MANIPULATION OF THE TELESCOPIC ALIDADE 
109 
of the plane table may deflect the needle and introduce grave 
errors into the work. The table should never be set up for com- 
pass orientation within 10 feet of a wire fence, within 15 feet 
of a pump-jack, less than 10 feet away from a pipe-line, buried 
or on the surface, closer than 10 feet to a railroad track, or nearer 
than 20 feet to an automobile. 
The plane-table sheet may also be brought into correct orien- 
tation without the use of the compass needle by means of fore- 
sight lines. This necessitates the planning of one’s work for 
days or even weeks in advance, but should be used in all triangu- 
lation work and may be used in orienting the table at any set-up 
where for some reason compass orientation is unwise. Assume 
that two stations are clearly visible, the one from the other, and 
that the table is set up in correct orientation at one, while the 
other has not yet been occupied. A line, the fore-sight, is drawn 
from the point representing the occupied station in the direction 
of the distant objective. It should be drawn the full length of 
the alidade base, extending from the plotted point away from the 
unplotted as well as toward it, even though the point when 
plotted will be only an inch or two away from the occupied 
station. Or if preferred, the fore-sight line may be discontinu- 
ous, covering the estimated location of the distant station on the 
map and including a line an inch or so in length at either end 
of the alidade straight edge. When, later, the station to which 
the fore-sight line has been drawn is reached in the progress of 
the field work, the table should be approximately oriented with 
the eye, the alidade base placed along the fore-sight line so that 
the telescope points back toward the bid station, and the table 
rotated until the distant signal is bisected by the vertical cross 
hair of the alidade. The most likely place for error to enter into 
this manipulation is in the adjustment of the alidade base to 
the ruled fore-sight line; a pocket magnifier should be used in 
the placing of the alidade, if the most accurate results are desired. 
Still another method of orientation makes use of the Baldwin 
Solar Chart. The angle between the apparent position of the 
sun and true north is graphically determined by means of this 
chart which is so constructed that when turned until the proper 
