CHAPTER IV 



THE MAIN TRIANGULATICN 



General — Making a Main Station — False Station — Sketch — Convergency 

 — Calculation — Co-ordinating Astronomical Observations with Triangu- 

 lation. 



Definition. The main triangulation has been already defined as " the 

 estabhshment by means of angles of a series of positions, from 

 which and to which, angles are afterwards taken, to fix the 

 secondary points of the survey." 



Main \]\ positions from which angles are taken, with the mtention 



of fixing other objects, are called " stations," the symbol for 

 which is A, but the ones with which we are immediately con- 

 cerned, that is, the first and important positions, are distin- 

 guished as the " main stations," and these collectively form the 

 '' main triangulation." 



The first object of main stations is to see other main stations, 

 and with this in view their positions are chosen accordingly ; 

 but angles to everything useful, secondary stations, marks, etc., 

 are, of course, taken as well. 



Secondary Secondary stations are those from which angles are taken 

 solely to fix the smaller marks and details, etc., of the survey. 

 They will be nearer together than the main stations, and may 

 often be perforce so placed as to be useless for any other object. 



Points. ^1 objects fixed and plotted on the skeleton chart are 



known as " points." A " point " may be a main station, a 

 secondary station, or simply a mark ; but when fixed and 

 plotted on the sheet, with the intention of using them in the 

 survey, they are one and all spoken of by the generic term of 

 " points." 



Main triangulations may be divided into two kinds : " calcu- 

 lated," in which the triangles are all worked out, so that the 



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