2. PLANS AND PREPARATIONS 



29 



quires exceptionally good weather. The Chief 

 of Party should make a study of the meteoro- 

 logical conditions of the project area. In 

 most areas there will be periods best suited 

 to certain operations which require good 

 visibility. Such periods often follow a storm 

 when clear weather will prevail for a few 

 days. In some localities there is a dependable 

 cycle of weather conditions during which a 

 period of calm or clear weather can be pre- 

 dicted. Much valuable information on weath- 

 er observations, weather forecasts, and cy- 

 clonic storms will be found in Chapters 37, 

 38, and 39 of H.O. Pub. No. 9, American 

 Practical Navigator (Bowditch 1958). 



2-28 Compass deviations. — For accuracy 

 of hydrographic surveys and the safety of 

 the survey vessel it is essential that the er- 

 rors of the magnetic compasses be kept to 

 a minimum and that their amounts be known. 

 Each survey ship and auxiliary vessel 

 equipped with a magnetic compass shall be 

 swung to determine the compass error, which 

 shall be compensated for, so far as prac- 

 ticable, at the following times: 



(a) After any extensive lay-up period in 

 port, before proceeding to sea. 



(b) On the working ground at the begin- 

 ning of each season's work. 



(c) During the field season whenever there 

 is evidence of an important change in the 

 deviations. 



The above rules apply even though the ves- 

 sel may also be equipped with a gyrocompass. 

 A compass log shall be kept in which should 

 be entered : 



(a) Dates when the vessel is swung, rec- 

 ord of compass adjustments made, and table 

 of deviations. 



(b) Record of azimuths observed. 



(c) Compass comparisons observed (at 

 least once each watch) . 



Instructions for swinging ship and com- 

 pensating compasses are contained in H.O. 

 Publication No. 226, Handbook of Magnetic 

 Compass Adjustment and Compensations. 

 The results of the ship swing and computa- 

 tions of deviations are recorded in Forms 

 354, 355, and 356. 



The deviations of a magnetic compass 



used in hydrographic surveying shall be en- 

 tered on Page 1 of the first volume of the 

 sounding records of each hydrographic sur- 

 vey, and, if changed during the survey, the 

 new values shall be entered in the appro- 

 priate volume, with the date of their appli- 

 cability given. Deviation tables are not 

 required for boat compasses, or when a gyro- 

 compass is used in hydrographic surveying. 

 A deviation table on Form 261, containing 

 the most recent values, shall be posted in 

 the pilothouse or chartroom of every survey 

 vessel. 



2-29 Navigation and seamanship. — The 



operation of sounding from a vessel, and the 

 various other operations performed by a 

 survey vessel underway, requires navigation 

 of the highest order and expert ship handling. 

 Many of the conditions which are encountered 

 almost daily by the hydrographer are met 

 by the average mariner only in emergencies. 

 A knowledge of how to navigate and handle 

 his vessel under all conditions and of the 

 proper use of lines and anchoring gear is 

 essential to the hydrographer. Launches and 

 other boats are used almost daily and it is 

 frequently necessary to retrieve them under 

 adverse conditions. Every officer engaged in 

 hydrographic surveying should endeavor to 

 perfect himself in the theory and practice 

 of navigation. The Commanding officer should 

 provide reasonable opportunity for officers 

 and men to become proficient in all phases 

 of the operation. Each officer's training and 

 experience should eventually qualify him for 

 command. 



2-30 Safety of survey ship. — The Com- 

 manding Officer of a survey ship has a grave 

 responsibility ; he is personally responsible 

 for the lives and safety of his officers and 

 crew and for the safety of an expensive 

 survey ship. 



The nature of hydrographic surveying re- 

 quires that more risk be taken at times than 

 would be necessary in commercial navigation ; 

 this is especially true when operating in an 

 unknown or previously unsurveyed area. To 

 prosecute such work expeditiously it is nec- 

 essary to run some risks that would other- 



