5. HYDROGRAPHY 



139 



vantages; first, the effects of current can 

 be observed directly and corrections applied 

 to maintain the desired line spacing ; and sec- 

 ond, sounding line spacing can be reduced by 

 running "splits" at any desired interval. 

 When properly used the maximum line spac- 

 ing for the depth can be used and the control 

 is so positive that this maximum should 

 seldom, if ever, be exceeded'. Since lines 

 can always be run exactly where they are 

 proposed, very few lines are wasted. 



When this system is used the hydrographer 

 has a choice of paralleling segments of two 

 sets of arcs. He should select the set which 

 will provide the most efficient coverage of 

 the area and best development of submerged 

 features. The system is most suitable for 

 inshore hydrography in wide passages, in 

 areas where offshore islands are available 

 for shore stations, and in wide bays or es- 

 tuaries. It may be used by ships in offshore 

 surveys where the bottom slope is slight, 

 but should generally not be used where 

 sounding lines pass over steep slopes at an 

 angle less that 45°. 



A system to provide automatic steering 

 along an arc has been developed but is not 

 in common use. Information is furnished by 

 the Shoran or Raydist to the gyro-pilot 

 which responds to bring the ship to the 

 proper distance from the shore station. 



5-22 Systenis of radiating lines. — Off 



points where there is a marked change in the 

 trend of the shoreline, in small bays and 

 around off-lying islets, radiating lines gen- 

 erally provide the most adequate develop- 

 ment. These are merely special cases of sys- 

 tems designed to provide sounding lines 

 normal to the shore, and, because of their 

 divergence, they are comparatively short 

 lines. It is sometimes convenient to develop 

 a shoal of small extent by running a system 

 of radial lines crossing in the vicinity of a 

 temporary marker buoy planted near the 

 center of the shoal. 



5-23 Cross lines. — In addition to the re- 

 gular system of sounding lines adopted to 

 accomplish the survey, a second, but not less 

 important system of lines must be run on 



most surveys. These are the cross lines 

 which are designed to disclose discrepancies 

 in the regular system of soundings. Such 

 discrepancies may arise from the use of a 

 faulty plane of reference, inadequate or 

 weak control, an operational failure of the 

 echo sounder, or unusual tide levels caused 

 by strong winds. Crosslines shall be run as 

 specified in 1-26. 



The shallow water sounds and bays along 

 the Atlantic and Gulf Coasts are subject to 

 unusual tides as a result of strong winds, if 

 a system of cross lines is run first in such 

 areas as directed in 1-26 (e), and under 

 normal condition of wind and sea, abnor- 

 mal tides will be reflected by poor crossings. 

 The tide observing program can be supple- 

 mented by a short series of observations 

 on a staff in the immediate vicinity, or an 

 arbitrary correction can be deduced with 

 reasonable accuracy by comparison of sound- 

 ings at crossings. 



5—24 System of sounding lines in chan- 

 nels. — The limits of narrow channels should 

 first be defined by a series of cross lines which 

 may be run normal to the axis or diagonally 

 across it. In either case, extreme care is 

 required in order to avoid displacement of 

 depth curves on steep slopes. Variations in 

 speed at the beginning and ending of short 

 cross lines, if not properly plotted, produce 

 inaccuracies which are indicated by unnat- 

 ural waves in the depth curves along the edge 

 of the channel. 



After the limits of the channel have been 

 established, it must be completely developed 

 by a system of closely spaced lines parallel 

 to the axis of the channel. If the channel 

 is marked by a range, it is essential that a 

 line of soundings be run exactly on the range 

 line. 



Most dredged channels are maintained by 

 the Corps of Engineers, but some are pri- 

 vately maintained. Nautical charts contain 

 the latest available information on dredged 

 channels as to project depths and controlling 

 depths on a certain date. Data on primary 

 channels are usually published in tabular 

 form in which the least depth is given for 



