That's Heavy - Mapping Gravity On An Imperfect Sphere
Earth's gravity is coming down on us at 9.81 m/s2, right? Wrong. Well, maybe. This figure is known in physics circles as the 'standard average', meaning that if the Earth were a perfect sphere, and there were no winds, currents, or tides (or a moon, for that matter), then this would be the amount of gravity experienced everywhere.
Fortunately - for the sake of alleviating boredom - the Earth does have winds, currents, and tides (and a moon), and it is by no means a perfect sphere.
Big deal, you say. Unless I'm a world-class athlete, why would I care if gravity is coming down at 9.80 m/s2 or 9.82 m/s2? Well, it is a big deal, in that it affects big things. Like the ocean. The ocean weighs roughly 1,500,000,000,000,000,000 tons, which is really, really heavy. So, when gravity is less, say, off the coast of India than it is off the coast of Iceland, there can be up to a 500 foot difference between where 'sea level' happens to be at each place.
Again, although this is fascinating, you may be thinking, So what, I'm a landlubber, so how does this affect me? Well, this also has has a big impact on ocean currents, ice floes, even the mechanisms that cause earthquakes, so if your lifestyle is at all affected by the climate or the ground moving beneath your feet, there is a good chance this has some relevance to you.
Which is why the European Space Agency (ESA) has devoted some attention to mapping the Earth's gravity with their low-flying Gravity and Ocean Circulation Explorer (GOCE) satellite. The result is the most accurate gravity anomaly geoid ever produced. A geoid being, of course (according to Wikipedia), "the equipotential surface which would coincide exactly with the mean ocean surface of the Earth, if the oceans were in equilibrium, at rest (relative to the rotating Earth), and extended through the continents." In other words, it shows how the Earth would look if everything was gravitationally equal.
So without further ado, here's the rotating image they have generated for your viewing pleasure: