5. HYDROGRAPHY 



145 



ject and the left and right-hand objects 

 should be longer than the distance from the 

 observer to the center object. 



Beginners should demonstrate the valid- 

 ity of the above rules by plotting examples 

 of each and their opposites. It should be 

 noted that a fix is strong if, in plotting, a 

 slight movement of the center of the pro- 

 tractor moves the arms away from one or 

 more of the stations, and is weak if such 

 movement does not appreciably change the 

 relation of the arms to the three points. An 

 appreciation of the accuracy required in meas- 

 uring the angles can be obtained by changing 

 one angle about 5 minutes in each example 

 and noting the resulting shift in the plotted 

 positions. 



Avoid a selection of objects which will 

 result in a "revolver or swinger." Do not 

 observe an angle between objects of different 

 elevation if the inclined angle would require 

 a correction of more than two minutes to 

 reduce it to a true horizontal angle (see 3 

 15). Indefinite objects such as tangents, 

 hilltops, and other poorly defined or located 

 points should not be used. 



5-39 Change of fix — Generally the strong- 

 est fix available should be used at each posi- 

 tion. However, there are other practicable 

 limits which govern selection of a fix. Fre- 

 quent changes of objects are conducive to 

 recorder's errors as well as observer's er- 

 rors. It is better to use signals which can 

 be positively identified rather than less dis- 

 tinct signals which would provide a some- 

 what stronger fix. Some observers tend to 

 use the same object long after a change to 

 a stronger fix should have been made. 



When the anglemen do not have easy ac- 

 cess to the boat sheet, it is advisable to 

 make a rough shoreline tracing showing the 

 location, name, and brief description of each 

 signal in the vicinity. This enables the an- 

 glemen to select strong fixes and positively 

 identify the stations used. 



5-40 Special problems in sextant fixes. — 



Sextant fixes at distances approaching the 

 limit of visibility of the signals are likely to 

 be weak because the angles are small. In 



such cases the angles change slowly and a 

 slight error in the angle affects the position 

 by a comparatively large amount. When sig- 

 nals are some distance away or are difficult 

 to reflect, a telescope should always be used 

 in the sextant, the sextants should be in per- 

 fect adjustment, and the angles should be 

 measured and read accurately (see 3-14). 



Without a multitude of small signals it is 

 impracticable to obtain a strong fix at the 

 inshore end of every line, as a survey vessel 

 close to the shore is nearly on line with 

 the signals (see 4-28). The sum of the 

 two angles will approach 180° with one 

 angle being very large and the other very 

 small. The angles change very rapidly 

 when the vessel is moving, and unusual care 

 must be taken to mark them simultaneously. 

 The effects of errors introduced by failure 

 to mark angles simultaneously is minimized 

 when the signals used are at short distances 

 from the observer. 



At times when it is impossible to obtain a 

 three-point fix, it may be possible to measure 

 two angles to four signals so as to fix the 

 position. This is known as a "split fix" be- 

 cause there is no common center object. A 

 split fix may occasionally be observed by 

 accident. If the signals are appropriately lo- 

 cated so that the loci of the angles intersect 

 at an angle greater than 45°, the fix is just 

 as strong as a three-point fix, but consider- 

 ably more time is required to plot it. The 

 locus of each angle must be plotted sep- 

 arately, the position being at the intersection 

 of the two loci. Such a fix should be taken 

 intentionally only when no satisfactory 

 three-point fix is available. 



5-41 Position fixing by Shoran. — The 



Shoran system used for control of hydrog- 

 raphy is described in Chapter 3, Sections 33 

 to 47 inclusive. The system was first used 

 in hydrographic surveying in 1945 on the 

 ship EXPLORER in the Aleutian Islands, 

 and has been widely used in all areas since 

 then. The Shoran equipment is designed to 

 measure two distances simultaneously be- 

 tween a mobile station and two fixed sta- 

 tions whose positions are known. The posi- 

 tion of the mobile station is determined by 



