Ortometric

Ortometric refers to a type of height measurement relative to a reference gravitational equipotential surface. In geodesy, this reference surface is often approximated by a geoid, which represents the mean sea level extended globally. Ortometric height is the distance along a plumb line from the geoid to a point above it. It is distinct from ellipsoidal height, which is measured perpendicular to a reference ellipsoid, a smooth mathematical surface that approximates the Earth's shape.

The concept of ortometric height is important for practical applications such as engineering and hydrology where the actual vertical separation from sea level is crucial. For instance, river flow and water table levels are best represented by ortometric heights. Calculating ortometric height requires knowing the geoid undulation, which is the difference between the geoid and the reference ellipsoid at a given location, and the ellipsoidal height of the point. The formula relating these is approximately H = h - N, where H is the ortometric height, h is the ellipsoidal height, and N is the geoid undulation.

The gravity field of the Earth is not uniform, meaning the geoid is an irregular surface. This irregularity makes the plumb line deviate slightly from the direction perpendicular to the reference ellipsoid. Consequently, the ortometric height and ellipsoidal height will differ. Accurate determination of the geoid is a complex task involving gravity measurements and sophisticated geodetic models.