28 



HYDROGRAPHICAL SURVEYING 



[chap. I. 



General 

 Rule for 

 observing 

 Angles of 

 a Fix. 



Size of 



Angles 

 Admissi- 

 ble. 



"Points" 

 in Transit. 



The angle should be observed between the two most distant 

 objects and between the two nearest objects. 



The size of angles admissible in a good fix depends on the 

 position of the three objects. If two objects are equi-distant, 

 the angle must not be small, for a slight error in the angle will 

 make a great difference in the position ; but if one object be 

 much farther off than the other, a very small angle between 

 these will suffice, so long as the third object is so placed as to 

 make a fairly large angle. 



An arrangement of the objects not yet considered, is when 

 two of them are in line from the observer's position. 



This is technically called " transit," and no transit of known 

 marks is allowed to take place without making use of it. 



One angle to a third object is here enough to fix the position, 

 which is one advantage, another being that if two angles are 

 taken and placed on the station pointer, the coincidence of the 

 position, as plotted by these two angles, with the transit line, 

 gives an excellent check. 



Here, Fig. 5, A and B are in line of transit ('A) ; H is a third 

 object. 



It will be evident that when the observed angle is on the 

 station pointer, and the latter is placed with one leg coinciding 

 with the line A B, we have only to move it up or down that 

 line, until H coincides with the other leg, which gives us X. 



