CHAP, v.] IRREGULAR METHODS OF PLOTTING 127 



that will subsequently have to be plotted by chords should, if 

 possible, lie well within the area covered by the main triangula- 

 tion. After la5dng down the first three stations, three distances 

 must be measured to plot each point on an intersection of the 

 arcs cutting each other at a sufficiently broad angle ; the 

 plotting of the main stations once begun must be completed 

 before distortion of the paper can occur from change of humidity 

 of the atmosphere. Plotting, whether by distances or by 

 chords, must be begun on as long a side as possible, so as to 

 plot with decreasing distances. 



IRREGULAR METHODS OF PLOTTING. 



We have up to the present been considering the plotting 

 of stations for a regularly triangulated survey. Let us now 

 look at some other methods. 



In surveys for the ordinary purposes of navigation, it 

 occasionally happens that a regular system of triangulation 

 cannot be carried out, and recourse must be had to a variety 

 of devices ; the judicious use of the ship in such cases is some- 

 times essential, and with proper care excellent results may be 

 obtained. A few examples will best illustrate some of the 

 methods used, but circumstances vary so much in every survey, 

 that it is only possible to meet them properly by studying each 

 case as it arises, and improvising methods. 



In plotting the points of a chart which is being constructed a Position 

 on the principle of do-with-what-you-can-get, which is very^y^*^®^^ 

 often what has to be done in marine surveys, it is frequently 

 found necessary to plot a position by its own angles, as, for 

 instance, where the ship, anchored or moored off a low coast, 

 has to be a main station, and only angles from aloft can be 

 obtained to objects inland, such as hills, conspicuous trees, etc., 

 already fixed. 



The use of "angles back," or "calculated angles," is one of 

 the most ordinary expedients for fixing a station that has been 

 shot up from another fixed point. A necessary condition 

 is that the " receiving angle " at the position (A) to be plotted, 

 between any two lines (direct or calculated), must be sufficiently 

 broad to gi\e a good cut ; also that the points from which the 



