TACHEOMETRIC SURVEYING - Civil Engineering Notes KPSC

TACHEOMETRIC SURVEYING 


• Tachometry is a branch of angular surveying in which the horizontal and vertical distances ofpoints are obtained by instrumental observations. 
• Chaining is entirely eliminated in tachometry. 
• This method is most rapid but less accurate. 
• The accuracy of tachometry is less than that of chaining 
• The primary object of tacheometry is the preparation of contoured map or plan. 
• The instruments used in tacheometry are a tacheometer and a leveling or stadia rod. 
• Tacheometer is a transit theodolite having a stadia telescope ie, a telescope fitted with two additional cross hairs called stadia hairs.
• Stadia rod is 5 cm to I5 cm wide and 3 to 4m long. It is graduated in meters, decimeters and centimeters. • For short sights up to 100m a stadia theodolite and a leveling staff may be used. But for long sights, say more than 300m a tacheometer and stadia rod are required. 

TACHEOMETRIC METHOD 

I. STADIA METHODS 

1. Fixed hair method: 
• This is the most commonly used method in tachecmetry, In this method the stadia hair interval is fixed and the staff intercept IS taken. 
• The required horizontal and vertical distances are computed from this staff intercept. 
• In case of inclined sights, the staff may be held vertically or normal to the line of sight 

2. Movable hair method : 
• This method is rarely used. 
• In this method, the stadia wires can be moved by means of micrometer screws. 
• The staff is provided with two targets fixed at a known distance apart, usually 3m. 
• The variable stadia interval is measured. The required distance may be computed from this value 

II. TANGENTIAL METHOD 
• This method is used when the telescope of the instrument is not equipped with a stadia diaphragm. 
• In this method a staff fitted with two targets or vanes at a fixed distance apart, usually 3m is held at the station. 
• Vertical angles to the two vanes are observed with a theodolite. 
• Calculation of distance in stadia method


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