30 



HYDROGRAPHICAL SURVEYING 



[chap. I. 



Choosing 

 a Station 

 Pointer. 



90° be unity, and the error in the angle D O A and distance 

 A remain the same, 



If D O A be changed to 30- 0° error in position changes to 2. 



19-75° ,, ,, „ 3 



which shows the rapid rate at which the error increases as the 

 cutting angle diminishes after it is less than 30°. This is very 

 important, and should be kept constantly in view. 



In choosing a station pointer, of which instrument Heather 

 gives but a meagre account, the first important thmg to look at 

 is that the smallest angle to be read on the leg which will not 

 come to zero, is as small as it should be. A well-planned 

 modern station pointer should allow this leg to return to 3° 

 or 4°, but old instruments frequently will not read less 

 than 19°. 



It is a great nuisance to find that the only angles you can take 

 cannot be plotted by means of your station pointer, and the 

 chance of this should therefore be minimised as much as 

 possible. 



Station pointers are generally made to allow the left angle to 

 come to 0°. 



When the angle on the right is too small to set, and the left 

 angle is more than 90°, the difficulty can be got over by setting 

 the small angle on the left leg and bringing the right leg round 

 to the left until the required left angle is made between it and- 

 the left leg. 



Another method when the above cannot be carried out, but 

 care must be taken not to employ it when exact accuracy is 

 required, is to set the angles on the station pointer reversed, 

 i.e. the right angle on the left, and vice versa. Place the legs 

 containing the larger angle on its " points," with the centre 

 near the supposed position, and make a prick. Move the centre 

 a Httle, right and left of the first prick, keeping the points on, 

 and make other pricks. A line drawn joinmg these pricks will 

 form an arc of the circle for those " points." 



