- Instrumental Errors
- Collimation Error
- Error due to Curvature & Refraction
- Other Errors
1. Instrumental error and Correction
- Collimation error
- Correction: Check before use and equalise sights.
- Under sensitive bubble.
- Errors in staff graduation
- Correction: Check
- Loose tripod head.
- Telescope not parallel to bubble tube
- Correction: Permanent adjustment.
- Telescope not at right angles to the vertical axis
- Correction: Permanent adjustment
2. Error of Collimation:
Collimation error occurs when the collimation axis is not truly horizontal when the instrument is level. The effect is illustrated in the sketch below, where the collimation axis is tilted with respect to the horizontal by an angle a:
3. Curvature & Refraction
For detailed explanation Click here...
Other sources of errors in levelling and their correction:
|Type of Error||Correction|
|1. Incorrect setting-up of instrument.|
|2. Movement of staff from position when changing level station.||
|3. Staff not held vertically.||
|4. Parallax: Instrument knocked or moved during backsight-foresight reading||
|5. Ground heating causes chaotic refraction of light||
|6. Tripod or rod settles between measurements e.g Bubble off center||
|7. Staff not properly extended and locked.|