The IGLD is based on slightly different principles tlian the NGVD, with small differences 

 in the elevations assigned by each system to the same point. The differences will rarely have 

 any practical importance in engineering calculations when only one system of elevations is 

 used. Recognizing tliat there is a difference between the systems and that both have a logical 

 basis will reduce the possibility of error. The different systems are discussed below; detailed 

 explanations are in Feldscher and Berry (1968), Bomford (1971), and National Oceanic and 

 Atmospheric Administration (1974; 1977b). 

 2. The Definition of Elevation. 



When two points at approximately the same latitude and longitude are displaced from 

 one another in the vertical, the elevation of the higher point above the lower is adequately 

 defined as the component of the distance between them along a Une normal to a level 

 surface passing tlirough one of the points. Tliis definition is not adequate when the two 

 points are separated by a considerable distance in the north-soutli direction. Difficulty arises 

 because the apparent gravity at the surface of the Earth is the resultant of two distinct 

 forces— a force toward the center of the Earth described by equation (3) and the centrifugal 

 force due to tlie Earth's rotation. The centrifugal force, Fp, acts outward along a Une 

 parallel to the equational plane. Its magnitude is expressed with good approximation by 



Fc = -<J- acos0 (23) 



where to is the angular frequency of the Earth's rotation, a the radius of the Eartli, 

 and the geographic latitude. As illustrated in Figure 19, tlie resultant force, i.e., the 

 apparent gravitational force, is directed toward the center of the Earth only at the poles 

 where the force, Fc, vanishes and at the Equator where Fc and Fg are parallel; the 

 apparent gravitational force is generally implied in engineering discussions unless it is clear 

 that this is not the case. 



Since the universal gravitational force is directed toward the center of the Earth, and the 

 centrifugal force is directed outward from the polar axis, tlie apparent gravity has its 

 maximum value at the poles and its minimum value at the Equator. 



A level surface is defined as a surface normal to the local gravitational vector. Since the 

 gravitational vector is not, in general, directed toward the center of the Earth, sea level 

 cannot be a spherical surface. Newton showed in 1687 that tlie figure of the Earth must be 

 given approximately by an ellipsoid of revolution (Feldscher and Berry, 1968). The exact 

 shape of this ellipsoid is determined by the requirements that the MSL be a surface which is 

 everywhere normal to the apparent force of gravity. The resulting surface has a radius which 

 is larger at tlie Equator, where the apparent gravitational force is a minimum, than at the 

 poles where the gravitational force is a maximum (as shown in Fig. 20). The difference 

 between the polar and equatorial radii of the level surfaces increases with elevation above 

 MSL. As a result, the differences between two specified level surfaces increase from pole to 



50 



