108 STADIA AND PLANE-TABLE SURVEYING 



determining the magnetic meridian on the drawing. The 

 plumbing arm e p p'e' serves for suspending a plumb-bob, 

 so that it will be directly under a point e on the paper 

 representing the point determined on the ground by the 

 plumb-bob. 



The plane table is used for preparing topographical maps. 

 In the survey of larger areas, reference lines forming a network 

 of triangles are run with a transit and platted on the drawing 

 of the plane table; the vertexes of these triangles, called 



P/ane Tab/e at A 



FIG. 2 



triangulation stations, are used for determining other points of 

 the survey by means of the plane table. 



Orienting. When the plane table set up over a point has 

 each line platted on it parallel to the corresponding line in 

 the field, it is said to be oriented. 



Let aft, Fig. 2, be the platted position of the line AB on the 

 ground, and assume that the plane table is to be oriented at A. 

 First, orient the table approximately by the eye and at the same 

 time, by means of the plumbing arm, bring the point c over A. 

 Then level the table and, with the edge of the alidade ruler 



