112 TOPOGRAPHIC SURVEYING 



TOPOGRAPHIC SURVEYING 



METHODS EMPLOYED 



In a topographic survey, the relative elevations or depressions 

 of points and objects are determined in addition to their posi- 

 tions. Three methods, differing with regard to the instruments 

 used, are employed in making topographic surveys; These 

 are the transit method, the stadia method, and the plane-table 

 method. 



TRANSIT METHOD 



The transit method is well adapted to surveys for the loca- 

 tion of railroads and to similar surveys that relate to lines 

 rather than to areas, and in which the topography is required 

 to cover only comparatively narrow strips of country contigu- 

 ous to the lines. In such surveys, the entire process is based 

 on the line of the survey, which is usually alined with a transit 

 and measured with a chain or tape. Along with the survey, 

 a line of levels is run with a leveling instrument and at suitable 

 intervals, generally every 100 ft., cross-sections are taken at 

 right angles to the line. For the latter purpose the hand 

 level and the clinometer are often used. 



Hand Level and Clinometer. The hand level, also called 

 the Locke level, is shown in Fig. 1. The bubble of the level 

 tube C can be seen through the opening D by means of a 



FIG. 1 



reflecting prism. A cross-hair placed in the main tube AB 

 serves to fix the object observed, and when this hair at the 

 same time bisects the bubble the line of sight is horizontal. 

 By means of a clinometer, one form of which is shown in 

 Fig. 2, the angle that a slope makes with a horizontal can be 



