Figure 8-9.-Difference in elevation.
you can see, when the center cross hair is sighted on a
rod graduation that is equal to the h.i. before reading the
vertical angle, then calculating the difference in
elevation is greatly simplified. Obviously, though, if the
line of sight is obstructed and you cannot sight on a rod
graduation that is equal to the hi., then you must sight
on some other graduation.
Another, although less frequent, occurrence in
topographic work using stadia is shown in figure 8-9,
view B. In this situation it is desired to determine the
difference in elevation between two points on the
ground (P1 and P2) from an instrument station (E) that
is located between the two points.
For this discussion, let us assume that a backsight is
taken on a rod held at P1 and then a foresight is taken to
P2. Now the difference in elevation (DE) between the
two points can be written as follows:
In reverse, if a backsight was taken to P2 with a foresight
to P1, then the expression for DE can be written as