MANIPULATION OF THE TELESCOPIC ALIDADE 
123 ‘ 
section and add that figure to 1/100 the horizontal distance to 
get the V. D. (vertical difference in elevation), which in this 
case, would be negative. If the bottom hair is still above the 
rod, note where it in turn intersects some fixed object in the 
field of vision and turn down the instrument until the top hair 
occupies its position. The bottom hair now appears to be 2/100 
the horizontal distance below the point intersected by the middle 
hair with level sight. The process may be repeated, noting how 
many steps’’ are used, until the bottom hair finally intersects 
the rod. 
Obviously, the same method may be utilized for determining 
elevations of stations above the instrument by stepping up” 
from the level sight until the top hair cuts the rod. Figure 10 
illustrates the method. In recording observations it is only 
necessary to note the observed distance, the number of steps, the 
final rod intersection, and the sign of the V. D., plus for stations 
above and minus for stations below the instrument elevation. 
For example, a sight to a station 1760 feet distant, recorded as 
“ +4 (steps) — 3.5”, indicates that the base of the rod is (4 X 
17.6) — 3.5, or 66.9 feet above the instrument. Or, a sight of 
1320 feet with the V. D. recorded ^‘—3 (steps) — 12.9” indicates 
that the base of the rod is — (3 X 13.2) — 12.9, or — 52.5 feet 
in relation to the altitude of the telescope. 
Attention should be directed again to the fact that the first 
^^step” is in reality only a “half step” for by it one of the outer 
hairs is moved to the position occupied by the middle hair, 
whereas, each step, after the first, involves the movement of 
one outer hair to the position occupied by the other outer hair. 
This is compensated by the fact that the reading of the rod 
intersection after the final “step” is a reading of the position of 
an outer hair, not that of the middle hair. This makes the 
final “step” really a “step and a half” for the hair, the inter- 
section of which is read, is a^half intercept above or below the 
middle hair. 
The “step” method when used by an experienced instrument 
man is very fast and fairly accurate. It is not, however, suffi- 
ciently accurate for important work, as there is wide margin of 
