Page 711 the smooth sheet 767 



shoals, or other dangers or features of importance. They must, therefore, be plotted 

 with extreme care. The plotting of positions on a continuous line of soundings is 

 automatically checked to a certain degree by the dead reckoning. There is no such 

 check on detached positions. The survey requirement for important detached positions 

 is that a third, or check, angle shall be taken at such positions. In plotting the smooth 

 sheet this check angle must always be used by the plotter to verify the correctness of 

 his protracting. The fact that this has been done must be indicated in the Sounding 

 Record by a colored pencil checkmark placed after the third or check angle. 



767. Use of Boat Sheet in SxMooth Plotting 

 76 7i. Comparison With Boat Sheet 



During the protracting of the smooth sheet, constant reference should be made 

 to the boat sheet, as a check on the smooth plotting. Generally, a visual comparison 

 is sufficient, but where difficulty in plotting is encountered or where numerous errors 

 are found in the recorded data, it is sometimes advisable to trace from the boat sheet 

 the positions of an entire line for comparison with the smooth plotting. Where a dis- 

 crepancy between the two is noted, it must be investigated. Such discrepancies may 

 arise from errors in the smooth plotting; from errors in the boat-sheet plotting; or from 

 errors in the recorded data. In determining the position most probably correct, 

 several facts must be kept constantly in mind, the most important of which is that the 

 boat-sheet position is usually plotted independently of the recorded data. The plotter, 

 himself, has usually observed one of the angles and knows which signals he used. The 

 other angle he obtains orally from the other angleman. Furthermore, the officer-in- 

 charge knows what he is trying to do and where the most probable position of the vessel 

 should be, irrespective of where the recorded data place the position. It frequently 

 happens, either intentionally or unintentionally, that an angle is measured to a different 

 signal from that previously used, but in the haste to plot the position, the change of 

 signal is not reported or is not entered in the Record when it is reported. (See also 

 3211.) 



Frequently when some error has been made in the observed data, such as an 

 angle observed on a wrong signal or an error in reading the sextant, the boat-sheet 

 plotter may not have sufficient time to determine what this error is. He plots the 

 position as best he can, and may mstruct the recorder to question either the entire 

 fix or a part of it. The smooth plotter should study such cases. With a little patience 

 he can often determine what the most probable error is, and can make the necessary 

 correction to the recorded data. Such corrections must be made with a colored pencil 

 (see 7741). 



7672. Positions From Boat Sheet 



There are cases in hydrographic surveys where it is impossible to obtain a three- 

 point fix or other position data. This frequently happens in narrow winding channels 

 and sloughs. The hydrographer, in such cases, spots his position on the boat sheet from 

 the adjacent features of the shoreline and from his sense of dead reckoning. The 

 smooth plotter should transfer such positions to the smooth sheet and note that fact 

 in the Sounding Record with his colored pencil. In addition, he should transfer the 

 curved connecting lines between positions, which in narrow channels usually conform 

 to the bends in the shoreline, it being impossible to run a straight course between 

 positions. (See 3252 and 3352.) 



