An ellipsoid is a shape like a sphere, but it may be longer in one or more directions, like a rugby ball. It is the 3 dimensional analogue of an ellipse.
![Thumb](http://upload.wikimedia.org/wikipedia/commons/thumb/b/b4/Ellipsoide.png/320px-Ellipsoide.png)
The Earth is shaped slightly like an ellipsoid. Its shape is called an oblate spheroid. An ellipsoid shaped like the Earth, called a reference ellipsoid, is often preferred to the geoid because it makes calculations simpler.
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