Ellipsoids are used to represent the shape of the Earth. For many applications, and especially in small-scale mapping, the Earth can be represented as a sphere. Most of the projections supplied in this package assume by default that the Earth is a sphere with a radius of approximately 6371 kilometers. However, because the Earth rotates on its axis, it is slightly flattened at the poles, and is therefore better approximated by an ellipsoid rotating on the polar axis. Ellipsoidal projections are used for accurate, large-scale maps and flat coordinate systems. However, in very large scale maps, representing a continent or the whole planet, we recommend that you use spherical projections. Indeed, the elliptical form of most of the projections in the package is accurate only for a few degrees of latitude or longitude around the projection center.