Correct Answer - Option 3 : Reiteration
Concept
Measurement of Horizontal Angles
a) Repetition Method.:This method is used for very accurate work. In this method, the same angle is added several times mechanically and the correct value of the angle is obtained by dividing the accumulated reading by the no. of repetitions. The No. of repetitions made usually in this method is six, three with the face left and three with the face right. In this way, angles can be measured to a finer degree of accuracy than that obtainable with the least count of the vernier.
b) Reiteration Method. This method is another precise and comparatively less tedious method of measuring the horizontal angles. It is generally preferred when several angles are to be measured at a particular station. This method consists of measuring several angles successively and finally closing the horizon at the starting point.
Note:
The method of repetition eliminates different errors present in the measurement of a horizontal angle as follows:
- The errors due to the eccentricity of Vernier’s and centers get eliminated as readings from both the Vernier’s are taken
- The errors due to inaccurate graduations get eliminated as the readings are observed at different parts of the circle
- The errors due to lack in the adjustment of line of collimation and the horizontal axis of the instrument get eliminated for considering both face readings
- Errors due to the inaccurate bisection of the object, eccentric centering, etc are eliminated partially as these get counter-balanced in different observations
Note: The errors due to slip, due to the displacement of station or its signal do not get eliminated and moreover, these errors are cumulative in nature.
