Corresponding to this new height concept is the concept of the telluroid, the collection of points Q whose normal potential is equal to the true geopotential of a point P on the terrain, and on the same plumb line. The separation between points P and Q, i.e., between topographic and telluroid surfaces, is called the height anomaly, and is, contrary to the geoid undulation N (with respect to the reference ellipsoid), defined without requiring density information throughout space, not only at sea level.