3842 HYDROGRAPHIC MANUAL PaGE 302 



Each method has its own advantages depending on the depth and the character 

 of the bottom. Fine sand will adhere readily to an armed lead, but is likely to be 

 entirely washed out from a snapper, especially if small pebbles or shells are mixed 

 with the sand and are caught between the jaws, holding them partly open while being 

 hoisted to the surface. Soft mud will scarcely adhere to an armed lead at all, but an 

 excellent sample of it may be obtained by the use of a snapper. 



Neither the armed lead nor the snapper will secure specimens of a rocky bottom 

 and the best possible indications of this are the feel of the leadline after the lead strikes 

 the bottom and abrasions which may be found on the bottom and sides of the lead. 



It must be realized that the specimen obtained on an armed lead is froiii the 

 surface layer of the bottom, which may be of very different character from that 

 immediately beneath. 



In addition to an examination of the specimens brought to the surface, leadsmen 

 and sounding-machine operators must be taught to gage the consistency of the bottom 

 material by feeling through the leadline or the wire the way in which the lead strikes 

 the bottom. 



The data on bottom characteristics determined during the sounding must be sup- 

 plemented by examinations of the material brought up on the flukes of the anchor or 

 on buoy anchors and these data should be recorded in the Sounding Record with their 

 positions given as closely as Icnown. 



3842. Frequency of Bottom Characteristics 



Wlien sounding with a handlead, the character of the bottom should generally be 

 determined on every fixed position and must be determined at least once on each page 

 of the Sounding Record. In harbors, anchorages, and channels the coverage should 

 be more complete than is necessary elsewhere. Along the open coasts and in large 

 bays and similar areas where tests have indicated that a sameness of bottom material 

 is to be expected, a lesser number is required. 



An attempt shall be made to determine the bottom characteristics on each^hoal 

 and bank within the range of visual fixes. On ofi'-lying shoals and banks of 50 fathoms 

 or less beyond the range of visual fixes, such bottom characteristics shall be determined 

 as are practicable in each case. If of exceptional importance one or more survey buoys 

 may be located for use in referencing spot stops for the purpose of obtaining bottom 

 data, especially if survey buoys are being used in the hydrographic control of the area. 



When soundings are taken by machine and wire, the character of bottom shall be 

 determined at least at each position, and if the soundings are, in comparatively deep 

 water and widely spaced, a determination should be made at each sounding. 



Since the use of echo sounding has become more prevalent in hydrographic sur- 

 veying, the character of the bottom cannot be determined with the ease and frequency 

 it could when handlead and wire soundings were more commonly used. When a re- 

 survey is being made of an area where sufficient determinations of the bottom were 

 made during a prior survey, it will be sufficient to test a small percentage of these to 

 determine whether there have been any changes, and if not, the coverage of the prior 

 survey in this respect may be considered a part of the coverage required for the new 

 survey. 



Echo-sounding surveys of harbors, anchorages, shoals, and banks in areas where 

 the hydrography is controlled by visual fixes shall be followed by a second coverage 

 of the area during which spot stops are made to secure the information relative to 

 bottom characteristics. Supplemented by data from prior surveys, the coverage in 



