768 HYDROGRAPHIC MANUAL Page 712 



There are occasional cases where position data, although recorded, are so erroneous 

 that they obviously cannot be used. In such cases the smooth plotter's last recourse 

 is the boat sheet. Often by placing the center of the protractor on the boat-sheet 

 position and trying various combinations of angles and signals, he will be able to de- 

 termine what the recorded data should have been. 



768. Identification of Positions 



Each hydrographic position, regardless of the method used in plotting, shall be 

 marked on the smooth sheet by a small prick hole and identified by the position number, 

 the prick hole being accentuated by a fine ink dot of the same color as the position 

 number and day letter (see 7681). At the beginning and end of each line, at every 

 intermediate fifth position, and at any pronounced change in the direction of the line, 

 the position number shall be accompanied by the day letter. 



7681. Position Numbers and Day Letters 



Beginning with number 1, consecutive numbers are assigned to positions during 

 each day, and each day's field work by each vessel is identified by a day letter (see 

 3311 and 3312). The day letters inked on the smooth sheet must correspond in color 

 and case with the entries in the Sounding Record, and the position numbers must corres- 

 pond in number with the entries in the Sounding Record, and in color with the day letters. 

 The colors to be used for this purpose are blue, purple, green, and red, preference being 

 given in that order for the predominant color on a sheet. The use of black and yellow 

 is forbidden (see 8121). A contrast is desirable between two colors used for this pur- 

 pose on the same sheet. 



Position numbers and day letters are usually inked as each sounding line or page 

 of the Sounding Record is protracted. This is generally often enough to avoid errors. 

 In congested areas, it is sometimes convenient to connect the prick holes representing 

 the positions with pencil lines on the overlay tracing, to assist in their subsequent identi- 

 fication on the smooth sheet. 



The position number must be so small that it can never be confused with a sound- 

 ing, and yet should be large enough so that it can ordinarily be read without a magnify- 

 ing glass. Vertical figures and letters shall be used, and these shall ordinarily not exceed 

 1 mm in height. In open areas, where the soundings will be inked larger than average, 

 the position numbers and day letters may also be slightly larger than the average, while 

 in areas of close development, where the soundings wiU have to be inked smaller than 

 average, the position numbers and day letters should be somewhat smaller than average 

 (see 7732 and fig. 163). 



Because of their small size, considerable care should be taken in inking the 

 position numbers and day letters, and in their placement on the sheet. Their purpose 

 is to permit ready comparison of the plotted data with the Sounding Record, and if 

 illegibility makes this difficult, the purpose is defeated. 



The position number should be placed preferably just below and to the right of the 

 position dot, with enough space left for the entry of the soimding so that when the latter 

 is inked the position number will be close to, but just below and to the right of it, con- 

 sideration being given to the size of the sounding to be inked at the position. (See also 

 7762.) 



In congested areas, and especially in those areas in which considerable development 

 has been done, it is usually advisable not to ink the position numbers and day letters 



