Page 219 hydrography 3334 



movement of the center of the protractor moves the arms away from one or more of the 

 stations, and it is weak if such a movement does not appreciably disturb the relation of 

 ' the arms to the three points. 



3334. Changes of Fix 



Generally the strongest fix available should be observed at each position. There 

 are other practical limits, however, to the frequency with which changes in fix should 

 be made. Frequent changes of objects are conducive to recorder's errors as well as 

 observer's errors. It is considerably easier for an observer to repeat angles between 

 the same two objects than it is to find two new objects in his sextant at each observation. 

 When signals are faint, due to distance, haze, etc., it is often much better to hang on to a 

 fix, rather than attempt a change to a stronger fix. Some observers, however, tend to 

 retain the use of the same three objects long after a change to a stronger fix should have 

 been made. 



3335. Sextant Fixes on Shore Objects 



In addition to the general information relative to strength of fixes and selection of 

 objects, there are specific points that should be guarded against when fixing the position 

 of the vessel by sextant close inshore and far oft'shore. 



Without a multitude of small signals it is impracticable to obtain a strong fix at 

 the inshore end of each sounding line, as a survey vessel close to the shore is nearly 

 on line with the signals and the sum of two angles will generally approach 180°, with 

 one of the angles being extremely large and the other extremely small. Such a fix 

 determines accurately the distance offshore but is very weak in its determination along 

 the shore. Since the angles of such a fix change rapidly when the vessel is moving, 

 unusual care must be taken to mark the two angles simultaneously; otherwise there 

 may be considerable error in the position. This also applies to inshore positions fixed 

 by using two signals close to the observer and one distant offshore signal. 



Where all the signals of a three-point fix are at short distances from the vessel, 

 the error introduced by not marking simultaneously has less effect on the position. 

 Wliere the center object is close, the effect on the position increases with the distance 

 of the right and left objects from the observers. 



At times when it is impossible to obtain a three-point fix, it is possible to measure 

 two angles to four signals so as to -fix the position. This is known as a split jfix because 

 there is no common center object. If the signals are appropriately chosen the fix may 

 be just as strong as a three-point fix but considerably more time is required to plot it. 

 It cannot be plotted by the three-arm protractor method — instead, the locus of each 

 angle must be plotted separately, the position being at the intersection of the two loci 

 (see 7625). Such a fix should be taken intentionally only when no three-point fix is 

 available. 



Fixes at the extreme limit of visibility are likely to be weak because the angles 

 between the only stations visible are generally small. In such cases the angles change 

 slowly, and a slight error of observation affects the position a comparatively large 

 amount because of the smallness of the angles. In such cases it is necessary to use 

 telescopes in the sextants, to have the sextants in perfect adjustment, and to mark and 

 read the angles with extreme accuracy. 



3336. The Use of Indefinite Objects 



Indefinite objects or those inaccurately located shall not be used to control inshore 

 or critical hydrography. For general use, when other objects are lacking, one indefinite 



