7622 HYDROGRAPHIC MANUAL PaGE 700 



7622. With Celluloid Protractor 



The use of the celluloid protractor (4536) shall be limited to areas of the smooth 

 sheet where the stations are never more than 10 inches from the positions, the sum of 

 the two angles is not less than 60°, and the smaller of the two angles is not less than 20°. 

 Positions based on stations farther away than 10 inches or based on angles smaller than 

 the above prescribed limits, must be plotted with a metal protractor. Critical positions 

 shall be plotted with a metal protractor, if practicable. 



7623. Positions Close to Control Stations 



Celluloid protractors of the present model permit plotting three-point fixes which 

 are quite close to the control stations, but because of the opaqueness of the protractor 

 near its center, it sometimes happens that even this cannot be used satisfactorily. In 

 such cases the angles of the thi-ee-point fix are plotted on a piece of tracing paper, or on 

 a transparent paper protractor (4539), wliich is then manipulated the same as a three- 

 arm protractor to plot the position. 



7624. Procedure for Protracting Positions 



The Sounding Record is the official record of the sounding and position data, and 

 the plotter should follow the recorded data strictly, unless it is apparent that an error 

 has been made. Since the system of field work requires that the sounding vessel pro- 

 ceed at as nearly a constant speed as practicable (see 3461), and since the hydrographer, 

 almost invariably, attempts to run straight lines, the dead-reckoning position of the 

 vessel provides a reasonable check on the recorded position data. The boat sheet pro- 

 vides the best check since the positions thereon were plotted by the officer-in-charge at 

 the time the work was executed, and he was in a position to know where his vessel was at 

 the time a particular position was taken (see 7671). 



Lines of soundings are run systematically, and it is usual for a number of positions 

 to occur on a comparatively straight line (see 314). The smooth plotter, therefore, 

 usually protracts all of the positions recorded on one page of the Sounding Record, or on 

 one straight sounding line, before comparing them with the boat sheet and connecting 

 them with penciled lines. When the plotter has assured himself that the positions of 

 the series have been correctly protracted, a checkmark in colored pencil must be placed 

 in the Sounding Record alongside each position number. A pencil of identical color 

 should be used by the smooth plotter throughout his work, in making entries and 

 corrections in the Sounding Records. 



As the protracting progresses, the notations in the "Remarks" column should be 

 noted in order that due attention may be paid to any remark which may affect the 

 smooth plotting, and for information regarding features to which cuts were taken or 

 which were otherwise located during the progress of the survey. As each such item 

 is applied to the smooth sheet, this should be indicated by placing a checkmark after 

 it in the Sounding Record. 



It is usual to protract all of the positions on the smooth sheet before the soundings 

 are penciled, including the detached positions and all the cuts and other data that 

 need to be located on the smooth sheet. When the protracting has been completed, 

 the smooth sheet should contain all position information and, in general, it should 

 not be necessary to use the protractor during the subsequent plotting of the soundings, 

 except for occasional verification of a position. 



