PLANE TABLE SURVEYING. 
99 
rays drawn from a will be the position of each object on the map. Fig. 1 
(p. 98) illustrates the manner in which the work is done. 
To continue the survey by obtaining fresh rays to objects from another 
station.— First orient the table correctly, and find the position of that 
station on the board. 
By orienting is meant placing the table in such a position that the 
north and south line on it shall correspond with the magnetic north and 
south; or, what is the same thing, so that the line drawn between any 
two stations on the board shall be parallel to the line between the 
stations on the ground. 
The position on the board of the station at which the board is set up 
can be found, and the board oriented in a variety of ways. 
(1.) When the station has been fixed by two rays from the ends of the base 
or from other stations , all that has to be done is to place a pin in the 
board at the station mark, lay the fiducial edge of the ruler against it 
and against the mark on the board indicating the most distant station 
from which a ray has been drawn, turn the board until the sights are in 
a line with A, and clamp the board, which is then oriented. 
(2.) To find the position when only one ray has been drawn to the station :— 
Set up the table over the station to be fixed, say D (Fig. 1, p. 98), and 
place the fiducial edge of the ruler along the ray that has been drawn, 
say a, d , turn the table until the sights align on A, clamp the table, which 
will then be oriented. Place a pin in at b on the table and turn the ruler 
about until it is aligned on B, and draw a line which will intersect the 
line already drawn at d on the table, the position required. 
Bepeating the last operation with other fixed stations will, if the lines 
intersect, give certainty to the new position. 
It may be mentioned that it is always preferable to choose a station 
which has one ray already drawn to it, to fixing by any of the following 
methods. 
(3.) To find the position when no ray has been drawn to it, but with the 
fixed points on the board , the following methods may be employed. 
With three visible stations, ABC (Fig. 2, p. 100), represented on the 
table by a b c, the table can be oriented, and the position of an unknown 
point x found. 
First Method .— In interpolation the surveyor should set up the plane- 
table at the desired spot, fixing it as level as possible. The compass 
h 2 
