136 



HYDROGRAPHIC MANUAL 



sandy beaches, lines should be run parallel 

 to the shore during periods of high tide and 

 calm weather. In areas of very small range 

 of tide, such as the Gulf of Mexico, there 

 may be a wide band of very shoal water 

 offshore from the low-water line which is 

 difficult and uneconomic to develop. In such 

 areas the inshore lines should be run as close 

 to the shoal as possible, and the hydrography 

 supplemented by a few widely spaced sound- 

 ings obtained from a skiff or by wading at 

 low water. 



On rocky coasts it is frequently imprac- 

 ticable to delineate the low-water line even 

 in part. Where it is dangerous for the sound- 

 ing launch to enter a rocky area along the 

 shore, or where kelp is so thick that the 

 sounding boat cannot navigate through it, 

 the facts should be stated in the record book 

 and the area accurately outlined on the boat 

 sheet with appropriate notes added. 



Before attempting to run sounding lines 

 in rocky inshore areas, the hydrographer 

 should examine the area at low-water, pref- 

 erably at low spring tide, and locate by sex- 

 tant fixes or cuts all breakers and exposed 

 rocks. Sounding lines may then be run at 

 high tide with a greater degree of safety. 



In all cases when the low-water line can- 

 not be delineated by the hydrographic sur- 

 vey, the areas should be fully described in 

 the descriptive reports with an explanation 

 of the conditions preventing the extension 

 of the survey close inshore. Copious notes 

 in the record book and on the boat sheet 

 should be made to show the limits of break- 

 ers, kelp, or foul areas which prevented 

 closer approach to the shore. 



5-18 Sounding. — The various devices by 

 which depth may be measured in hydro- 

 graphic surveying are described in Chapter 

 3, and the general depths in which each is 

 to be used is specified in 1-35. Soundings 

 should always be obtained by a graphic re- 

 corder whenever possible, since this type of 

 instrument yields a permanent record and a 

 continuous profile of the bottom. When sound- 

 ing in areas where kelp or other varieties of 

 bottom cover partially or totally obscure the 

 bottom trace, a leadline or sounding pole must 



be used to supplement or replace echo sound- 

 ings. Hand lead or wire soundings must be 

 true vertical measurements. When bottom 

 samples are being taken, the soundings should 

 not be recorded if the wire or leadline is slop- 

 ing. Such soundings only cause confusion in 

 later processing unless the discrepancies are 

 fully explained in the record. 



Echo sounders record soundings with a 

 consistency and accuracy which is directly 

 related to the care with which the instru- 

 ments are maintained and operated. If there 

 is doubt as to the accuracy of the soundings, 

 or control, for whatever reason, the sound- 

 ing line should be broken immediately and 

 should not be resumed until all uncertain- 

 ties have been resolved. To continue on line 

 under such circumstances is usually a waste 

 of time and often introduces unnecesssary 

 complications in processing (see 5-75). The 

 hydrographer should remember that the re- 

 cording of a sounding is only the first opera- 

 tion in a lengthy process of publishing it on 

 a nautical chart. 



The depths of water shall be measured 

 with the greatest accuracy consistent with 

 efficient progress. No depth-measuring in- 

 strument or method shall be used to sound 

 over relatively even bottom or in critical 

 depths which does not measure depths less 

 than 11 fathoms accurately within one half 

 foot, and greater depths within one percent, 

 unless specifically authorized by the Director. 



In rapidly changing depths and over ir- 

 regular bottoms the requirements may be 

 lowered to 1 foot for depths less than 11 

 fathoms. It is recognized that echo sound- 

 ings in a submarine valley or on a steep 

 slope may be less than the vertical depth 

 under the vessel, particularly when a low 

 frequency echo sounder is used. Slope cor- 

 rections are not required. 



5-19 Systems of sounding lines. — It is 



obviously impossible to measure the depth 

 at every point in the water area. A me- 

 thodical and systematic examination of an 

 area is best accomplished by running a sys- 

 tem of parallel sounding lines (see 1-24). 

 The purpose of the regular system of lines 

 is: first, to furnish a realistic representa- 



