More precisely, the surface of the Earth is usually approximated by an ideal oblate ellipsoid, for the purposes of defining precisely the latitude and longitude grid for cartography, as well as the "center of the Earth". In the WGS-84 standard Earth ellipsoid, widely used for map-making and the GPS system, the radius of the Earth is assumed to be at the equator and center-to-pole; meaning a difference of in the radii and in the diameters, and a relative flattening of 1/298.257223563. The sea level surface is much closer to this standard ellipsoid than the surface of the solid Earth is.