Orthometric

Orthometric is a term used in geodesy and surveying to describe quantities referenced to the direction of gravity and to a vertical reference surface. The core concept is the orthometric height, which is the distance along the local plumb line from the geoid to a point on the Earth's surface.

Orthometric height, denoted H, represents the elevation above mean sea level used in most civil engineering and national height datums. It is conceptually measured by leveling and is influenced by gravity variations along the vertical. The geoid is an equipotential surface of the Earth's gravity field that roughly coincides with mean sea level, and the orthometric height corresponds to the length of the vertical segment from the geoid to the point.

In geodetic terms, ellipsoidal height h is the distance along the line perpendicular to a reference ellipsoid, while the geoid undulation N is the difference between the ellipsoid and the geoid (N = h − H). Consequently, H = h − N. This relationship underpins how modern measurements convert GNSS-derived ellipsoidal heights into practical orthometric heights using geoid models.

An orthometric surface is an idealized surface of equal orthometric height, locally aligning with mean sea level. In practice, orthometric heights are obtained through leveling, gravimetric measurements, and, with GNSS data, by applying geoid models to derive N and thus H.

Applications of the orthometric concept include land surveying, mapping, construction, hydrology, and flood-risk assessment. The term derives from Greek roots meaning “straight measure.”