Because of the bumpy and irregular surface of the geoid, another surface was developed to help smooth the transition from 3D to 2D, and this is called a spheroid. A spheroid is a mathematical term for what is essentially a sphere that has been compressed or expanded in one axis. Geodesists create spheroids to closely match the geoid to a given extent. There are many spheroids in the world, and each one was created to handle different areas around the world, including spheroids for the whole of Earth. The spheroid represents a reference system for horizontal measurement.