This basic principle is applied in different ways in different tacheometric
methods. There are basically three systems of tacheometric measurements such as
stadia system, tangential system, and subtense bar system.
This is the more extensively used system of tacheometry particularly for
detailed work, such as those required in engineering surveys. In this system,
a tacheometer is first set up at a station, say P, and a staff is held at station
Q, as shown in Figure 2.2. The difference of upper hair reading and lower
hair reading is called staff intercept s. All the three hairs including central
cross hair are read, and s is determined. Vertical angle, θ, corresponding to
the central hair is also measured. These measurements enable determination
of horizontal distance between P and Q and their difference in elevation.
There are two different types of systems in stadia method. These are as
Fixed Hair Method
In this method, the distance between the upper hair and lower hair, i.e.
stadia interval i, on the diaphragm of the lens system is fixed. The
staff intercept s, therefore, changes according to the distance D and
vertical angle θ.
Movable Hair Method
In this method, the stadia interval ‘i’ can be changed. The stadia hairs
can be moved vertically up and down by using micrometer screws.
The staff intercept s, in this case, is kept fixed. Two vanes (targets)
are fixed on the staff at a fixed interval of 2 m or 3 m.
The fixed hair method is the one which is commonly used and, unless
otherwise mentioned, stadia method means fixed hair method.
Movable hair method is not in common use due to difficulties in
determining the value of i accurately.
Figure 2.2 : The Stadia System