he called this surface equipotential and he was pleased to find it was identical to the mathematician's ellipsoid.
7. A geologist arrived on the scene, but thinking that all new words should begin with "geo", he coined the word geoid for the equipotential surface.
8. The geologist liked big lumps of rock and so chucked a few into the rotating body creating local anomalies. The geoid was no longer an elegant mathematical shape, but it had bumps in the surface. The physicist could still roll his apple over all the bumps without consuming or gaining any energy. However the equipotential surface was no longer identical to the mathematician's ellipsoid.
9. Next came the chemist who thought that the body would look much nicer covered with sodium chloride solution, but because he had not stirred the solution very well, the liquid was not of uniform density. This meant that the liquid surface did not fit the geologist's geoid exactly. The new surface became know as the chemist's level (or C-level for short).
10. It was now the Meteorologist's turn. He added winds, temperature differences, and lots of other nasty things. This changed the shape of C-Level to the Meteorologist's Special Level (or MSL for short).
11. Not to be outdone, the Oceanographer, who had been circulating, then chipped in that he had just discovered currents (and because he discovered them he called them Ocean currents) - but this just made the whole story even more complicated.
12. And the last to appear was the Environmentalist with his dire warnings of global warming. He talked of melting ice and great changes to the MSL. He said that in 100 years time everyone else would be wrong anyway!
13. The Geologist, having become bored with the conversation, had stepped out
of the room. When he returned a few millenia later he proudly announced that
some places had gone up in the world and some had gone down.
And if you want to know your height, it all depends in whose shoes you stand and when you stand in them.
How GPS Utility handles altitude