Our earth isn't perfectly spherical. So when we try to approximate the shape of earth, we need to choose an appropriate reference model. An ellipsoid is such a model or reference surface.

There are many different kind of ellipsoid models in the world. A commonly known ellipsoid is WGS84, which is also used in GPS. Another important ellipsoid is the GRS80 ellipsoid that is used for measuring coordinates in the United States and Canada. WGS84 and GRS80 has the same distance at the equator but the GRS80 ellipsoid is a little bit more flattened.

From the figure below, you will have a general idea about the difference between when different ellipsoid models are applied to the earth surface (WGS84 vs GRS80).

The following table lists the primary ellipsoid parameters of GRS80 and WGS84.