MANIPULATION OF THE TELESCOPIC ALIDADE 
107 
compass needle comes to rest in the line of its indicators. The 
orientation line is then ruled along the alidade base. The angle 
between that line and the meridian of the map will be deter- 
mined by the local magnetic declination as modified by any 
divergence which may be present between the line of sight of 
the alidade, the fiducial edge of the alidade base, and the line 
defined by the compass needle and its indicators. These three 
lines need not be parallel; the work will not be affected by diver- 
gence between them. 
Another method of orienting the table before drawing the 
orientation line makes use of a Brunton or other compass, care 
being taken to make allowance for the departure of magnetic 
from true north, and depends upon previous knowledge of the 
magnetic declination in the region. 
Or, again, it may be necessary or desirable to draw the orien- 
tation line on the plane-table sheet before it is oriented for the 
first time. It must then be assumed that the line of sight, the 
compass line, and the alidade straight edge are parallel; the error 
in orientation thus involved will in most cases be of no conse- 
quence. The orientation line is plotted so that the angle be- 
tween it and the meridian line is equal to the magnetic declina- 
tion.* This angle may be scaled off by means of a protractor, or 
if more accurate drafting is desirable, a trigonometric function 
may be used. Draw a line 10 inches long in the desired position 
of true north; at one end erect a perpendicular such that its 
.length in inches is 10 times the tangent of the angle of declina- 
tion; connect the other end with the end of the perpendicular 
by a line which is the desired orientation line. This is more 
accurate than plotting the angle with a protractor which has a 
radius of only 3 or 4 inches for in that case any error in plotting 
is greatly multiplied when the line is extended to the necessary 
length of 12 or 15 inches. 
The whole matter of accurate situation of the orientation line 
is of slight importance when working upon a plain sheet, but is 
of the greatest importance if the work is to be done upon a sheet 
on which the land net has been previously plotted, or upon a 
base map prepared by other surveyors, as for example an enlarge- 
