MANIPULATION OF THE TELESCOPIC ALIDADE 
121 
the necessity for speed. The more commonly used methods will 
be described in the order of their simplicity. 
1. Direct readings on the rod. Occasionally the difference in 
elevation between plane table and rod is so slight that with level 
telescope the middle cross hair intersects the rod. The vertical 
distance from the bottom of the rod, or from any other selected 
point on it, to the point of intersection may be read directly 
from the graduations on the rod’s face. Care must be exercised 
to prevent the confusion of the top and bottom stadia hairs with 
the middle cross hair. It is the visual ray projected by the 
middle hair that is parallel to the striding level and therefore is 
horizontal when the level bubble is centered. 
But, suppose that when the instrument is level, the middle 
hair falls above or below the rod while one of the other hori- 
zontal hairs cuts the rod. In practice it is customary first to 
read the rod intercept for distance and second to determine the 
vertical difference between instrument and rod; therefore, the 
operator has just measured the vertical distance between the 
visual rays projected by the cross hairs at the position occupied 
•by the rod. The bottom hair, in figure 8, for example, cuts the 
rod at a point, the distance of which below the point where the 
middle hair would intersect the rod, were the rod long enough, 
has just been determined. Similarly, the top hair in figure 9 
intersects the rod at a known distance above the middle hair. 
Hence, if any of the three hairs rests on the rod when the tele- 
scope is level, the vertical difference between instrument and rod 
may be measured directly. 
Examples are illustrated in figures 8 and 9. In the former, 
the horizontal distance has been read as 1050 feet and with level 
telescope the bottom hair intersects the rod 9.6 feet above its 
base, which is therefore 14.85 feet below the elevation of the 
instrument. In the latter, the horizontal distance has been 
determined as 1960 feet and with level telescope the top hair 
intersects the rodT.8 feet above its base, which is therefore 8 
feet above the alidade. 
In practice it is only necessary to record the horizontal dis- 
tance and the rod intersection, noting which hair intersects the 
