Equator. The surface of a large water body which is not influenced by wind or currents will 

 be level, whether at sea level or at a plateau several hundred meters above sea level. The 

 vertical distance between the surface of an elevated body of water and sea level, however, 

 win be less when measured at the northern extremity than when measured at the southern 

 extremity. It would be awkward to quote different elevations for a single level surface as a 

 function of tlie path used by surveyors in measuring the elevation. Therefore, corrections 

 have been developed to account for the divergence of level surfaces from pole to Equator. 

 These corrections, called orthometric corrections, lead to the assignment of the same value 

 to the height of a level surface regardless of the latitudes at which it is measured. The 

 correction is a function of the mean latitude, the difference in latitude, and the mean 

 elevation of the Une of levels used in the measurement. Orthometric corrections are used in 

 the definition of the NGVD to reduce the uncertainties of surveying the network of 

 geodetic Unes. 



Orthometric corrections do not provide all of the information needed for hydrauhc 

 calculations, since the gravitational force active on any level surface is higher at the poles 

 than at the Equator. Thus, the energy required to lift a given mass between two level 

 surfaces increases from Equator to pole. It is convenient in hydraulic calculations to define 

 the difference in elevation of two points in such a way tliat the energ)^^ required to lift a unit 

 mass from the lower elevation to the higher one is independent of the path followed in the 

 elevation changes. Heights computed in this manner are called dynamic heights. The 

 dynamic height difference, H, between two level surfaces is given by 



H = 



h 

 f g(0)dh (24) 







where g is the value g at a latitude of 45°, g(0) the actual gravity at latitude 

 0, and h the orthometric height. Wlien tlie concept of dynamic heights was first 

 introduced and when running the levels used to define the IGLD, values of gravity 

 computed according to standard formulas for g as a function of latitude were used in 

 computing dynamic heights. With recent improvements in gravimeters, it would be possible 

 to use observed values of gravity in computing dynamic heights. 



Note from the above explanation that if the orthometric and dynamic elevations of a 

 point at the Equator are equal and both are 1,000 meters above sea level, a point with the 

 same dynamic elevation over the pole wiU be beneath the point with tlie same orthometric 

 level, which wiU be only 995 meters as measured above sea level as measured at tlie pole. 

 3. The International Great Lakes Datum (1955). 



The Great Lakes Basin and St. Lawrence River were treated as an integral system in 

 defining the IGLD. The zero of the system was established as the average of all hourly water 



52 



