128 



HYDROGRAPHICAL SURVEYING 



[chap. v. 



Use of 

 Tracing- 

 paper. 



Only Two 



Angles 



available. 



Moun- 

 tains In- 

 valuable. 



Use of 



True 



Beaxings. 



*' angles back " arc calculated should not be situated at too 

 great distances from A, considered absolutely and relatively 

 to the distance between A and the station shooting it up. 



A station pointer, generally, has some small errors of 

 centring, etc., that prevent it being used where exactness is 

 required, and, moreover, only two angles can be laid off at a 

 time by this instrument. In this case, then, it is better to 

 plot all the angles obtainable on to tracing-paper, using chords 

 for the purpose, and being very careful to make a very minute 

 hole at the centre from which they radiate. If the objects 

 are fairly well placed, a very exact position will be obtained, by 

 laying this tracing on the sheet, and pricking through for the 

 position. This vnW be much assisted if but one Une can be 

 got from a fixed station, as the angles can then be plotted on 

 this line, supposm^g that in this case back angles cannot be 

 calculated. 



The line from the fixed station forms a gooS zero for laying 

 off angles to other objects. 



Again, it may sometimes be found necessary to carry on 

 the main stations with a point plotted by only two angles ; 

 but if this happens, efforts must be made to check this, by 

 getting an angle back from stations plotted on by means 

 of this doubtful position, to some old well-fixed station, as a 

 distant mountain ; or if this is not to be had, a regular be- 

 ginning must be made again by plotting two stations with 

 two angles, pricking one, and then laying the angle from that 

 to the fourth, as practised at the commencement of the chart, 

 which ^^^ll give a certain amount of check. 



A well-defined mountain, though miles inland and never 

 visited by the surveyors, will often prove the very keystone of 

 a chart that cannot be regularly and theoretically triangulated. 

 When once well fixed, it will remain to get angles to, long after 

 all the other first points of the survey have sunk below the 

 horizon as the work progresses. 



The bearings of this uill often be useful, and these can be 

 laid off from the mountain by applying the convergency. 



Let us take an example, which Avill perhaps explain what is 

 required easier by means of a diagram. 



We hope that we have made it plain, by what has gone 

 before, that if a distant object bears, say, N. 47° 20' W., we 



