333 HYDROGRAPHIC MANUAL P AGE 218 



the positions of the two range objects must be very accurately known and plotted, or 

 comparatively large errors in position will result. It should also be noted that, while 

 such a fix continues strong as long as the near object is in the center, it may become 

 very weak as soon as the vessel moves to a position where the more distant object is 

 the center object. 



Avoid using at close range an indefinite object or one whose position is uncertain. 

 Errors from such sources decrease rapidly as the distance of the uncertain object in- 

 creases relative to the other two. In other words, two nearby definite objects whose 

 positions are accurate may be combined to good advantage with a right- or left-hand 

 distant object which is indefinite or whose position is not so accurately known. 



The definiteness of the objects and their accuracy of position being equal, nearer 

 objects should always be preferred to more distant ones, since slight errors in the angles 

 affect a position less. Possible distortion of the plotting sheet also makes it preferable 

 to use nearer objects. For inshore hydrography, control stations on the adjacent shore 

 provide greater accuracy than distant stations as, for instance, those on the opposite 

 side of a wide channel or large bay. 



There are numerous advantages in using off-lying signals, or those on an opposite 

 shore, however. In many cases angles are easier to observe and plot. It is not neces- 

 sary to change objects so often as when using nearby signals and the necessity for 

 numerous closely spaced signals is avoided. The extent to which such signals can be 

 used is therefore important and requires good judgment. In general, for ordinary 

 hydrographic surveying it is satisfactory, and in fact preferable to use such signals, 

 provided their positions are accurately determined, that they are so distributed that 

 strong fixes may be obtained from them, and that they are not so far away as to require 

 the use of extension arms in plotting. Under these conditions one or more distant 

 signals can be used advantageously in conjunction with nearby signals. The strength 

 of an inshore fix may be increased considerably if one distant offshore signal is used in 

 conjunction with two signals along the shore close to the observer, the survey vessel 

 being inside the triangle formed by the stations. The angle between the alongshore 

 stations determines the distance of the vessel offshore, the angle to the offshore station 

 determining the position along the shore. 



If practicable, the distances between the center and the right- and left-hand objects 

 should be longer than the observer's distance from the center object. But a very near 

 center object is a disadvantage when used in combination with distant right- and left- 

 hand objects from a moving vessel because of the rapidity with which the two angles 

 change. Unless the observers stand close together and mark their angles accurately 

 and simultaneously, there is likelihood of introducing a relatively large error in the 

 sum angle which will affect the position considerably. In such cases, however, there 

 will be less likelihood of appreciable errors of position being introduced if one of the 

 observers measures the sum angle and it is used in combination with either the right or 

 left angle. (See 7625.) 



Avoid a selection of objects which results in a revolver (see 3332). Sometimes near 

 the inshore end of a line there is no other choice; but in such a case a third angle should 

 be taken, if practicable, to a distant definite feature such as a tangent or point of land. 

 Wlien the hydrographer expects such a situation, a constant course and speed should 

 be maintained from the previous position, so that the dead reckoning may be used in 

 conjunction with the angles in plotting the position in its correct location. 



A weak fix is indicated when both angles change slowly as the survey vessel moves 

 along the sounding line. It should be noted that the fix is strong if. in plotting, a slight 



