PRIMARY TRI ANGULATION. 49 



inaccuracy in the measurement of the base is multiplied, this work must 

 be planned and executed with greater care than the body of the triangula- 

 tion requires. 



A base line measured as above prescribed requires little expansion, 

 since from the extremities of an 8 or 10 mile base one can observe 

 directly on points 12 to 15 miles away, a distance as great as the average 

 side of a triangle. Ordinarily, from the ends of the base, the surveyor 

 can observe directly upon stations in his scheme of triangulation. 



In the western mountain region, where the sides of triangles may be 

 20 to 50 miles in length, an expansion is required. 



SELECTION OF STATIONS. 



In the selection of triangulation stations two different sets of require- 

 ments must be served. 



First. They must be so selected as to afford what is known as strong 

 figures, in order to reduce to a minimum the errors which will creep into an 

 extended system. In order to insure intervisibility, they should, if possible, 

 be located upon hill or mountain summits, the most commanding in the 

 neighborhood. No triangle upon which dependence is placed for the loca- 

 tion of a station should have at that station an angle of less than 30° or 

 more than 150°. 



The stations should, if practicable, be grouped into simple figures, as 

 quadrilaterals, or pentagons with an interior station, etc. In cases where an 

 area is being covered with triangulation, such groupings naturally occur, 

 but in certain cases the triangulation takes the form of narrow belts of fig- 

 ures, and then the belt may consist of simple triangles or quadrilaterals, as 

 more complex figures are rarely desirable. 



Second. Since the sole object of this triangulation is the control of the 

 topographic map, the location of stations must, as far as is consistent with 

 accuracy, be adjusted to the needs of the topographers. This requirement 

 affects most seriously the distance between stations. Every atlas sheet 

 must contain at least two primary stations, and a third is desirable. Thus, 

 for controlling the sheets on the scale 1:62500, the stations should not be 

 more than 10 or 12 miles apart, and should be located with direct reference 

 mon xxii 4 



