CHAPTER I ! . 



CLASSIFICATION OF WORK. 



The work involved in making a map usually comprises several opera- 

 tions, which may in practice be more or less distinct from one another. 

 They are enumerated as follows: 



First. — The location of the map upon the earth's surface, by means of 

 astronomic observations. 



Second. — The horizontal location of points. 



This is usually of three grades of accuracy, primary triangulation, or 

 primary traverse, in cases where triangulation is not feasible; secondary 

 triangulation for the location of numerous points within the primary triangu- 

 lation; and ordinary traverse, for the location of details. 



Third. — The measurement of heights, which usually accompanies the 

 horizontal location, and which may, similarly, be divided into three classes, 

 in accordance with the degree of accuracy. 



Fourth. — The sketching of the map. 



Nearly all of the geometric work of the Survey, the work of location, 

 is executed by five instruments. 



Theodolites, of a powerful and compact form, used in the primary 

 control. 



Plane tables, with telescopic alidades of the best type, used for second- 

 ary triangulation and height measurements. 



Plane tables, of crude, simple form, with rider alidades, used for 

 traversing and minor triangulation. 



Odometers, for measuring distance. 



Aneroids, for the measurement of details of heights. 



