CHAP, v.] PLOTTING 125 



remember that as a standing rule the smallest angles both 

 give less trouble and present least chance of error. 



The ordinary way of marking the points is to ring a small Marking 

 circle of carmine round them. Larger circles can conveniently ** 

 be used to distinguish the main stations. 



It will be found in the course of plotting that the paper stretch- 

 will vary so much, expanding at one time and contracting at paper, 

 another, that the arcs of radii once measured and scratched 

 on the paper, cannot be considered as so done once for all. If 

 some hours have elapsed since marking any radius, it must 

 be remeasured, to ascertain if it has altered. 



In getting angles for plotting stations of all kinds, it must Calcuiat- 

 be remembered that two angles of a triangle mil always give Angle. 

 the third, and that as far as mere plotting goes, it is not 

 necessary to waste time in observing the third angle. If the 

 two observed angles have been got fairly accurately, the double 

 error which will be thrown into the third angle deduced from 

 them should not be enough to show in plotting, and if it does, 

 it will soon make itself apparent by not intersecting. An angle 

 from a fourth station mil show which of the other three angles 

 is wrong. 



Thus if we have observed at a station C, which we want to 

 plot, the angle between A and B, and also the angle at A 

 between B and C, the angle at B which is wanted to draw 

 a line to C can be calculated without the trouble of visiting B. 

 It is indeed a blessed circumstance for the marine surveyor 

 that the three angles of any triangle equal 180°. 



It is a great assistance when plotting, to note in red ink in the Entering 

 " Main Angle Book," each " calculated angle " as it is obtained jat^d""' 

 placing it under the heading of the particular A from which the Angles " 

 angle is calculated. The angle should be referred whenever Angle 

 possible to the same zero that has been used at that A, and Book, 

 then it can be treated as if it had been actually observed. 

 The A through which the angle has been obtained being also 

 stated in brackets, the origin of a calculated angle can be 

 traced. Such angles must, of course, be used with great 

 caution, but when an angle can be calculated through two or 

 three different stations, and the results are in agreement when 

 reduced to a common zero, it becomes assured within much 

 smaller limits than would otherwise be possible. 



