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FARM DEVELOPMENT 



to good advantage. After having carefully leveled 

 up the telescope of the instrument, direct it to the meas- 

 uring rod standing on the point chosen for a bench mark 

 near the outlet of the proposed drain; indicate to the 

 rodman who is holding this rod, to move the disk up or 

 down until its center is in line with the eye and the 

 horizontal cross hair in the telescope. The rodman 

 should then read on the measuring rod the exact dis- 

 tance from the bench mark up to the center of the disk 

 and record same in field notes, as in Figure 81. This 

 gives the height of the instrument above the bench 



Figure 81a. Showing manner of using leveling instrument in planning a draia. 



mark. It is a simple matter now to assume that the 

 bench mark is 100 feet above an imaginary level plane, 

 which, for convenience, is termed " datum plane " or 

 " datum." The height of the instrument above this 

 imaginary plane is 100 feet plus the distance above the 

 bench mark to the disk on the measuring rod, which 

 is level with the eye at the instrument. To make 

 the illustration more complete, we will assume that the 

 reading in a given case on the measuring rod was 5.32 

 feet. Adding this figure to the height of the bench 

 mark above datum, i. e., to 100 feet, we have the height 

 of the instrument above datum 105.32 feet. 



