Thus small corrections must be applied to field measurements to obtain the orthometric height usually used in engineering.

In Austria orthometric height is used, while its neighbours use other height systems, which leads to differences.

The orthometric height of a point is the distance H along a plumb line from the point to the geoid.

Alternatives to orthometric height include dynamic height and normal height.

Alternatives are: orthometric heights and dynamic heights.

The reference surface for orthometric heights is the geoid, an equipotential surface approximating mean sea level.

Orthometric heights are usually used in the US for engineering work, although dynamic height may be chosen for large-scale hydrological purposes.

Elevation may be specified as normal height (above a reference ellipsoid), orthometric height, or dynamic height which have slightly different definitions.

Dynamic height is a way of specifying the height of a point above a reference, as opposed to orthometric height or normal height.

The definition of NAVD88 uses the Helmert orthometric height, which calculates the location of the geoid (which approximates sea level) from modeled local gravity.

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.

Since gravity isn't constant over large areas the orthometric height of a level surface isn't constant, and NGS orthometric heights are corrected for that effect.

Normal gravity values are easy to compute and "hypothesis-free", i.e., one does not have to know, as one would for computing orthometric heights, the density of the Earth's crust around the plumbline.

As gravitational potential cannot be neglected for applications with high accuracy requirements, the Swiss national height network 1995 (LHN95) created a new orthometric height vertical reference point, fixed to the geoid.

They are periodic changes of atmospheric pressure, temperature and orthometric height in a current of air caused by vertical displacement, for example orographic lift when the wind blows over a mountain or mountain range.

GPS measurements give earth-centered coordinates, usually displayed as height above the reference ellipsoid, which cannot be related accurately to orthometric height above the geoid without accurate gravity data for that location.

Orthometric height is for practical purposes "height above sea level" but the current NAVD88 datum is tied to a defined elevation at one point rather than to any location's exact mean sea level.

For example, gravity is 0.1% stronger in the northern United States than in the southern, so a level surface that has an orthometric height of 1000 meters in Montana will be 1001 meters high in Texas.

As a result there were changes in both the old West German normal orthometric heights (new methods of calculation) and the normal heights of East Germany (referred to the new Normaal Amsterdams Peil datum.

Practical applications must use a model rather than measurements to calculate the change in gravitational potential versus depth in the earth, since the geoid is below most of the land surface (e.g., the Helmert Orthometric heights of NAVD88).

The North American Vertical Datum of 1988 (NAVD88) is the vertical control datum of orthometric height established for vertical control surveying in the United States of America based upon the General Adjustment of the North American Datum of 1988.