A geographic coordinate system is not a perfect system for measuring locations on the earth, however, because the earth is not a perfect sphere. It bulges at the equator because of its rotation. The bulge can be modeled mathematically using a spheroid, an oblong sphere with a major and minor axis. (The bulge in Figure 11.3 is highly exaggerated.) Latitude-longitude locations defined on a spheroid are more accurate than those defined on a sphere. The spheroid becomes a part of the GCS definition. (Some people use the term ellipsoid rather than spheroid.)