80 A MANUAL OF TOPOGRAPHIC METHODS. 



The plane table is essentially very simple, consisting of a board upon 

 which is fastened a sheet of drawing paper. This board is mounted upon 

 a tripod, which, in the more elaborate forms of the instrument, possesses 

 great stiffness and stability. It should be capable of being leveled, of 

 being turned in azimuth, and of being clamped in any position. Upon the 

 paper is produced directly in miniature a representation of the country. 

 When set up at various places within the area in process of being mapped 

 the edges of the board must always be placed parallel to themselves — that 

 is, a certain edge of the board must always be set at the same angle with 

 the north and south line. This is called orienting the board. 



Directions are not read off in degrees and minutes, but platted directly 

 upon the paper. The instrument used for this purpose is known as the 

 alidade, and consists of a ruler with a beveled edge, to which are attached 

 for rough work two raised sights, and for the higher class of work a tele- 

 scope turning on a horizontal axis. This telescope carries also a delicate 

 level and a vertical arc for the measurement of angles in the vertical plane, 

 from which relative heights are obtained. The method of using this instru- 

 ment is extremely simple in principle, and becomes difficult in practice only 

 when a high degree of accuracy is required. 



The work of making locations from intersections obtained by means of 

 the plane table requires that the instrument have the utmost stability con- 

 sistent with lightness ami portability. It requires an alidade equipped with 

 a telescope of considerable power and good definition. In short, it requires 

 that the plane table be in every respect of the best modern type in order 

 that the highest degree of accuracy possible to represent upon the paper be 

 attained. Various forms of plane-table movement have been in use, includ- 

 ing the heavy and cumbersome but stable movement of the Coast and Geo- 

 detic Survey, and the light but unstable movement used by the same 

 organization in its less important work. At present a table is in general use 

 which was invented by Mr. W. I). Johnson, of this Survey, which combines 

 the elements of stability, lightness, and facility of operation in a remarkable 

 degree. (See Fig. 8.) This movement is essentially an adaptation of the 

 ball-and-socket principle, so made as to furnish the largest practicable 

 amount of bearing surface. It consists of two cups, one set inside the other. 



