452 HYDROGRAPHIC MANUAL PaGE 368 



Angles may be measured with a theodolite for this purpose and subsequently 

 measured by sextant from the same position. Angles of as many different sizes as 

 practicable should be measured, preferably angles approximating 10°, 20°, 30°, 40°, 

 etc. Assuming that the angles measured by theodolite are correct, the differences 

 are the sextant errors due to eccentricity and graduation. From these errors a table 

 or curve may be constructed from which the error for an^^ angle may be found by 

 interpolation. 



Errors of graduation may also be detected by placing the zero of the vernier in 

 exact coincidence with selected graduations of the arc and observing whether the last 

 numbered division of the vernier also coincides with a division of the arc. This test 

 should be made at numerous positions of the graduated arc, and the agreement should 

 be perfect in ever}^ case. 



452. Sextant Angles 



4521. Horizontal Angles 



To measure the angle between two terrestrial objects in a horizontal plane the 

 sextant is held horizontally, handle down, with the telescope pointed toward the left- 

 hand object. This object is viewed directly over the top of the horizon mirror, while 

 the index arm is moved until the image of the right-hand object is seen, by double 

 reflection, in the horizon mirror directly below. With both objects in view, and in 

 approximate coincidence, the index arm is clamped and the objects are brought into 

 exact coincidence by means of the tangent screw. 



This measurement of horizontal angles is the basis of fixing the position of the 

 survey vessel by the three-point problem in hydrographic surveying. 



Little experience is required to measure unchanging angles between prominent 

 objects where the sextant is mounted on a steady base or even where held in the hand 

 by an observer on land. In hydrographic surveying, however, the angles to be meas- 

 ured are always changing more or less, the survey vessel is frequently far from steady, 

 and the objects are often small or faint. Considerable practice is required to enable 

 one to observe sextant angles quickly and accurately under these conditions. 



Where the objects are prominent and readily visible, and especially when the angle 

 is changing rapidly or the survey vessel is unsteady, many observers prefer to use the 

 sextant without its telescope because of the larger field of view. 



Where the objects are distant, indistinct, or indefinite, when the angles are changing 

 slowly, and particularly where a small error in the angle will affect the position con- 

 siderably, a telescope should always be used. 



When the angle to be observed is changing appreciably, the usual procedure is to 

 bring the object seen direct and the reflected image into approximate coincidence and 

 maintain them so by moving the free index arm until it is about time to mark the angle. 

 Just before the mark the arm is clamped and the two objects are brought into and main- 

 tained in exact coincidence by the use of the tangent screw until the word mark. When 

 the angle is changing rapidly the tangent screw cannot be moved fast enough for this, 

 but in this case so much accuracy is unnecessaiy. For such angles many observers 

 set the clamping screw partly, so that there is a little friction when the index arm is 

 moved, the angle being taken without using the tangent screw. The magnifying glass 

 should be adjusted beforehand and no part of the arm should be touched after marking, 

 neither should the sextant be set down, until after the value of the observed angle has 

 been read. This requires care, but is generally preferable to clamping the arm after 



