17a (Change 1) 



HYDROGRAPHIC MANUAL 



Stations observed at only one depth. — In ro- 

 tary currents, any period during wiiich the 

 meter has been inoperative shall be made up at 

 the end of the series. In revei-sin_g currents, a 

 break of less than V2 hours in the series of obser- 

 vations may be ignored and the series may be 

 discontinued at the end of the specified period: 

 if the break exceeds 12 hours, resumed observa- 

 tions shall continue through the unobserved 

 part of the series plus an additional 25 hours. 

 In general, observations shall be for the number 

 of complete tidal days (25 hours each), not 

 necessarily in succession, specified in the jiroject 

 instructions. 



tStatlons observed at more than one depth. — A 

 break of less than 12 Iiours in obsei'vations at 

 one depth is not critical if an unbroken series is 

 obtained at other depths. If the break exceeds 

 12 hours, observations shall continue at all 

 depths as outlined for similar breaks in one- 

 depth ohservatious of reversing currents. 



Groups of stations observed concurrently. — 

 After a break in observations at any one station 

 or depth, determine the additional period of 

 observations as for a single station and continue 

 observations at all stati(ms of the group for the 

 same additional period. 



In order to obtain the maximum possible 

 velocity from the current observations, the 

 Commanding Officer shall make every effort to 

 observe the current during the period of the 

 month when astronomically the velocity should 

 be at its strongest. In waters where there is a 

 semidiurnal or mixed type of tide, the period of 

 maximum velocity is near times of new or full 

 moon (spring tides) at perigee (moon nearest 

 the earth). Where the tide is diurnal, the 

 maximum velocity occurs near the times of the 

 moon's greatest north and south declinations. 

 Astronomical data for a particular year can be 

 obtained from any tide table or tidal current 

 table for that year. 



Dubious prospects of completing a full pe- 

 riod of observations shall not be allowed to 

 interfere with occupation of a station; any sta- 

 tion occupied, even for a period less than that 

 specified, will contribute to the oceanographic 

 knowledge of the area. When adverse weather 

 causes obvious distortions of plotted current 

 data, station observations shall be discontinued 

 until the return of normal weather. 



Each station occupied shall be accurately lo- 

 cated by sextant fixes or other acceptable means 

 and plotted on a section of the largest-scale 

 chart of the area. This chart section shall be 

 attached to the first record book for the par- 

 ticular station. 



^AHien the Roberts Radio Current Meter is 

 used, all current tapes shall be scaled and 

 checked promptly on removal from chrono- 

 graph or current-meter recorder. It is also im- 

 jjortant that observers properly mark all tape 

 sections used in determining the velocity and 

 direction of the current and the seconds of time. 

 Do not discard tapes that show no velocity or 

 direction signals because of equipment failure 

 or because of a zero velocity at slack water. A 

 tape for each entry in the log book shall be 

 forwarded to the Washington Office. 



The field party shall maintain a running plot 

 of the half-hourly observations of current 

 velocities and directions for each meter. Cor- 

 rective measures shall be taken if any data 

 appears questionable. 



The observer must be thoroughly familiar 

 with the chronograph and its operation if he is 

 to obtain good tapes. The Roberts Radio Cur- 

 rent Meter Operating Manual says that fast 

 tape si>eed is required to permit good scaling 

 of rapid signals (strong currents) ; slow tape 

 speed is advised for slow signals (weak cur- 

 rents) to condense the record to convenient 

 lengths. 



