Introduction
Adjustments of a surveying compass refer to the processes of aligning, calibrating, and fine-tuning the instrument to ensure it provides accurate and reliable measurements. These adjustments are crucial in the field of surveying for several reasons:
- Accuracy: Proper adjustments ensure that the compass readings are as close to true values as possible, minimizing errors in surveying data.
- Reliability: Well-adjusted compasses provide consistent results, which is essential for creating dependable survey maps and plans.
- Precision: Even small misalignments can lead to significant errors over large distances. Adjustments help maintain the high level of precision required in surveying work.
- Longevity: Regular adjustments help identify and correct minor issues before they become major problems, extending the lifespan of the instrument.
- Adaptability: Adjustments allow the compass to be used effectively in various environmental conditions and terrains.
- Compliance: Many surveying standards and regulations require instruments to be properly adjusted to ensure the quality and legality of survey results.
Surveyors need to understand and perform these adjustments to maintain the integrity of their work. The adjustments are divided into two main categories:
i.Temporary adjustments: which are performed at each setup.
.Permanent adjustments: which areii done less frequently to correct fundamental issues with the instrument.
a) Temporary Adjustment
Temporary adjustments, also known as station adjustments, are those that need to be made every time the instrument is set up at a new location. The temporary adjustments compensate for the variations in instrument position and local conditions that occur as the survey progresses from one point to another.
The process of making temporary adjustments is relatively quick but requires attention to detail. It typically takes a few minutes at each new station but saves considerable time and improves accuracy in the long run. These adjustments ensure accurate readings for each survey point. They include:
i.Centering
The process of centering the instrument, i.e., positioning the pivot exactly vertically over the ground station mark, is a crucial step in setting up a compass for surveying. This procedure, known as centering, ensures accurate measurements and bearings.
To begin, the compass is securely mounted on top of a tripod. The tripod provides a stable platform and allows for precise adjustments. Centering is achieved by carefully manipulating the legs of the tripod. This may involve extending or retracting individual legs and adjusting their spread to achieve the desired position.
There are several methods to verify proper centering:
- Plumb bob method: A plumb bob, which is a weight suspended on a string, can be hung from the centre of the circular compass box. When the instrument is correctly centered, the tip of the plumb bob should align exactly with the ground station mark.
- Pebble drop method: If a plumb bob is not available, a simple alternative is the pebble drop test. A small pebble is dropped freely from the center of the bottom of the circular box. If the compass is perfectly centered, the pebble will fall directly onto the ground station mark.
- Optical plummet: More advanced instruments may feature an optical plummet, which is a small telescope built into the instrument that allows the surveyor to look directly down the vertical axis to align with the ground mark.
- Laser plummet: Modern digital compasses and total stations often include a laser plummet, which projects a visible dot onto the ground for easy alignment.
It’s important to note that centering is not a one-time process. Throughout the survey, the instrument’s position should be periodically checked and adjusted if necessary, as vibrations, wind, or unstable ground conditions can cause slight movements that affect accuracy.
Additionally, after centering, the instrument must be leveled using bubble levels or electronic tilt sensors to ensure that the horizontal circle is truly horizontal. This two-step process of centering and leveling is essential for obtaining reliable survey data.
ii. Levelling
Levelling is a critical process that ensures the compass is held in a perfectly horizontal position, allowing its graduated ring to swing freely and accurately. This step is essential for obtaining precise measurements and bearings.
Methods of Levelling:
- Eye Judgment: The most basic method of levelling is by eye judgment. This requires experience and a keen eye to visually assess when the instrument appears level.
- Ball and Socket Arrangement: Most modern compasses are equipped with a ball and socket arrangement attached to the tripod. This mechanism allows for quick and precise levelling of the instrument. The surveyor can make fine adjustments by manipulating the ball within the socket until the compass appears level.
