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HYDROGRAPHIC MANUAL 



tuate the differences in shading of the traces. 

 Since it is not always possible to determine 

 the line of separation, doubtful soundings 

 should be investigated by a handlead. It is 

 not practical to investigate all such sound- 

 ings in a generally foul area, but the limiting 

 features must be carefully sounded, and 

 any indication of a pinnacle in or adjacent 

 to a navigable waterway must be examined. 



Difficulties of fathogram interpretation 

 when kelp or grass is present, increase with 

 increased transducer frequency. Echoes 

 from kelp and echoes from the bottom are 

 distinguishable on many 808 fathograms, 

 but are considerably less discernible on 

 EDO-255 records, and bottom echoes are 

 completely masked when very high frequency 

 transducers are used. In consequence more 

 leadline or pole soundings are required to 

 substantiate fathogram interpretation when 

 transducer frequency is increased. 



In areas of heavy grass the bottom trace 

 may be completely obscured, as in Figure 

 73D. Usually the top of the profile will 

 have a ragged appearance in contrast to the 

 smooth bottom, however, in some instances 

 the top of the grass will appear no more 

 irregular than many ragged bottom profiles. 

 In Figure 74(A) the bottom can be easily 

 identified in the gaps between traces from the 

 grass, and the bottom profile can be drawn 

 with assurance. Unless a bottom sounding 

 can be identified with sufficient frequency 

 for a basic survey, supplemental leadline or 

 pole soundings must be obtained. 



Side echoes produce traces which may be 

 detached from the bottom trace, blend with 

 the bottom trace, or mask it completely. It 

 is not always possible to distinguish between 

 a side echo and echoes from fish or turbu- 

 lent water. Occasionally a side echo is the 

 only indication of a boulder, pinnacle, or 

 other submerged obstruction. Side echoes 

 should not be ignored, neither should they 

 be recorded as true depths. Such soundings 

 should be noted in the remarks column and, 

 unless they come from previously identified 

 shoaler features, additional lines should be 

 run to obtain the least depth on them. Typi- 

 cal side echoes are shown in Figure 74(B). 



Where the side echo represents the shoalest 

 depth obtained on an isolated feature, it 

 should be accepted and plotted as the least 

 depth thereon. 



Irregular profiles resulting from swells or 

 choppy seas are similar to profiles of sand 

 waves. When the irregular profile is caused 

 by sea conditions, readings should be taken 

 along a line representing the mean depth, 

 and not from the top of the peaks. The 

 sounding record and fathogram should con- 

 tain notes establishing the cause of the ir- 

 regular profile. When this information is 

 lacking, an examination of the fathogram 

 will usually reveal the cause. Chop or swell 

 will be apparent over the whole line, and 

 the character of the graph will be changed 

 when going with or into the sea. 



5-123 Final reduced soundings. — After 

 all check scanning of fathograms has been 

 completed and all reducers entered in the 

 sounding record and verified, the recorded 

 soundings shall be reduced by the algebraic 

 sum of the corrections and the corrected 

 soundings, entered in the column headed 

 "Field." These soundings shall always be en- 

 tered in the same unit used for recording 

 them, either feet or fathoms. Since only one 

 unit may be used on a smooth sheet, it is 

 sometimes necessary to convert recorded 

 soundings to the unit to be used in plotting. 

 The conversion shall be made as shown in 

 Table 12 and the results entered in the dou- 

 ble column headed "Office." 



The reduction of soundings and conver- 

 sions, if any, shall be checked. To indicate 

 that this has been done, a check mark shall 

 be placed opposite each verified correction 

 entry and at the bottom of each column of 

 reduced or converted soundings. The initials 

 of the persons who made the original en- 

 tries and of those who checked them shall 

 be entered in the appropriate spaces of rub- 

 ber Stamp No. 38. 



Special Types of Surveys 



5-124 Wire-drag surveys. — Wire-drag 

 equipment and survey methods are described 

 in the Wire Drag Manual, Publication 20-1. 



