STADIA AND PLANE-TABLE SURVEYING 107 



PLANE-TABLE SURVEYING 



Plane Table. Fig. 1 shows a Johnson plane table, whicn 

 is the one most generally used in private work. Its essential 

 parts are: (1) a drawing board mounted on a tripod, with con- 

 trivances for leveling the board and for turning it horizontally, 

 called the movement, and (2) an instrument for sighting and 

 transferring the line of sight to the paper on the board, 



FIG. 1 



called the alidade. The latter consists of a telescope provided 

 with a level tube, a vertical circle, and stadia wires. The 

 telescope is carried by an upright resting on a metal ruler. The 

 vertical plane in which the line of sight of the telescope is 

 moving is parallel to the edge of the ruler. The declinator C is 

 a compass mounted on a base whose edges are parallel to the 

 line joining the zero marks of the compass; it serves for 



