Dynamic height
Dynamic height is a way of specifying the height of a point above a reference, as opposed to orthometric height or normal height.
Dynamic height is the most appropriate height measure when working with the level of water over a large geographic area, and is used by the Great Lakes Datum in the US and Canada. [1]
Dynamic height is constant if one follows the same gravity potential as they move from place to place. Because of variations in gravity, surfaces having a constant difference in dynamic height may be closer or further apart in various places. Dynamic heights are usually chosen so that zero corresponds to the geoid.
When optical leveling is done, the path corresponds closely to following a value of dynamic height horizontally, but to orthometric height for vertical changes measured on the leveling rod. Thus small corrections must be applied to field measurements to obtain either the dynamic height or the orthometric height usually used in engineering. US National Geodetic Survey data sheets [2] give both dynamic and orthometric values.
Dynamic height can be computed using the normal gravity at 45 degree latitude and the locations geopotential number.