ABSTRACT
The actual surface of the Earth is not very cooperative. It’s bumpy. There is not one nice, smooth figure that will fit it perfectly. It does resemble an ellipsoid somewhat, but an ellipsoid that fits Europe may not work for North America. And one applied to North America may not be suitable for other parts of the planet. That’s why, in the past, several ellipsoids were invented to model the Earth. There are about 50 or so still in regular use for various regions of the Earth. They have been, and to a large degree still are, the foundation of coordinate systems around the world. But things are changing. And many of the changes have been perpetrated by advancements in measurement. In other words, we have a much better idea of what the Earth actually looks like today than ever before, and that has made quite a difference.
