Explain Temporary Adjustment Of Theodolite
When surveyors and engineers work with precision instruments, one of the most essential skills they must master is the proper adjustment of a theodolite. This process ensures accurate angular measurements, which are critical in mapping, construction, and civil engineering projects. Among the different methods of preparing the instrument, temporary adjustment of theodolite stands out as a necessary step every time the instrument is set up for use. Without this process, even the most advanced theodolite would fail to deliver correct results, making it vital to understand each stage in detail.
What is Temporary Adjustment of Theodolite?
Temporary adjustment of theodolite refers to the set of operations carried out each time the instrument is placed at a new station. Unlike permanent adjustment, which is concerned with long-term calibration of the instrument, temporary adjustment is performed every time before actual observations begin. The purpose is to set the instrument into its working condition so that measurements can be made accurately.
Objectives of Temporary Adjustment
The temporary adjustment serves several objectives. The most important include
- To ensure the theodolite is securely placed and stable on its tripod.
- To position the instrument so that it is approximately level and centered over the survey station.
- To establish the line of sight and reading scales in proper condition for precise measurements.
- To prepare the instrument in a way that minimizes errors during fieldwork.
Steps Involved in Temporary Adjustment of Theodolite
The procedure for temporary adjustment can be divided into three main steps. Each must be carefully executed to guarantee reliable readings.
1. Centering
Centering is the process of placing the theodolite exactly over the survey station point. This is usually done with the help of a plumb bob or optical plummet. The steps include
- Mount the theodolite on the tripod loosely so that its position can be adjusted.
- Place the plumb bob string over the station mark or use the optical plummet to align with the station point.
- Adjust the legs of the tripod until the plumb bob is nearly centered above the station mark.
- Finally, use the shifting head screws of the theodolite to bring the instrument precisely over the point.
Accurate centering ensures that all angular measurements taken will correspond correctly to the survey station.
2. Leveling
Once the theodolite is centered, the next step is leveling. Leveling ensures that the vertical axis of the theodolite is truly vertical. This is critical because any tilt can result in errors in angular measurement. The procedure involves
- Position the telescope parallel to one pair of leveling screws and adjust them simultaneously to bring the bubble of the level tube to the center.
- Rotate the telescope by 90 degrees so that it lies over the third leveling screw, and then adjust this screw to center the bubble again.
- Repeat the process until the bubble remains centered in all directions.
When leveling is done correctly, the theodolite’s vertical axis becomes perpendicular to the horizontal plane, ensuring that readings are free from tilt errors.
3. Focusing and Elimination of Parallax
The last step in temporary adjustment is focusing, which eliminates parallax error. Parallax occurs when the image of the object and the crosshairs of the theodolite are not in the same plane, causing inaccurate observations. The procedure is as follows
- Adjust the eyepiece by pointing the telescope at a bright surface and turning the eyepiece until the crosshairs appear sharp and distinct.
- Focus the objective lens on the target object by rotating the focusing screw until the image becomes clear.
- Ensure that there is no relative movement between the image and the crosshairs when the observer shifts their eye slightly.
Once parallax is eliminated, the instrument is ready for precise measurement.
Importance of Temporary Adjustment in Fieldwork
Carrying out the temporary adjustment of theodolite is not a formality but a necessity. Fieldwork often requires accuracy up to seconds of an angle, and even a small error in setup can lead to significant deviations in results. By following proper centering, leveling, and focusing, surveyors can trust their readings, reducing the risk of costly mistakes in construction or mapping.
Common Mistakes to Avoid
While the process may seem straightforward, there are common errors that must be avoided
- Improper centering of the instrument, which leads to systematic displacement of all observations.
- Inaccurate leveling, which results in incorrect horizontal and vertical angle measurements.
- Failure to check for parallax, leading to blurred or shifting crosshair alignment during observations.
- Rushing through the process without double-checking the bubble and alignment.
Attention to detail during temporary adjustment ensures the theodolite functions as designed.
Temporary Adjustment vs Permanent Adjustment
It is important to distinguish between temporary and permanent adjustment of theodolite. Temporary adjustments are performed every time the instrument is set up, while permanent adjustments involve correcting the fundamental errors of the instrument, such as misalignment of the optical axis or collimation errors. Temporary adjustments are more frequent and necessary for routine use, while permanent adjustments are usually done by skilled technicians at specific intervals.
Applications of Theodolite After Temporary Adjustment
Once properly adjusted, the theodolite can be used for a wide range of tasks in surveying and engineering
- Measuring horizontal and vertical angles with precision.
- Establishing straight lines and accurate right angles on the ground.
- Conducting topographic surveys for mapping terrain features.
- Setting out construction works such as roads, bridges, and buildings.
- Checking alignment in large-scale engineering projects.
Without proper temporary adjustment, none of these applications could be executed with the required accuracy.
The temporary adjustment of theodolite is an essential process that ensures reliability and precision in surveying tasks. By carefully following the steps of centering, leveling, and focusing, surveyors prepare the instrument to perform its role effectively. This process might appear routine, but it directly affects the quality of results in projects ranging from small-scale land surveys to massive infrastructure developments. A well-adjusted theodolite is the foundation of accurate fieldwork, making temporary adjustment a skill that no professional in the field can overlook.