Current Surveys 
93. Systematic tidal current surveys of the principal waterways of the country, 
which involve considerable expense and equipment, are undertaken from time to time. 
Several vessels are employed and a number of stations are occupied. One station is 
selected as a primary or control station to be maintained throughout the entire period 
of the survey. At the secondary stations observations have frequently been limited 
to 2 or 3 tidal days, but longer series are greatly to be desired, especially at points 
where the current is of considerable importance Whenever practicable, observations 
at important points should cover a period of not less than 2 weeks and preferably a full 
lunar month. 
Selection of Stations 
94. Previous to the field work, the office prepares a plan for the survey which 
includes the proposed current stations plotted on a large scale chart. This preliminary 
selection of stations is based upon the requirements for general information and also 
upon any special purpose for which the survey is being undertaken. Stations located 
in harbor entrances and in main channels are especially important for the purposes of 
navigation, but stations located in secondary channels and at different points in the 
cross section of a stream will also supply information relating to the general circulation 
of the water in a harbor which is of special value to engineers. 
95. After the survey party has reached the working grounds, it may be found 
impracticable on account of local conditions to occupy each station as plotted in the 
office. Traffic conditions or very swift currents may make the occupation of a station 
too hazardous, or the presence of a submarine cable, a pipe line, or an unsuitable 
bottom may preclude anchoring altogether. In such cases the chief of party should 
ascertain if there is a more suitable location available. While current observations are 
usually taken from a floating vessel they may be taken also from a fixed support such 
as a bridge or a platform supported by piles. Observations taken from a fixed support, 
being free from the effects of the pitching and tossing of a vessel, are in general to be 
preferred. 
Organization and Equipment 
96. A current survey party usually consists of a number of units so that several 
stations can be occupied simultaneously. The number of units will depend upon cir- 
cumstances. A typical party consisting of four units requires four boats for observing 
and one tender to carry supplies and enable the chief of party to keep in close contact 
with all of the units. In general, each unit is organized with a personnel of 6 observers 
and a cook, one of the observers being a junior officer who is also in charge of the entire 
unit. As the work is carried on both day and night, it has been found convenient to 
have 4-hour watches with two observers on each watch, one of whom is held responsible 
for the work during the watch. 
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