Grid System of the Earth

meridians and parallels on geographic maps and globes serving to mark off the geographic coordinates of points on the earth’s surface—the latitudes and longitudes. All points on a given meridian have one and the same longitude and all points on a parallel, the same latitude.

In geodesy the figure of the earth is taken to be an oblate ellipsoid of revolution on which the meridians are ellipses passing through the earth’s poles and the parallels are small circles whose planes are perpendicular to the earth’s axis of rotation and parallel to the earth’s equator. Owing to the compression of the earth’s ellipsoid, the linear distance between parallels projected in an equal number of degrees increases slightly from the equator toward the poles. On the geoid the meridians and parallels are lines of double curvature although they very closely resemble ellipses and circles, respectively.