5. HYDROGRAPHY 



149 



are used for boat sheet plotting within the 

 limits imposed by the length of the arms. 

 The plotting is done very rapidly so that 

 the course may be adjusted as required to 

 follow the prescribed line. However, the po- 

 sitions should be plotted as accurately as 

 time and circumstances permit. If it is an- 

 ticipated that the smooth plotting will be 

 done by the film positive transfer method 

 (see 6-47) the boat sheet positions must be 

 plotted accurately. 



The hydrographer usually estimates where 

 he will be at the next fix on the basis of 

 his course and the distance run during the 

 time between fixes. This will assist in plot- 

 ting the fix, since he can center the protrac- 

 tor at the estimated position, and if there is 

 an appreciable difference between the esti- 

 mated and plotted positions, the fact is im- 

 mediately apparent. If the distance be- 

 tween plotted positions is correct, but the 

 last position is slightly off the proposed line, 

 a change of course is indicated. If the dis- 

 tance between positions is significantly dif- 

 ferent, or if the last position falls far off 

 course it is probable that the position was 

 incorrectly plotted or the position data are 

 incorrect. The plotter should immediately 

 request the anglemen to verify the sextant 

 readings and the objects used in the fix at 

 the same time checking to see that he has 

 correctly laid off the angles on the protrac- 

 tor. If no errors are discovered the fix 

 should be questioned in the record book and 

 another fix observed as soon as possible. At 

 the end of the day each questioned or doubt- 

 ful fix must be examined and a decision 

 reached as to its accuracy. The final action 

 taken shall be noted in the record book over 

 the initials of the hydrographer. Jumps in 

 the line which are not otherwise explained 

 are probably caused by an incorrectly lo- 

 cated signal. 



When electronic distance measuring de- 

 vices are used to control hydrography, dis- 

 tance circles are drawn on the sheet (see 

 5-11) and positions are plotted by an Odes- 

 sey protractor (see 3-22). The protractor 

 is placed on the sheet so that its center 

 marks the increment or decrement from the 



nearest distance circle for one station, the 

 partial distance being measured by the close- 

 ly spaced concentric circles. The interpolated 

 concentric circle on the protractor will be 

 tangent to the distance circle on the sheet. 

 The protractor is then moved until the cen- 

 ter marks the measured distance from the 

 second station while keeping it in its correct 

 relationship to the first distance circle. The 

 position is then pricked by a needle through 

 a small hole at the center of the protractor. 

 Questionable position data must be treated 

 in the same manner as that prescribed for 

 doubtful sextant fixes. 



5-51 Supervision by the Chief of Party. 



— The number of men required to operate a 

 survey unit varies with the size of the ves- 

 sel, area of operation and type of control 

 being used. On a large survey ship engaged 

 in surveys on a 24-hour basis at least three 

 watches are required, while a launch party 

 normally has a complement just adequate for 

 operations during an 8-hour day. 



The Commanding Officer, or Chief of 

 Party, is responsible for all the work being 

 done by the party. He plans the work and 

 supervises all operations. He must inspect 

 the boat sheets and records daily, if possible, 

 to assure himself that the work is being ac- 

 complished in accordance with the instruc- 

 tions, that it is complete, that the data are 

 clearly and correctly recorded, that all indi- 

 cations of shoals are examined, and that all 

 navigable channels are developed (see 1-43). 

 From this daily examination of the boat 

 sheet, the Chief of Party indicates to the 

 hydrographer any additional sounding lines 

 to be run or what practices and procedures 

 are sub-standard, if there are any. A final 

 examination of the completed boat sheet 

 must be made before leaving the working 

 ground to ensure the adequacy and complete- 

 ness of the survey. 



The Chief of Party need not sign the 

 sounding records, but in his final approval 

 of the survey he shall state the amount of 

 his personal supervision given to the field 

 and office work (see 7-11). 



5-52 Officer-in-charge. — The officer-in- 



