MANUAL OF TIDE OBSERVATIONS 77 



mary tide station suitably located from which the necessary data for 

 the comparison may be obtained. Form 657 for the comparison of 

 monthly means may be used for this reduction. 



245. The name "mean sea level" should be applied only to the 

 datum derived from observations taken on the open coast or in adja- 

 cent waters having free access to the sea. The average of the hourly 

 heights taken in a river is called "mean river level" and is higher than 

 the mean sea level because of the river slope. The plane of half -tide 

 level derived from the high and low waters approximates very closely 

 to the mean sea level or mean river level determined from the hourly 

 heights. For any one station the difference remains nearly constant 

 from month to month and affords a convenient check on the work 

 when both planes are computed. 



246. Mean low water. — ^Mean low water is generally adopted as a 

 datum for hydrographic operations along the Atlantic coast of the 

 United States. It may be defined as the mean of all low waters 

 over a considerable period of time. The datum may be derived inde- 

 pendently from a long series of observations, but from a short series 

 can best be obtained from a comparison of simultaneous observations 

 at a nearby standard station (pars. 240-241). For the longer series, 

 the range of tide is corrected for the longitude of the moon's node 

 (pars. 237-239) , and one-half of the corrected range is then subtracted 

 from the half-tide level to obtain the corrected mean low water. 

 When reduction is made by a comparison of simultaneous observa- 

 tions both range and half -tide level are subject to correction, and the 

 corrected mean low water is obtained by subtracting one-half the cor- 

 rected range from the corrected half -tide level. 



247. Mean lower low water. — This datum is generally adopted for 

 hydrographic operations along the Pacific coast of the United States 

 and may be defined as the mean of the lower of the two low waters 

 of each day over a considerable period of time. When determined 

 independently from a long series of observations, the mean range 

 and diurnal low water inequality must be corrected for the longitude 

 of the moon's node as explained in paragraphs 237-239. The cor- 

 rected mean lower low water is then obtained by subtracting from 

 the half -tide level height the sum of the corrected half raige and the 

 corrected diurnal low water inequality. For a short series of obser- 

 vations, the mean lower low water datum may be computed by means 

 of form 248 for the comparison of simultaneous observations (pars. 

 240-241). 



248. Mean low water spriiigs. — While datums approximating this 

 plane have been rather generally used by foreign countries, its use by 

 this Survey is limited to the Pacific coast of the Panama Canal 

 Zone. The datum may be defined as the mean of the low waters of 

 the spring tides which occur within a day or two after the moon is 

 new or full, and may be obtained by subtracting one-half the spring 

 range of tide from the half-tide level. Because of the limited use of 

 this datum it is not regularly obtained at all tide stations. The most 

 satisfactory method of obtaining the spring range of tide is from an 

 harmonic analysis, an involved process not adapted to field use. 

 From such analyses it has been found that the ratio of spring range to 

 mean range is fairly constant over wide areas. For Balboa, Canal 



735445 O - 47 - 6 . 



