The Elevation Mirage

Mountain heights shift depending on how you measure gravity because a mountain's true elevation is not a simple matter of distance from the center of the earth. It is determined by the geoid, an equipotential surface of Earth's gravity field that corresponds to undisturbed global sea level. Because of lateral and vertical variations in Earth's mass, this invisible baseline undulates relative to a reference ellipsoid by up to 100 meters. GPS receivers, meanwhile, measure height relative to a geocentric reference ellipsoid rather than this undulating gravity field, requiring mathematical corrections to arrive at a true orthometric height.

Yet even if our gravity measurements were static, the ground itself refuses to cooperate. Tectonic plate movement continuously alters landforms and mountain structures at rates of two to fifteen centimeters per year. On top of this slow creep, large earthquakes can cause rapid, localized vertical or horizontal ground shifts of several feet in a matter of seconds, turning static elevation measurements into a constantly moving target.