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Observations and Collections at Sea
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OBSERVATIONS OF TIDES

Observations of the tides are necessary to establish reference levels for depths and elevations, for reducing soundings to the local reference level (p. 10), for the preparation of tables of predicted tides for use in navigation, and to further the scientific study of tidal phenomena. The rise and fall of the sea surface associated with tidal movements is discussed in chapter XIV, where it is shown that in any locality the range of tide, character of tide, and time of tide with reference to the meridional passage of the moon must be obtained by observation. When adequate data are assembled, the tides at that locality may be predicted with great accuracy.

The essential data are a series of measurements of the elevation of the sea surface at certain time intervals referred to a standard time system, and from these measurements the rise and fall of the sea surface associated with the tide may be plotted as a function of time. In all such studies the elevations must be referred to one or more bench marks, which may or may not be connected with other points of tide observations by accurate leveling. Bench marks are necessary so that the


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measurements of elevation, mean sea level and other reference planes may be referred to some arbitrary standard. If the difference in elevation between the bench marks at two stations is known, the departure of the true sea level from the ideal sea level can be obtained (p. 458).

The simplest device for measuring the rise and fall of the tide on shore is a tide staff. A tide staff is a stout plank that is graduated in feet and tenths of feet or according to the metric system and which is securely fastened to some permanent structure such as a rocky cliff, cement dock, or piling. It must be of sufficient length to extend above the highest tide and below the lowest tide. The graduation must be adjusted with reference to the bench mark, so that if the staff is removed for repairs or replaced, the readings can be referred to a common base. If observations are made every hour, a complete marigram (tide curve) can be constructed, but in many cases observations are made only at certain selected times of the day, or only high and low water are measured. By comparison with the conditions at nearby stations, such random observations are sufficient to determine the character of the local tide. The tide staff is quite accurate in protected water where the waves are small, but on an open coast waves and swells may make it difficult to obtain accurate observations (Rudé, 1928).

In localities where waves cause difficulties, tape gauges are used for visual observations. A float is suspended in a well, commonly a large pipe with small openings below the lowest tide level, and attached to the float is a graduated tape that passes over a pulley, a counterweight being on the other end. The rise and fall of the surface due to waves is largely eliminated, and the tape may then be read with reference to some arbitrary level at appropriate time intervals (Rudé, 1928).

The principle of the tape gauge may be adapted for obtaining a continuous automatic record of the tide level. In the standard automatic gauge used by the U. S. Coast and Geodetic Survey (Rudé, 1928), the float, which is suspended in a well, is attached to a wire that turns a pulley mounted on a threaded rod. As the pulley turns, a carriage with a pencil moves back and forth along the threaded rod that is mounted at right angles to a clockwork-driven roller carrying a sheet of paper. The paper is driven ahead about one inch per hour, and the device thus traces the marigram automatically. Suitable reduction is obtained by varying the size of the pulley and the pitch of the threaded rod. An accurate clock makes a special mark every hour, and a fixed pencil traces a reference line. Short-period waves are largely eliminated because of the damping in the well, but seiches and disturbances of the sea surface lasting several minutes or more are recorded (p. 542). From the marigram the hourly heights and the levels and times of high and low water are easily read off. The standard gauge carries enough paper for one month, but the clocks must be wound once a week, and each day the


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instrument must be checked against a staff or tape gauge to ascertain whether it is functioning properly and to make sure that the holes in the float well are free from seaweed and other detritus. A portable recording unit that can be used by field parties is operated on the same principle.

The devices so far described can be used only on shore or where some rigid structure extends above the sea surface. Many types of pressure-recording devices for use on the sea bottom have been designed. In some the pressure element only is placed below the sea surface, with the recording device on land; in others, such as those to be used far from shore, the recording device is an integral part of the instrument, which may be placed on the sea floor or anchored and left in position for a week or more. Such open-sea tidal recorders are not beyond the experimental stage. Descriptions of the various types may be found in the Hydrographic Review. The character of the tide over shoals in the open sea may be determined from an anchored ship by means of repeated wire or sonic soundings, if the bottom is flat enough to warrant such observations.


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