National Tidal Datum Control Network-A network composed of the primary control tide 

 stations of the NOS. Distributed along the coasts of the United States, this network 

 provides the basic tidal datums for coastal boundaries and chart datums of the United 

 States. Tidal datums obtained at secondary control tide stations and tertiary tide stations 

 are referenced to the network. Terrestrial leveling between stations is not a requirement 

 of the network. 



National Tidal Datum Epoch-The specific 19-year cycle adopted by the NOS as tlie official 

 time segment over which tide observations are talcen and reduced to obtain mean values 

 (e.g., mean lower low water, etc.) for tidal datums. It is necessary for standardization 

 because of apparent periodic and apparent secular trends in sea level. The present epoch 

 is 1941 throu^ 1959. It wiU be reviewed for possible revision every 25 years. 



neap rise— The height of neap high water above the elevation of reference or datum of chart. 



neap tides or tidal currents— Tides of decreased range or tidal currents of decreased speed 

 occurring semimonthly as the result of the Moon being in quadrature. The neap 

 range (Np) of the tide is the average semidiurnal range occurring at the time of neap 

 tides and is most convenientiy computed from tiie harmonic constants. It is smaller than 

 the mean range where the type of tide is either semidiurnal or mixed and is of no 

 practical significance where the type of tide is diurnal. The average height of tlie high 

 waters of the neap tides is called neap high water or high water neaps (MHWN) and the 

 average height of the corresponding low waters is called neap low water or low water 

 neaps (MLWN). 



node cycle— Period of approximately 18.61 Julian years required for the regression of the 

 Moon's nodes to complete a circuit of 360° of longitude. It is accompanied by a 

 corresponding cycle of changing inclination of the Moon's orbit relative to tlie plane of 

 the Earth's Equator, with resulting inequalities in the rise and fall of the tide and speed 

 of the tidal current. 



node factor (f)— A factor depending on the longitude of the Moon's node which, when 

 applied to the mean coefficient of a tidal constituent, will adapt the same to a particular 

 year for which predictions are to be made. 



nodical month— Average period of the revolution of the Moon around the Earth in respect 

 to the Moon's ascending node. It is approximately 27.212220 days in length. 



Nj— Larger lunar elliptic semidiurnal constituent. See L2. 



2N— Lunar elliptic semidiurnal, second order, constituent. 



orthometric corrections— Corrections to the measured difference in elevation to account for 

 the nonparaUelism of level surfaces. Orthometric corrections to the measured differences 

 in elevation are needed to make the measured differences in elevations independent of 

 the route followed in making the geodetic survey. See Section V of this report. 



105 



