5. HYDROGRAPHY 



189 



varied by an amount sufficient to require 

 correction and all necessary corrections 

 should be entered in the record book at the 

 appropriate places. If soundings have been 

 obtained on more than one scale (phase) 

 all changes of scale shall be correctly labeled 

 on the fathogram and noted in the sounding 

 record. If a template is used when scaling 

 the soundings, the paper speed should be 

 checked at each setting of the template. If 

 a template is not used, several speed checks 

 should be made at random during the day's 

 work. If there is evidence of incorrect speed, 

 a detailed examination is necessary to de- 

 termine where corrections are required and 

 their amounts. 



The soundings should then be scaled at 

 the selected interval and compared with the 

 recorded soundings, correcting the latter 

 when necessary. All significant peaks and 

 deeps which occur between the soundings 

 recorded at regular intervals shall be scaled 

 and entered in the record book and the time 

 or fraction of sounding interval recorded. 

 When the bottom is very irregular and it is 

 not practicable to scan or plot soundings to 

 show every irregularity, soundings should be 

 selected to show the general character of the 

 bottom. 



Poor scanning techniques, careless scan- 

 ning, scanning by inadequately trained per- 

 sonnel, and improper interpretation are 

 among the major causes of defects encoun- 

 tered in verification of smooth sheets. This 

 phase of the processing shall be closely 

 supervised and inspected in order that a 

 high standard of accuracy may be main- 

 tained (see 1-34). 



5-122 Interpretation of fathograms. — 



Although depth recorders have been used 

 for many years, the correct interpretation 

 of fathograms remains a major problem on 

 many surveys. Where recorded traces on 

 the fathogram cannot be attributed with 

 considerable certainty to reflections from 

 the bottom or from obstructions, they should 

 not be recorded as soundings. Herein lies 

 the principal source of difficulty, since there 

 are occasional differences of opinion regard- 



ing the identification of such traces even 

 among the most experienced observers. 



The hydrographer should investigate a 

 number of representative strays with a 

 handlead in order to verify his interpretation 

 of the fathogram. This procedure will not 

 assure infallible interpretation of fatho- 

 grams, but it will provide tangible evidence 

 on which to base the interpretation. 



In many areas, the depth recorder is an 

 imperfect means of obtaining soundings and 

 it should not be relied on as the sole depth 

 measuring device. Supplementary handlead 

 or pole soundings are required for clarification 

 of certain traces, substantiation of doubt- 

 ful least depths, and, in areas of dense grass, 

 even for the basic development. 



It is not feasible to illustrate all the vari- 

 eties of traces which may appear on fatho- 

 grams, but the following reproductions of 

 fathograms are examples of typical records 

 requiring interpretation. 



Where strays are the result of erratic 

 operation of the depth recorder or originate 

 from other causes of which only the hydrog- 

 rapher has personal knowledge, they should 

 be adequately annotated on the fathogram 

 or in the sounding record. When the gain is 

 too high, strays from various sources may be 

 recorded on the graph and are generally 

 identifiable as such (see Fig. 73A). 



A section of the same fathogram is re- 

 produced in Figure 73B. Note the unbroken 

 bottom trace and the depression below the 

 unidentified echo indicating a scour around 

 a submerged obstruction. Subsequent wire- 

 drag investigation revealed the presence of 

 a wreck with kelp on top. 



Kelp recordings probably cause the great- 

 est difficulty in fathogram interpretation. 

 Since kelp seldom grows in depths greater 

 than 15 fathoms, difficulties from this source 

 are encountered in relatively shoal water, 

 and usually in areas of irregular bottom. 

 The traces may resemble side echoes and be 

 detached from the bottom trace as at (1) 

 in Figure 73C or they may blend with the 

 bottom trace as at (2). The bottom profile 

 can usually be traced through the kelp by 

 holding the fathogram at a slant to accen- 



