datum (vertical) or datums— For marine applications, a base elevation used as a reference to 

 reckon heights or depths. It is called a tidal datum when defined by a certain phase of 

 the tide. Tidal datums are local datums and should not be extended into areas which 

 have differing topographic features without substantiating measurements. In order that 

 tliey may be recovered when needed, such datums are referenced to fixed points known 

 as bench marks. See chart datum. 



declination— Angular distance north or south of the celestial Equator, taken as positive (+) 

 when north and negative (— ) when south of the Equator. The Sun passes through its 

 decUnational cycle once a year, reaching its maximum north declination of 

 approximately 23V2° about June 21 and its maximum soutli declination of 

 approximately —2Vli about December 21. The Moon has an average decUnational cycle 

 of 27V3 days which is called a tropical month. Tides or tidal currents occurring near the 

 times of maximum north or soutli declination of the Moon are called tropic tides or 

 tropic currents and those occurring when the Moon is over tlie equator are called 

 equatorial tides or equatorial currents. The maximum decUnation reached by the Moon 

 in successive months depends upon the longitude of the Moon's node, and varies from 

 28V2° when tlie longitude of the ascending node is zero to 18V2° when the longitude of 

 the.node is 180°. The node cycle or time required for the node to complete a circuit of 

 360° of longitude is approximately 18.6 years. See epoch (2). 



diurnal— Having a period or cycle of approximately 1 tidal day. Thus, the tide is considered 

 diurnal when only one high water and one low water occur during a tidal day, and the 

 tidal current is considered diurnal when there is a single flood and single ebb period in 

 the tidal day. A rotary current is diurnal if it changes direction through all points of the 

 compass once eacli tidal day. A diurnal constituent is one which has a single period in the 

 constituent day. The symbol for such a constituent is usually distinguished by tlie 

 subscript 1. 



diurnal range of the tide— See great diurnal range. 



dynamic elevation or dynamic height— The difference in geopotential between two level 

 surfaces. The dynamic height difference between two points A and B is J^g dh. 

 If g is measured in kUogals (one kilogal = an acceleration of 10^ centimeters per square 

 second) and h in meters, the dynamic elevation is given as geopotential units (GPU). 

 The numerical values of the dynamic heights are about 98 percent of the geometric 

 height value. 



elevation— The vertical distance of a point above or below a reference surface or datum. In 

 the fundamental horizontal control survey of tliis country, the datum for elevations is 

 the National Geodetic Vertical Datum (NGVD) of 1929, formally called the Sea Level 

 Datum of 1929. 



elevation, adjusted— The elevation resulting from the application of an adjustment 

 correction to an orthometric elevation. Also the elevation resulting from tlie application 

 of botli an orthometric correction and an adjustment correction to a preliminary 

 elevation. 



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