Question
Face Left and Face Right observations using a vernier theodolite will eliminate
Theodolite Observations: Error Elimination
Performing observations with a vernier theodolite in both Face Left (FL) and Face Right (FR) positions is a standard surveying practice. This technique aims to minimize or eliminate systematic errors inherent in the instrument or the observation process.
Understanding Face Left (FL) and Face Right (FR)
- Face Right (FR): The vertical circle is on the right side of the observer when the telescope is in the normal position.
- Face Left (FL): The vertical circle is on the left side of the observer when the telescope is in the normal position (i.e., the instrument is plunged).
By taking readings in both positions and often averaging them, certain instrumental errors can be significantly reduced.
Errors Eliminated by FL/FR Observations
Taking readings in both FL and FR positions primarily helps eliminate errors related to:
- Index Error: This is the error in the vertical circle reading when the telescope is truly horizontal. By observing in both FL and FR, the index error gets averaged out. If the index error is $ \epsilon $ in FR, it becomes $ -\epsilon $ in FL (approximately), and the average reading is closer to the true value.
- Graduation Error: This error arises from inaccuracies in the divisions marked on the horizontal and vertical circles. Observing the same angle using opposite ends of the graduated circle (which happens naturally when switching between FL and FR) helps compensate for these inaccuracies.
Errors Not Fully Eliminated
- Eccentricity Error: This error occurs if the centre of the vernier scale does not coincide exactly with the centre of the graduated circle. While FL/FR observations reduce its effect, it may not be completely eliminated without specific procedures or assumptions.
- Atmospheric Error: Errors caused by atmospheric refraction, temperature gradients, etc., affect the line of sight. FL/FR observations do not directly eliminate these environmental errors.
Therefore, using both Face Left and Face Right observations effectively eliminates index error and graduation error.
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Important Questions from Theodolite
- Which of the following errors is/are eliminated by taking both left and right face observations in a theodolite?
- When conducting a survey using a total station, a zenith angle is measured as $84°13′56″$ in the direct mode. What is the equivalent zenith angle in the reverse mode?
- "Transit the telescope" of a theodolite involves
- Choose the CORRECT statement(s).
- An angle of $90^\circ$ is to be laid out with a theodolite having a least count of $30''$. The angle was measured by repetition method and was found to be $90^\circ 00' 25''$. The offset value at a distance of 300 m from the theodolite to set-out the correct angle is _______ m. (Rounded off to 3 decimal places).
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