LEVELING 83 



signals from the leveler. For ordinary work, the type first 

 mentioned is preferred by engineers, the target rod being used 

 where very accurate work is required. 



It is very important that the rod should be held truly vertical 

 when sighted at. Different devices are employed for this 

 purpose, and for work requiring great accuracy, such as bridge 

 foundations, a rod level that fits closely to the angle of the rod 

 and carries two small spirit levels is used to plumb it accurately. 

 For ordinary work, however, this is not required. The leveler 

 can plumb the rod across the line of sight by observing whether 

 it coincides with the vertical cross-hair of the instrument, and 

 he can obtain good results by making the rodman slowly tip the 

 rod backwards and forwards in the direction of the line of sight 

 and then taking the shortest reading. 



FIELD WORK IN LEVELING 



Example in Direct Leveling. The principles of direct level- 

 ing are illustrated in the accompanying illustration. 



Let A be the starting point, which has a known elevation 

 of 20 ft. The instrument is set at B, leveled up and sighted 

 to a rod held at A. The target being set, the reading, 8.42 ft., 

 called a backsight, is the distance that the point where the 

 line of sight cuts the rod is above the point A, and is to be 

 added to the elevation of the point A; 20.00+8.42 = 28.42 

 is called the height of instrument and is designated by H. I. 

 The instrument being turned in the opposite direction, a point 

 C is chosen, which must be below the line of sight. This point 

 is called a turning point, and is designated by the abbreviation 

 T. P. Drive a peg at C, or take for a turning point a rock or 

 some other permanent object upon which the rod is held. The 

 first reading on a turning point is a foresight, and is to be 

 subtracted from the height of instrument at B to find the 

 elevation of the point C. Let the rod reading be 1.20 ft. 

 Then, 28.42 1.20 = 27.22 ft., is the elevation of the point C. 

 The leveler. carries the instrument to D, which should be 

 of such a height above C that, when leveled up, the line of 

 sight will cut the rod near the top. The backsight to C gives 

 a reading of 11.56 ft., which, added to 27.22 ft., the elevation 

 of C, gives 38.78 ft., the height of instrument at D. The 



