Traditionally, the earth's surface has been modeled as a sphere, but the World Geodetic System of $1984($WGS$-84)$ uses an ellipsoid as a more accurate model. It places the center of the earth at the origin and the north pole on the positive z$-$axis. The distance from the center to the poles is $6,356\mathrm{.}523$ km and the distance to a point on the equator is $6,378\mathrm{.}137$ km$.$