14 A MANUAL OF TOPOGRAPHIC METHODS. 



than those by traverse. As a rule, they are selected points chosen because 

 each controls positions in a certain area. On the other hand, traverse loca- 

 tions are not, as a rule, chosen for their control value, but only for intervisi- 

 bility on roads. Furthermore, the great majority of traverse stations are of 

 no service whatever beyond carrying the line forward, so that in estimating 

 the total amount of control in a certain area where the control is made up 

 in whole or in part of traverse lines, less weight should be given to them 

 than to locations by triangulation. 



The third element of accuracy, the distribution of locations, is a point 

 concerning which it is equally difficult to speak definitely. Other things 

 being equal, the distribution should be uniform over the area, but it will 

 necessarily vary with the character of the surface. The accompanying 

 diagram shows the amount and distribution of control in a typical piece of 

 work. In general, in the mountain regions, locations by angular measure- 

 ments are frequent and accompany the ranges or ridges, and such locations 

 are few in number in the valleys, being supplemented there by traverses. 



The fourth of the elements of the correctness of the n^ap depends upon 

 the artistic sense of the topographer, upon his ability to see things in their 

 proper relation, and his facility in transferring his impressions to paper. 

 This is by far the most important and the most difficult to meet. 



The education of the topographer, therefore, consists of two parts — the 

 mathematical and the artistic. The first may be acquired largely from 

 books, and this book knowledge must be supplemented by practice in the 

 field. The second, if not inherited, can be acquired only by long experi- 

 ence in the field,, and by many can be acquired only imperfectly. In fact, 

 the sketching makes the map, and, therefore, the sketching upon the Geo- 

 logical Survey is executed by the best topographer in the party, usually its 

 chief, whenever it is practicable to do so. 



