3452 HYDROGRAPHIC MANUAL PaGE 262 



if three-point fix control is used, the three objects to be observed are selected and 

 agreed upon. 



Where the beginning of the proposed line is so close inshore that, for purposes of 

 safety, it is necessary to start it from a standstill, that fact must be entered in the 

 Sounding Record. A complete time record of the changing speed of the vessel must 

 be kept, and another position fixed as soon as the vessel is traveling through the water 

 at a standard and uniform speed. 



3452. Trial Positions 



A trial position is a position taken to determine the progress or course of the vessel, 

 or its position relative to the proposed sounding line, but which is not recorded as a part 

 of the official record. Such positions are, of course, frequently necessary before starting 

 a line and often between lines. Their use should be umiecessary while running a sound- 

 ing line. The purpose of takmg positions is to enable the profile being sounded over to 

 be plotted accurately. If the regular mterval between positions is so long that the 

 position of the vessel is so uncertain as to require a trial position, the regular interval 

 should be reduced. If an intermediate or additional position is believed advisable to 

 verify the vessel's course or speed, it should be taken and recorded as a part of the official 



record. 



3453. Ranges and Distance Angles 



In surveying with the handlead, and at other times when an exceedingly slow speed 

 is required, ranges will be found very useful, especially when there is a cross wind or 

 current. Two shore objects separated by some distance should be selected for use as 

 range marks to assist in running the proposed lines (if practicable, without unduly 

 delaying the work). When a range is desired the correct point on the shoreline may 

 be determmed from the offshore end of the sounding line by measuring with the pro- 

 tractor on the boat sheet the angle between some control station and the azimuth of 

 the line being run. This angle is set on a sextant, and with the signal to which the 

 angle was measured in view, a search may be made for a suitable object along the 

 shore which may serve as a front range mark, the corresponding rear range mark bemg 

 farther inland. 



Starting at the inshore end of a line the selection of a front object is easy but it is 

 only after the vessel has advanced some distance offshore that a rear object can be 

 selected to give the correct range. 



Distance angles may be used for the same purpose as ranges except that the result- 

 ing lines will be flat curves instead of straight Imes. Ranges used for systems of radiat- 

 ing lines are described in 3142 and the use of distance angles is discussed in 3143. 



3454. Turns at Ends of Lines 



At the end of a sounding line, which is one of a system of parallel Imes, the survey 

 vessel turns and runs to the next proposed line. Most systems of sounding lines should 

 be planned so that soundings are not required while the vessel is turning between adja- 

 cent lines. The exact positions of such soundings are usually in doubt and there is less 

 confusion if they are omitted. When soundings around turns are not recorded or are 

 not intended to be used on the smooth sheet, a note should be. entered in the "Remarks" 

 column of the Sounding Record opposite the last position of a line that the "Line ends," 

 and opposite the first position of the next line a note should be entered that the "Line 

 begins." The expression "Line turns right" or "Line turns left" should be entered 



