Mf (lunar fortnightly constituent)— This constituent expresses the effect of departure from a 

 sinusoidal declinational motion. 



misclosure— The amount by which a value of a quantity obtained by surveying operations 

 fails to agree with another value of the same quantity held fixed from earUer determi- 

 nations or with a theoretical value of the quantity. See error of closure, traverse; error of 

 closure, triangle; error of closure, levels; and error of closure, horizon. 



mixed (tide)— Type of tide with a large inequality in either the high or low water heights, 

 with two high waters and two low waters usually occurring each tidal day. In strictness, 

 aU tides are mixed but tlie name is usually appUed to the tides intermediate to those 

 predominantly semidiurnal and those predominantly diurnal. See type of tide. 



Mm (lunar monthly constituent)— This constituent expresses the effect of irregularities in 

 the Moon's rate of change of distance and speed in orbit. 



MSf— Lunisolar synodic fortnightly constituent. 



Ml (smaller lunar elliptic diurnal constituent)- This constituent, with Jj , modulates the 

 amplitude of the decUnational Kj for the effect of the Moon's elliptical orbit. A sUghtly 

 slower constituent, designated (Mi), with Qi , modulates the ampUtude and frequency 

 of the dechnational Oi for the same effect. 



M2 (principal lunar semidiurnal constituent)— This constituent represents the rotation of tlie 

 Earth with respect to the Moon. 



M3 (lunar terdiurnal constituent)— A shallow-water compound constituent. See shallow- 

 water constituent. 



nadir— The point of tlie celestrial sphere that is directly opposite tlie zenith and vertically 

 downward from the observer. 



National Geodetic Vertical Datum (NGVD)— A fixed reference adopted as a standard 

 geodetic datum for heights. The datum was derived for land surveys from a general 

 adjustment of the first-order level nets of both the United States and Canada. In tlie 

 adjustment, 21 tide stations in the United States and 5 in Canada were held as fixed. The 

 geodetic datum now in use in the United States is the NGVD of 1929. The year indicates 

 the time of the last general adjustment. The geodetic datum is fixed and does not take 

 into account the changing stands of sea level. Because there are many variables affecting 

 sea level, and because the geodetic datum represents a best fit over a broad area, the 

 relationship between the geodetic datum and local mean sea level is not consistent from 

 one location to another in either time or space. For this reason the NGVD should not be. 

 confused with mean sea level. 



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