This model assumes that the ionosphere is a thin, uniform-density shell about the Earth, located near the mean altitude H of maximum TEC (approx. 350 km). Using geometry, a slant intersection with this shell model can be determined and a vertical TEC measurement inferred. The intersection between line of sight and this shell is called the ionospheric pierce point (IPP). The perpendicular projection onto the Earth's surface is called the subionospheric point.