Topographic elevation is the vertical distance above the reference geoid, a precise mathematical model of the Earth's sea level as an equipotential gravitational surface. Topographic prominence is the elevation difference between the summit and the highest or key col to a higher summit. Topographic isolation is the minimum great circle distance to a point of higher elevation. This article defines a major mountain peak as a summit with at least 500 metres (1640 feet) of topographic prominence. An ultra prominent peak is a summit with at least 1500 metres (4921 feet) of topographic prominence.