CHAPTER VII 



INSTRUMENTS FOR LEVELING 



So far our discussion has been confined to instruments 

 used for measuring horizontal distances, or those necessary 

 to obtain areas. In farm practice, however, it is necessary 

 in connection with drainage practice, road construction, etc., 

 to determine vertical distances, or the height of one point 

 above another even though these points be at some hori- 

 zontal distance from each other. 



DEFINITION OF TERMS 



A level surface is one that is perpendicular to a plumb 

 line at every point in the surface. It is not a plane nor is it 

 a true oblate spheroid, owing to the fact the earth is not a 

 homogenous body and the center of mass does not conform 

 with the center of form. 



A level line is one that lies wholly within a level surface. 



A leveling instrument is one by which a level plane or a 

 level line may be accurately determined. The three appli- 

 ances upon which leveling instruments depend are the plumb 

 line, a tube filled with liquid, and the bubble tube. 



A datum plane or a datum is the initial plane to which the 

 height or elevation of points may be referred. A datum 

 plane in common use is that of sea level. 



The elevation of a point is the distance of the point above 

 or below the datum plane. 



A leveling rod is a graduated measuring rod or staff 

 used for* measuring vertical distances between a point on 

 which the lower end of the rod may rest and a line indicated 



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