Page 277 hydrography 362 



(g) Although the position of tlie danger is correctly shown on the boat sheet, the positions of the 

 survey vessel may not be relatively correct, due to distortion of the boat sheet, weak control, inac- 

 curately located or plotted control, weak three-point fixes, etc. 



(h) A different method of control, or plotting, may make relative differences between the plotted 

 positions of the vessel and the position of the shoal; such as R.A.R. versus three-point fix control, 

 buoy control versus control from distant mountain peaks, plotting by circles versus three-point fixes. 



(i) A combination of small errors and inaccuracies may assume a magnitude sufficient to prove 

 serious when surveying in close proximity to a danger' — errors such as small errors in sextant angles, 

 small protractor errors, small errors in the plotted control, distortion in the boat sheet, the inherent 

 limitations of a small scale. 



362. Sources of Evidence 



The existence of dangers and shoals is disclosed by four generally different 

 procedures : 



(a) Through indications obtained during the systematic survey. 



(b) Through local reports. 



(c) By the shoal itself, or evidence of it, being seen. 



(d) By the examination of air photographs. 



As such indications are discovered, their probable locations should be noted on the 

 boat sheet for further examination to determine their limits and the least depths over 



them (see 3242). 



3621. Evidence From the Survey 



The spacing of the systematic sounding lines and the intervals between soundings 

 along the line are selected with a view to giving at least an indication of the dangers 

 and shoals within the area. It must not be assumed that the least depth will thus be 

 determined in any case. In fact, it is extremely unlikely that any line of soundings 

 will pass directly over the shallowest part of a shoal so that the least water will be 

 obtained at that time. It is, however, expected that some indication of the existence 

 of a shoal will be shown on the nearest line or lines. Such an indication will occur as 

 a break m the continuity of the slope of the bottom. A more positive evidence of the 

 existence of a shoal is found where two adjacent lines of soundmgs each contain similar 

 indications. Splits should be run to determine its extent more accurately. 



Where a sounding is obtained on one of the regular lines which shows even a slight 

 change from the average depth it should be regarded as a possible indication of a shoal. 

 Where such indications occur a very careful and thorough development of the area 

 shall be made to locate more accurately the probable position of the least depth regard- 

 less of any prearranged system of lines. The indications obtamed which require further 

 investigation should be marked in red on the boat sheet by the hydrographer or the 

 Chief of Party to indicate whether additional splits only are required or an investiga- 

 tion is necessary. This decision as to the number of split lines or the amount of devel- 

 opment or investigation required is a particularly important one requiring extensive 

 experience in hydrography. The Chief of Party is finally responsible for the thorough- 

 ness of the survey. 



Rocky bottom being felt by the leadsman in an area of predominantly mud or 

 sand bottom is an indication requiring examination. 



The hydrographer or the Chief of Party must examine the boat sheet periodically 

 with the utmost care and select the soundings requiring further attention. In many 

 localities it is out of the question to examine every shoal indication, nor is this required. 

 In making the selection the importance of the locality and the types of shoals or dangers 

 to be expected must be considered. 



