Page 701 the smooth sheet 7625 



a. errors in recorded data 



Although every effort is made to have all data recorded with the strictest accu- 

 racy, the smooth plotter must keep constantly in mind the fact that in many cases the 

 hydrograpliy is performed under circumstances conducive to errors in recording. The 

 recorder is frequently working under continuous pressure, and since the hydrography 

 proceeds at a constant rate and it is his duty to record all phases of it, he may fail 

 to correct known errors, or question suspected data. Launch hydrography is par- 

 ticularly susceptible to recorded errors. Hydrographic launches are often propelled 

 by gasoline engines in which the noise level is quite high. Angles and station names are 

 being called by the observers, soundings are being called by the leadsman, and orders 

 are being given, all in a loud voice, and the recorder is supposed to make an accurate 

 record of all of these at the time they occur. 



As the smooth plotter becomes experienced, he will learn the kinds of errors which 

 are most likely to occur in the recorded position data. The following list is not in- 

 tended to be complete, but will serve as a guide to an inexperienced smooth plotter: 



(1) One or more station names may be incorrect — such errors are likely to occur where station 

 names have similar sounds. 



(2) The station names are correct, but two or more of them have been transposed. 



(3) The right angle and left angle may be reversed. 



(4) The degrees and minutes of an angle may be reversed. 



(5) The observer may have read his sextant wrong, the most frequent erroTs being 10°, 30', 

 1°, and 20°, in that order. 



(6) The recorder may have misunderstood the angle, when called out, as for example, when 15 is 

 mistaken for 50, 7 for 11, and vice versa. 



(7) The recorded data appear complete, but actually the right and left angles were not observed 

 to a common center object. 



Corrections to recorded data should be made according to 7741 and figure 164. 

 7625. Protracting Inadequate Fixes 



It frequently happens in three-point fix hydrography that the position data 

 obtained are inadequate for plotting the positions by the conventional three-point 

 method. It is then necessary to use special methods of plotting, or the data must be 

 supplemented by other information before the position can be plotted. Such cases 

 occur, (a) where only one angle has been observed, (6) where the two angles are not 

 observed on a common center object, (c) where the three-point fix is weak, and (d) 

 where the position is located by estimation. 



In (a) , the procedure is first to plot the locus of the single angle. This is accomplished by setting 

 the angle on the protractor and placing the center near where the position is likely to be. The two 

 arms are then made to pass through th^ two stations involved. The center of the protractor will 

 then be a point on the locus of the angle. Several such points are plotted in the vicinity of the po- 

 sition sought and a short pencil arc is drawn through them. The exact position of the vessel is then 

 determined by the use of the dead reckoning in conjunction with the plotted locus. If the locus is 

 nearly perpendicular to the course, the next following position on the line should be protracted and 

 connected with the previous position by the dead reckoning. The intersection of the dead-reckoning 

 track and the locus will then give the position. Where the locus is nearly parallel to the course, 

 a strong determination of the direction of the sounding line is obtained, but the position must be 

 determined on a basis of proportional time between the preceding and following positions, assum- 

 ing there has been no change in speed. Thus, if positions 5A, QA, and 7A were taken at 9:04, 9:07, 

 and 9:11 respectively, then position 6^1 would be plotted at three-sevenths the distance between 5.4 

 and 7 A, on the locus of the observed angle. 



