5. HYDROGRAPHY 

 Table 12. — Conversion of reduced soundings 



193 



In areas of rocky bottom or where sub- 

 merged obstacles such as wrecks are pre- 

 sent, the surest and most practical method 

 at present of determining that all obstruc- 

 tions or dangers have been found and least 

 depths over them obtained is by use of a 

 wire drag. 



Most wire-drag surveys will be accom- 

 plished by parties especially organized for 

 that purpose. But survey ships should be 

 equipped with a short wire drag which can 

 be handled by launches in order that limited 

 drag investigations can be made when 

 necessary. 



5-125 Reconnaissance surveys. — A recon- 

 naissance survey is a preliminary survey of 

 an area to obtain information in advance of 

 a detailed survey. It may vary in character 

 from few exploratory sounding lines with 

 little or no control, to an extensive search 

 of a channel or anchorage at an indefinite 

 scale and only approximate relation to the 



adopted geodetic datum. The results of such 

 a survey, if released, must contain a note 

 indicating its quality. 



5-126 Large-scale surveys. — It is occa- 

 sionally necessary to make a hydrographic 

 survey at a large scale, sometimes as large 

 as 1:1,000. These surveys are usually in- 

 tended to furnish detailed information for 

 dredging or other types of harbor improve- 

 ment. Sextant angles can be used to control 

 surveys at scales as large as 1 :2,500, but 

 the control stations must be located precisely 

 and the sextant angles must be marked si- 

 multaneously. To avoid distortion in the 

 sheet, the survey should be plotted on alum- 

 inum mounted paper. For scales larger than 

 1:5,000, parallel ranges should be established 

 along the shore. If a prominent object in 

 the distant background can be seen through- 

 out the area to be surveyed, it may be used 

 as the rear range to a series of front ranges 

 established along the shore. 



