1. GENERAL REQUIREMENTS 



17 



the area and the change in time and range 

 of tide from place to place. If, on arrival at 

 the working grounds, the selected sites are 

 found to be impracticable, the Chief of Party 

 may make necessary subsitutions, but he 

 must inform the Office of such changes and 

 the reasons for them (see 2-49 to 52). 



Instructions for installation, maintenance 

 and removal of tide gages are contained in 

 Special Publication 196, Manual of Tide Ob- 

 servations. When practicable, observations 

 at each secondary station shall be continued 

 over a period of at least 29 days. The hourly 

 heights of the tide required for reduction 

 of soundings shall be tabulated before the 

 marigrams are forwarded to the Washington 

 Office. Hourly heights from standard gage 

 rolls will be furnished by the Office on 

 request. 



The time meridian used should be clearly 

 marked on the first marigram. When the 

 observations at any station are terminated, 

 a notation of the hour and date of discon- 

 tinuance should be entered on the last mari- 

 gram taken from the gage. The exact loca- 

 tion of each tide station shall be shown on 

 hydrographic sheets (see 6-71). 



1-47 Current observations. — The loca- 

 tion of current stations will usually be indi- 

 cated in project instructions. Observations 

 at each current station shall be continuous 

 for the number of hours specified in the 

 instructions, except that any station at 

 which the current fails to attain a velocity 

 as ?reat as 0.4 knot during the first 25 hours 

 of observation shall be discontinued at the 

 end of the 25-hour period. 



Observations shall be taken half-hourly — 

 on the hour and half-hour — with the current 

 pole and Price meter, Roberts Radio Current 

 meter, or Current Meter Recorder. All tapes 

 should be scaled and checked promptly when 

 taken from the chronograph or recorder. A 

 velocity curve shall be plotted as observa- 

 tions progress. Instructions for current ob- 

 servations are contained in Special Publi- 

 cation No. 215. 



If a vessel or current buoy is to be 

 anchored in or near traffic lanes, adequate 



advance notice shall be published in local 

 and national Notices to Mariners. 



1-48 Oceanography. — Periodic measure- 

 ments of temperatures and salinity are re- 

 quired to compute velocity corrections to 

 echo soundings (see 5-114 to 118), except 

 in areas where satisfactory bar checks can 

 be obtained to the maximum depth of hy- 

 drography. The frequency of observations is 

 a matter that must be left to the hydrog- 

 rapher's judgment, but it must be borne in 

 mind that, to comply with the requirements, 

 the average temperature from surface to 

 bottom used to correct any sounding must be 

 within 2 degrees of the actual mean tempera- 

 ture. At least one serial temperature and 

 salinity should be observed in the deepest 

 part of the area surveyed each month (see 

 3-116). 



Nansen bottles and precision type revers- 

 ing thermometers should be used when avail- 

 able and salinities determined by titration. 

 The Sigsbee water cup with hydrometers and 

 the Tanner-Sigsbee reversing frame and 

 thermometer shall be used when the heavier 

 equipment is not available. 



The project instructions may require 

 oceanographic observations in addition to 

 the temperature and salinity observations 

 required for reduction of soundings. Repeat 

 observations should be made during the sea- 

 son to provide data on seasonal changes. The 

 program of observations at each station 

 shall be in accordance with instructions con- 

 tained in Chapter 2, H.O. Publication 607 

 and outlined as follows : 



(a) Nansen-bottle samples at Interna- 

 tional Nansen-bottle intervals from surface 

 to bottom or to a maximum depth of 2,500 

 meters. Note that oxygen samples must be 

 processed immediately. Chlorinity samples 

 may be retained indefinitely. 



(b) Bottom sample at all stations regard- 

 less of depth. Samples by Phleger Corer are 

 preferred. All samples shall be retained for 

 future analysis. 



(c) Secchi disc readings. 



(d) Sea and swell observations in accord- 

 ance with H. 0. Publication 606-e. 



