Explain the system of Tacheometric Measurements.

There are basically three systems of tacheometric leveling:

•  Stadia hair system
•  Tangential system
•  Subtense bar system

THE STADIA HAIR METHOD:

 This is the most extensively used system of tacheometry, particularly for detailed work, such as those required in engineering surveys. Stadia hair system may be of two types:

•  The fixed hair method
• The movable 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 `theta`. when the staff intercept is more than the length of the staff, only half intercept is read, which is equal to the length of the difference between central stadia hair reading and the lower/upper stadia hair reading.

There are basically three systems of tacheometric leveling:

•  Stadia hair system
•  Tangential system
•  Subtense bar system

THE STADIA HAIR METHOD:

 This is the most extensively used system of tacheometry, particularly for detailed work, such as those required in engineering surveys. Stadia hair system may be of two types:

•  The fixed hair method
• The movable hair method

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 `theta`. when the staff intercept is more than the length of the staff, only half intercept is read, which is equal to the length of the difference between central stadia hair reading and the lower/upper stadia hair reading.

fig: Fixed hair method

This methods can be useful even when horizontal sights are not possible. For inclined sights, reading may be taken by holding the staff either vertical or normal to the line of sight.some important features of this method are:

• Distance between hairs is constant.
• The staff intercept varies.
• The apex angle is constant.
• The distance varies with the staff intercept.

Movable 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 can be varied.The staff intercept,`s` is kept fixed. The stadia hairs can be moved vertically up and down by using micrometer screws. Movable hair method is not in common use due to difficulties in determining the stadia interval accurately. Some important features of this method are:

• Top and bottom hairs are movable.
• Staff intercept is kept constant.
• This method is rarely used.