What is a spheroid / ellipsoid?
There are exact mathematical definitions of these two terms, which I won’t go into too deeply here, because they aren’t what map makers mean when they use them. It is enough to say that in mathematics, a spheroid is a type of ellipsoid, one that is made by rotating an ellipse, in the third dimension, around either its long (semimajor) or short (semiminor) axis. When rotated about its semiminor axis, a spheroid is called an oblate spheroid. The Earth is (roughly) elliptical in cross-section, rotates about its short axis, and is therefor (approximately) an oblate spheroid. Map makers tend to be less anal retentive than mathematicians, and are quite happy using the terms ‘ellipsoid’ and ‘spheroid’ interchangeably. When a map maker uses either term, he (or she) is referring to one of a number of oblate-spheroidal models of the Earth. Many people confuse spheroid and datum. It’s probably easiest to remember that a spheroid is an Earth model, a datum is the practical application of the model
