CHAPTER IV. 



SECONDARY TRIANGULATION. 



The work of making secondary locations by intersection is done mainly 

 by plane table. The use of the theodolite for this purpose is restricted to 

 those cases where but little of this kind of location can be effected, and 

 where, therefore, it seems scarcely worth while to prepare plane-table sheets. 



By means of the primary triangulation, two or three points are usually 

 located upon each atlas sheet. Within this primary triangulation, and 

 depending upon it, are then located a large number of points, either by 

 intersection, by traverse, or by both methods, forming a geometric frame- 

 work upon which the sketching of the map depends. 



Location by intersection should be carried as far as practicable — that is, 

 all points capable of being located in this manner should be so located in 

 order to afford the most ample control possible for the traverse lines, by 

 which the intervening areas are to be tilled in, it being understood that the 

 location by intersection is more accurate and more rapid, and consequently 

 in every way more economic, than location by traverse. 



THE PLANE TABLE. 



Much misapprehension exists, especially in this country, regarding the 

 character and application of this instrument. This arises, apparently, from 

 the fact that it is little known. For making a map the plane table is a uni- 

 versal instrument, It is applicable to all kinds of country, to all methods 

 of work, and to all scales. For making a map it is the most simple, direct, 

 and economic instrument; its use renders possible the making of the map 

 directly from the country as copy, and renders unnecessary the making of 

 elaborate notes, sketches, photographs, etc., which is not only more expen- 

 sive, but produces inferior results. 



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