CHAP. IV.] THE MAIN TRIANGULATION 79 



length of any side, or the distance between any two main Varieties 

 stations, can be found ; and " plotted," in which the main gyjations 

 stations are simply the first points laid down on the paper. 



A calculated triangulation is used in any detailed survey, Caicu- 

 in plans, or whenever from circumstances it is convenient to Tnangu- 

 have different parts of the same survey on separate sheets, lations. 

 which can therefore afterwards be put together in the 

 engraving, witho'ut any fear of their not fitting into one 

 another. 



Bases for plans, on a larger scale than the rest of the chart, 

 can often be taken out of a calculated main triangulation 

 without measuring separate small lengths. 



Plotted triangulations mav further be subdivided into Plotted 

 " regular " and " irregular." ^ JSiionf 



A plotted regular triangulation will be when triangles have 

 been obtained which could, if requisite, be calculated trigono- 

 metrically. As, however, a calculated triangulation is of great 

 service as a record, and for future resurveys, it is expected to 

 be furnished with every chart. 



It is more satisfactory that triangulation should be regular if 

 possible, but it very much depends upon the nature of the coast 

 to be surveyed, in what manner it can be carried out. 



In many extended surveys, where, for instance, the land is irregular 

 low and densely wooded, or perhaps bordered by reefs to a great TriaiT^- 

 distance from the shore, a regular triangulation is hardly lations. 

 possible, or would entail so much loss of time as would not 

 justify its being undertaken. 



The main points must be plotted in these cases by all sorts 

 of means. The ship enters largely into the scheme, and 

 frequently boats also. Stations may have to be fixed solely 

 by angles observed at them. True bearings are freely used in 

 the construction of the chart, and any regular system of 

 triangles disappears. 



A large proportion of existing charts have been, and many 

 more are now being, constructed, by means of irregular plotting. 



A survey can often be commenced with a regular triangula- 

 tion, when it will be found necessary, after having advanced 

 a certain distance, to have recourse to irregular means to fix 

 main stations. 



Here it is, when ordinary rules and systems fail, that the 



