6. THE SMOOTH SHEET 



217 



of hydrographic surveying — foggy or hazy 

 visibility, the distortion of conversation 

 caused by wind, sea, and motor noise and 

 occasional confusion resulting from rapid 

 mobile operations. 



In many cases an erroneous position can be 

 salvaged by recognizing the nature of the 

 error and correcting the recorded data. The 

 data shall never be erased or obliterated ; re- 

 vision shall be entered in colored pencil 

 and the erroneous data crossed out. The 

 boat sheet may show the position correctly. 

 Errors which are most likely to occur in 

 recorded position data are as follows : 



(a) The station name may be incorrect 

 because (1) a change in the observed station 

 was not reported to the recorder, (2) the 

 recorder misunderstood the name for a simi- 

 lar name, or (3) the wrong object was ob- 

 served because of misidentification of the 

 station, in which case one angle was rejected. 



(b) The angles or the degrees rnd min- 

 utes may be reversed in the record. 



(c) The sextant might have been read 

 erroneously. Common-type of errors are the 

 misreading of an adjacent division, either 

 20' or 30' where either of these values is 

 the smallest arc division ; or 10' on the drum 

 of the Navy-type sextant; 1° on either sex- 

 tant; 2°, 4°, or 6° when a 1°, 2°, or 3°- 

 increment is erroneously applied on the 

 wrong side of the longer 5° or 10°-division; 

 5° and 10° for the same reason as for 

 10', 20', 30' and 1°. Although past expe- 

 rience at time of sextant observation and 

 in subsequent plotting attest to repetition 

 of these errors, they shall not be corrected 

 during plotting unless the substituted value 

 is absolutely confirmed by other factors, 

 such as time and course, boat-sheet plotting, 

 reference hydrography or objects, etc. 



(d) The recorder may have misunderstood 

 the value of the angle and recorded 15 for 

 50, 7 for 11, 5 for 9, and vice versa. 



(e) A station position is in error, such 

 as a weakly located hydrographic or supple- 

 mental station, in which case hydrographic 

 discrepancies are traceable to its repeated 

 use. Sometimes more careful evaluation of 

 original observations and search for later 



observations in the records will result in 

 a corrected position. 



(f ) Electronically controlled positions, may 

 be in error because of deficiencies in cor- 

 rection values, including corrections for 

 inclined distances where the ship is close 

 in to a highly elevated shore station. The 

 latter may occur off coasts where the inshore 

 limit of ship hydrography makes junction 

 with visually controlled launch hydrography. 



6-51 Position identification. — Each plot- 

 ted position should be carefully pricked 

 through the overlay sheet to the smooth sheet. 

 The needle point should not be pushed 

 through the smooth sheet, but should make a 

 small identifiable indentation in the surface 

 of the paper. After a line or several lines of 

 positions have been plotted, the cover sheet 

 shall be rolled back and the positions on 

 the smooth sheet identified by number and 

 day letter. Each position point shall be ac- 

 centuated by a fine dot of ink of the same 

 color as the day letter and number and shall 

 correspond to the color used in the sounding 

 record. Pencil lines shall be drawn lightly 

 between positions as guides for plotting 

 soundings. 



Position numbers are usually added in pen- 

 cil at intervals sufficiently frequent to permit 

 positive identification of all positions when 

 the protracting has been completed. It is 

 especially important to defer inking position 

 numbers in areas of closely spaced hydrog- 

 raphy, but it is equally important that the 

 positions shall not be misidentified when the 

 numbers are inked. 



Position numbers are inked in color and 

 case as shown in the sounding record (see 

 5-34 and 35), with vertical letters and nu- 

 merals about 1 mm high (Fig. 82). Each 

 position shall be numbered and the day letter 

 shall be shown at the first and last position 

 on a line, at each significant bend in the 

 line, at each position whose number is a 

 multiple of 5, and at each detached position. 

 If possible the number should be placed be- 

 low and to the right of the position dot 

 with enough space between them to permit 

 inking the sounding without crowding or 

 obscuring the position number. If a number 



