Page 227 hydkography 336 



336. Bearings and Distances 



Inasmuch as a bearing from the survey vessel to a control object is a line of position, 

 it is obvious that the position of the vessel may be determined by any additional data 

 which will provide a second line of position to intersect the bearing at an appropriate 

 angle. The more nearly this mtersection approaches 90° the stronger w411 be the deter- 

 mination of the position, assuming an equal reliability in the data. 



The line of position to intersect the bearing may be provided by data obtained 

 in any one of several ways: For example, by a horizontal sextant angle measured 

 between two control stations so situated that the locus of the angle intersects the 

 bearing appropriately; by an R.A.R. distance from the station to which the bearing is 

 measured or from any other station so located that the distance arc will intersect the 

 bearing at an appropriate angle; or by a distance based on a vertical or depression 

 angle, or measured by rangefinder. 



Where a survey is controlled principally by three-point sextant fixes on survey 

 buoys or by R.A.R. methods using sono-radio buoys, useful positions are frequently 

 obtained by the combination of a bearing and distance measured to a buoy being 

 passed at a short distance. Distances measured with a 1-meter rangefinder for this 

 purpose should not much exceed 1 nautical mile; longer distances are likely to be in 

 error more than a position controlled by other methods. (See 455.) 



3361. Bearings 



Directions to the survey vessel from shore positions can be observed quite accur- 

 ately but are practicable only when the survey vessel is plainly visible from shore. 

 They are ordinarily used only in connection with the large-scale surveys described in 

 334. The observation on board the vessel of all of the position data is an important 

 consideration. 



Bearings from the survey vessel to control stations are frequently useful to supple- 

 ment other methods of control in special circumstances, but they are notably untrust- 

 worthy from distances greater than a few miles. 



Bearings from the survey vessel to control stations are ordinarily measured by 

 means of some form of a pelorus (see 4434 to 4437). Their accuracy depends directly 

 on the stability of the survey vessel, and on the accuracy with which the magnetic 

 variation and the compass error are known, unless the vessel is equipped with a gyro- 

 compass. For best results a pelorus equipped with a telescopic alidade should be used 

 (see 4435), although a small theodolite mounted on the wing of the bridge of the vessel 

 may be substituted therefor with good results (see 4436). In any case, tests must be 

 made to ensure that the pelorus or other instrument is correctly alined with the lubber's 

 line of the compass (see 4437). 



Successive bearings should be observed to a distant control station to strengthen 

 the plot of a dead-reckoning sounding line where more accurate control is lacking. 

 The simplest case is, of course, the well-known bow-and-beam method used in navi- 

 gation for determining the position of the vessel when it passes abeam of an object, 

 the distance off being equal to the distance run by the vessel between the two obser- 

 vations. Where bearings such as these are used in connection with dead reckoning, 

 it is usually more convenient to take them at regular intervals when the log is read 

 and to adjust the read-reckoning plot graphicaUy to the plotted bearings (see 3378). 



Inasmuch as a bearing is an angular measm^ement, any inaccuracy will affect the 

 horizontal positions by a proportionally greater amount as the distance of the vessel 

 from the object increases. Even at considerable distances, however, the data may be 



