ion HYDROGRAPHICAL SURVEYING [chap. iv. 



Thus, to get position of F, we have — 



A B .... N. 09° 10' 37" W 10-2468 miles. 



B C .... N. 12 20 56 E 19- 1502 miles. 



C F . . . . N. 1 24 17 W 2-5691 miles. 



From which we calculate difference of latitude and departure 

 in the ordinary manner. 



We thus get the mercatorial bearing of A F, N. 12° 32' 45" W., 



and distance 25-5269 miles. 



Caicuia- It will be understood that it is by no means necessary to 



Tr^nsu- ^^ork out all the triangulation as just described when com- 



lation not mencing the plotting. All that is then required is as long a 



necessary ^'^^ ^^ ^'^ ^^^ g®* ^^ which to begin. The main triangulation 



early in can be calculated afterwards, and in many instances must be, 



^"^^^' as the whole of the angles will not be obtained till later on. 



In some nautical surveys it will not be necessary to calculate 



any triangulation at all. 



Triangles It will be observed that we have a triangle C D E with a 



ing^Smaii ^®^T small angle. This not being a receiving angle does not 



Angiesnot matter in the least. We are obtaining the position of F from 



iii^Mindi- ^' ^^^ I^' which are already fixed, and the angle of intersec- 



tloned. tion at F being nearly a right angle, the change of position in 



F, resulting from a small error in the angle at either C or D, 



will be as small as is possible, and much less than if, the angle 



at C being the same, that at D was 60°, which would result 



in the intersection at F being more acute, and any error would 



consequently change the position of F to a greater degree. 



If we were obtaining D from C and E, such a small angle 

 would not be admissible for a moment, as it is evident that 

 any small error at C or F would result in a great change of 

 position in I). 



It would be awkward and inconvenient to have many such 

 triangles in the main framework of the triangulation, as the 

 small side is of no use in carrying on the chain, and we should 

 be forced to multiply triangles in consequence ; but we are, 

 notwithstanding, sometimes obliged to include some such in 

 our work, from the he of the land and other causes, and as 

 long as we use them as in the example they will not affect the 

 result, as far as chance of accuracy goes, and should not be 

 under these circumstances considered as " ill-conditioned." 



