Orthometric height

The orthometric height is supposed to be the actual distance from a given point to the geoid measured along a vertical line. Since gravity isn't constant, two points that are level with each other may have slightly different orthometric heights.

Orthometric heights are what are usually used in the US for ordinary engineering work. Values for measured points can be obtained from the National Geodetic Survey data sheets. This data was gathered over many years of precise optical leveling.

Orthometric heights relate roughly to "height above sea level" but the current NAVD88 datum is tied to a specific elevation at a chosen point rather than to any location's exact mean sea level.

GPS measurements give earth-centered coordinates, usually as height above the reference ellipsoid, which cannot be related accurately to orthometric height above the geoid unless accurate gravity data is available for that location. NGS is undertaking the GRAV-D program to obtain such data.

Alternatives to orthometric height include dynamic height and normal height.