MANUAL OF TIDE OBSERVATIONS 



PURPOSES OF TIDE OBERVATIONS 



1. The tidal work of the Coast and Geodetic Survey is carried on 

 for the purpose of serving the needs of the mariner, the engineer, the 

 scientist, and the public generally. The work had its origin in the 

 necessity for reducing to a common level or datum plane, soundings 

 taken at different stages of the tide, and this still constitutes one of the 

 important purposes. Among other outstanding purposes are (a) the 

 determination of tidal datum planes for general engineering work, 

 (&) the derivation of data for the prediction of tides, (<?) the securing 

 of information pertaining to the mean and extreme rise and fall of the 

 tide which may be necessary in the construction of piers, bridges, and 

 other structures for 'which the tidal condition is an important factor, 

 (d) the securing of data for the study of crustal movements in the 

 earth. A continuous series of tide observations in any region also 

 provides data for the reduction of shorter series of observations in 

 nearby areas and furnishes information often required in connection 

 with legal cases involving maritime interests. 



2. Tide stations may be classified as primary and secondary. Pri- 

 mary stations are those at which observations are to be continued for 

 a number of years for the purpose of deriving basic tidal data for the 

 locality. A secondary station is one which is operated over a very 

 limited period of time to obtain tidal information for a particular 

 purpose and the length of series will depend upon that purpose. 



TIDE GAGES 



3. A tide gage is an instrument for measuring the height of the tide. 

 Tide gages may be divided into two groups — nonregistering gages 

 which require the presence of an observer to take and record the 

 height of the tide, and self -registering or automatic gages which auto- 

 matically record the rise and fall of the tide while unattended. 



4. The first group includes the tide staff and some types of float and 

 pressure gages. The second group includes a variety of types, some 

 of which record the rise and fall of the tide in the form of a graph, 

 others by printed figures, and others photographically. The two prin- 

 cipal kinds of automatic tide gages used by the Coast and Geodetic 

 Survey record by means of graphs. One of these, known as the 

 "standard automatic tide gage," is designed for use at primary tide 

 stations or where observations are to be continued for a considerable 

 period of time. The other, known as the "portable automatic tide 

 gage," was designed for use at tide stations which are to be continued 

 for only a short period and where ease of installation is a desired 

 factor. 



