THE PLANE-TABLE. 83 



Upon the plane-table sheet is constructed a projection upon the scale 

 of the field work, and upon that are platted such of the primary points as 

 fall xipon the sheet, each plane table sheet being made to correspond to an 

 atlas sheet. These primary points are first occupied by the plane tabler. 



The instrument is set over one of these stations, leveled, and clamped. 

 The ruler edge of the alidade is then laid upon the line connecting this 

 station with a neighboring one upon the sheet, and the table turned until 

 the other station is upon the vertical wire in the telescope. The instrument is 

 then oriented, and, after clamping in azimuth, is ready for work. Keeping 

 the ruler upon the occupied station on the sheet, the telescope is then turned 

 upon other objects which it is desirable to locate, and lines are drawn, in 

 turn, toward them. The instrument is then taken up and moved to a second 

 station, where it is again set up, leveled, and oriented, as before. A sight is 

 then taken, and a line drawn in the direction of each point sighted from the 

 first station, and the intersection of each pah* of sight lines is the true position 

 of the corresponding point upon the map. In this way, station after station 

 is occupied by the plane table, and numerous points are located by inter- 

 section. If possible, each point thus located should be intersected from at 

 least three stations in order to verify its location. 



Any point thus located on the map may be used afterward as a station. 

 In case it is necessary to occupy a point toward which no line has been 

 drawn, or which has not been located, the simplest and best plan for effect- 

 ing its location is as follows: 



Fasten upon the plane-table board, which necessarily has not yet been 

 oriented, a piece of tracing linen, or ,in default of that, a piece of tracing 

 paper. Assume a point upon this linen to represent the station, take sights 

 upon, and draw lines to all located points within the range of vision ; and 

 then, loosening the linen from the board, move it about over the map until 

 these sight lines fall upon the proper points upon the map. Then prick 

 through the position of the station from the lineu to the map underneath. 

 This location should then lie tested by sighting from the point thus found 

 to the various objects to see if the sight hues fall upon the points as marked 

 upon the map. 



