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Distance Between Two
Points
Pacing
Chaining
Temperature
Standardisation
Catenary
Slope
Tension
Tachaeometry
Stadia System
Measurement of
Tacheometric Constants
Refraction and Curvature
Theodolite
Tachaeometry
Field Tachaeometry
Electronic Tachaeometry
Stadia System
There are two types of instruments used for stadia surveying. In the first type the distance between the two stadia hairs in the theodolite telescope is fixed. In the second type of equipment the distance between the stadia hairs is variable, being measured by means of a micrometer.
The most common method used involves the fixed hair tacheometer, or theodolite. 
The notes below shows the calculation of the distance (D) from the centre of the fixed hair tacheometer to a target.
From the diagram, triangles AOB, aOb are similar
|
OX
|
=
|
U
|
=
|
AB
|
|
Ox
|
V
|
ab
|
Also if OF = f = focal length of object lens
|
then
|
1
|
+
|
1
|
=
|
1
|
(lens equation) and multiply both sides by (Uf). |
|
U
|
V
|
f
|
|
u -
|
U
|
.f + f |
|
V
|
|
u =
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AB
|
.f + f |
|
ab
|
AB is obtained by subtracting the reading given on the staff by the lower stadia hair from the top one and is usually denoted by s (staff intecept), and ab the distance apart of the stadia lines is denoted by i. This value i is fixed, known and constant for a particular instrument.
|
U =
|
f
|
.s +f |
|
i
|
|
D =
|
f
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.s + (f + c) |
|
i
|
The reduction of this formula would be simplified considerably if the term f/i is made some convenient figure, and if the term (f + c) can be made to vanish.
D = Cs + k
In practice, the multiplicative constant generally equals 100 and the additive constant equals zero. This is certainly the case with modern instruments by may not always be so with older theodolites.