97 



If then the harmonic components at a station have been computed, 

 and a good determination made of the mean low water datum, 

 formula (109) generally affords a satisfactory determination of the 

 mean low water of spring tides. After a satisfactory determination of 

 mean low water of spring tides has been made at one station, that at 

 other stations m the vicinity may be derived by comparison (par. 202). 



185. Use. — Mean low water of ordinary spring tides is the reference 

 plane for the British Admiralty charts and generally for works of 

 harbor improvement in the British Empire. It is used in some other 

 countries as well. In Canada, low water datum is taken as from 0.5 to 

 1.5 feet below the mean of the lowest low waters of spring tides. In 

 the United States, mean low water of spring tides is used as a datum 

 by the Coast and Geodetic Survey only on the Pacific coast of the 

 Panama Canal Zone, where the range from springs to neaps is marked 

 and regular. 



MEAN HIGH AND LOW WATERS OF NEAP, PERIGEAN, APOGEAN AND 



TROPIC TIDES 



186. These datums are determined in the same manner as the high 

 and low waters of spring tides. Thus the mean high water of neap 

 tides is taken as the mean of the successive pairs of high waters nearest 

 the time of neap tides, and is approximately equal to MHW— S2, the 

 neap range being approximately equal to Mn— 2S2. Mean high 

 water of perigean tide is similarly the mean of the successive pairs of 

 high waters nearest the time of perigean tide, as determined by adding 

 the parallax age to the time of lunar perigee. It is approximately 

 equal to MIIW+N2, whUe mean high water of apogean tide is approxi- 

 mately equal to MHW— N2. The lower low, higher high, higher low, 

 and lower high waters of tropic tides are the averages of the lower low, 

 higher high, higher low, and lower high waters at the time of tropic 

 tides as derived from the diurnal age. As has been stated, these 

 datums are rarely if ever used as reference planes for charts. The 

 elevations of mean high and low waters of neap tides, are however of 

 importance at stations having a marked and regular range from springs 

 to neaps, and especially at ports where navigation is on the tide. 



MEAN LOWER LOW AND HIGHER HIGH WATERS 



187. These planes are sometimes called declinational planes, since 

 the lower low and higher high waters vary with the declination of the 

 moon and sun. Mean lower low water is the average height of the 

 lower of the two daily low waters of tides of the semidiurnal and mixed 

 types. Since the lunar day is longer than the calendar day, occasion- 



