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HYDROGRAPHIC MANUAL 



taken less frequently when steering ranges 

 or when the lines are being run parallel 

 to distance arcs. In areas of even bottom the 

 distance between successive fixes along dis- 

 tance arcs may be slightly increased, but 

 should never exceed 2 inches on the sheet. 



In most cases a survey ship or launch pro- 

 ceeds along a sounding line at full speed. 

 If the scale of the survey is small, the posi- 

 tions may be taken at intervals of several 

 minutes, however, if the scale is large the 

 intervals will be short and it may be neces- 

 sary to reduce the speed of the vessel. 



Sextant fixes can be observed and plotted 

 very rapidly particularly where the fixes are 

 strong and the signals are nearby. Record- 

 ing of an excessive number of fixes should 

 be avoided in order to reduce the labor of 

 smooth plotting the survey. For example, a 

 11/2-niinute interval between fixes should not 

 be used if a 2-minute interval is adequate. 



In addition to the evenly spaced positions 

 along a sounding line, numbered positions 

 shall be recorded under all of the following 

 circumstances, whether or not accompanied 

 by control data, when this is practicable: 



(a) At the beginning and ending of each 

 line. 



(b) Whenever the speed of the sounding 

 vessel is changed appreciably. 



(c) At all changes of course larger than 

 10°. When the vessel is small and the change 

 in course is immediately effective, the posi- 

 tion may be taken at the middle of the change. 

 Otherwise, a position should be recorded 

 just before the course is altered and just as 

 soon as the vessel is on the new course. 



(d) At each detached sounding, partic- 

 ularly when determining the least depth on 

 a submerged feature. 



(e) Each time a bottom sample is ob- 

 tained with the vessel stopped, whether or 

 not a sounding is recorded. 



(f) Each time a position is fixed for any 

 purpose in connection with the survey, such 

 as for determining the position of a floating 

 aid, an obstruction, or a signal. Positions 

 recorded for the sole purpose of calibrating 

 electronic equipment need not be numbered. 



5-37 Three-point sextant fix. — Most in- 



shore hydrographic surveys are controlled 

 by the three-point fix method using sextants 

 to measure two angles between points whose 

 geographic positions are known (see 3-13). 

 The position at which the angles are ob- 

 served is fixed by the intersection of two 

 circles generated by the loci of the angles. 

 In practice the three-point problem is solved 

 mechanically by using a three-armed pro- 

 tractor. The fix is strongest when the circles 

 intersect at an angle of about 90°, and is 

 weakest when the two circles approach 

 coincidence. 



5-38 Selection of objects. — An experi- 

 enced hydrographer can estimate the strength 

 of a fix at a glance, and is able to select the 

 strongest available fix immediately. Begin- 

 ners often have diflflculty is visualizing the 

 problem and may select a weak fix when 

 good ones are available. The following gen- 

 eral rules apply in selection of objects to be 

 used: 



(a) The strongest fix is when the observer 

 is inside the triangle formed by the three 

 objects. And in such case the fix is strongest 

 where the three objects form an equilateral 

 triangle, the observer is at the center, and 

 the objects are close to the observer. 



(b) The fix is strong when the three ob- 

 jects are in a straight line, or the center 

 object lies between the observer and a line 

 joining the other two, and the center object 

 is nearest to the observer. 



(c) The sum of the two angles should not 

 be less than 50°, better results being ob- 

 tained when neither angle is less than 30°. 



(d) The fix is strong when two objects a 

 considerable distance apart are in range and 

 the angle to the third is not less than 45°. 



(e) A fix is strong when at least one of 

 the angles changes rapidly as the survey 

 vessel moves from one location to another. 



(f) Small angles should be avoided as 

 they result in weak fixes in most cases and 

 are difficult to plot. However, a strong fix 

 is obtained when two objects are nearly in 

 range and the nearest one is used as the 

 center object. The small angle must be meas- 

 ured accurately. 



(g) The distance between the center ob- 