- Plate Levels: More advanced surveyor’s compasses often feature two plate levels positioned at right angles to each other. These are small spirit levels (bubble levels) integrated into the compass body. To level the instrument:
- Adjust the ball and socket arrangement until both bubbles are centered in their respective vials.
- This dual-axis levelling ensures the compass is truly horizontal in all directions.
Ball and Socket Arrangement and Plate Levels are actually two separate but complementary methods that are often used together in modern surveying compasses.
- Electronic Levels: Some modern digital compasses incorporate electronic tilt sensors that provide a numerical readout of the instrument’s inclination. These offer even greater precision in levelling.
Levelling Procedure:
- Set up the tripod and mount the compass securely.
- If using plate levels, align one level parallel to two of the footscrews and the other level perpendicular to these.
- Adjust the footscrews to center the bubbles in both plate levels.
- Rotate the compass 90° and readjust if necessary.
- Repeat until the bubbles remain centered in all positions.
It’s important to note that levelling should be checked periodically during the survey, especially if the instrument is moved or if working in unstable conditions. The combination of proper centering and accurate levelling forms the foundation for reliable compass surveying.
iii. Focussing the prism (Applicable only to prismatic compass)
Focusing the prism is a step specific to prismatic compasses and involves adjusting the prism’s position to obtain a clear, sharp view of the compass card’s figures and graduations. This process is crucial for accurate readings and reducing parallax error. The prism, typically mounted on a sliding or threaded mechanism, is moved up or down until the observer sees the sharpest image through it. Proper focus ensures accurate bearing readings, reduces eye strain, and minimizes the risk of misreading graduations. Factors like individual eyesight, lighting conditions, and prism cleanliness can affect focus quality. Regular maintenance of the prism is important for optimal performance.
b) Permanent Adjustment
Permanent adjustments of a compass are essential to establish fundamental relationships between its various components.
- These adjustments are typically made by manufacturers or skilled technicians using specialized tools and procedures.
- Periodic checks of these adjustments are crucial to maintaining the compass’s accuracy over time.
- If any of these relationships are found to be out of adjustment, the compass should be sent for professional servicing.
- Proper care and handling of the compass can help maintain these adjustments for extended periods.
These permanent adjustments are the foundation of the compass’s accuracy and reliability in surveying operations. They differ from temporary adjustments, which are performed by surveyors in the field, as they establish the baseline precision of the instrument
The following fundamental relationships between different parts of a compass are established through permanent adjustments to ensure the instrument’s accuracy:
Vertical Axis Alignment:
- When the plate bubbles (if provided) are centered, the vertical axis of the compass should be truly vertical. This ensures that the compass is level when the bubbles are centered, providing a stable base for accurate measurements.
Sight Vane Verticality:
- When the instrument is properly leveled, the sight vanes should be vertical. This ensures that the line of sight is truly perpendicular to the horizontal plane of the compass, which is essential for accurate sighting.
Needle and Pivot Alignment:
- The ends of the needle and the center of the pivot should lie in the same vertical plane. Proper alignment allows the needle to swing freely and accurately point to magnetic north.
Pivot and Graduated Ring Centering:
- The center of the pivot should coincide with the geometric center of the graduated ring. This ensures that the needle rotates concentrically with the graduated ring, leading to accurate angle readings.
1.Vertical Axis Alignment (Adjustment of the Plate Levels)
Objective:
To ensure that the vertical axis of the compass is truly vertical when the plate bubbles are centered.
Test Procedure:
- Set up the compass on a tripod, ensuring it is placed on firm, stable ground.
- Align one of the levels parallel to a pair of leveling screws. Center the bubble by turning both screws simultaneously either inwards or outwards (see Fig. (a) below).
- Rotate the instrument 90° about its vertical axis until the level tube is parallel to the line joining the third screw and the midpoint of the first two screws. Center the bubble using the third screw (see Fig. (b) below).
- Rotate the instrument 180°. If the bubble remains centered, the instrument is properly adjusted.
Adjustment Procedure:
- If the bubble does not remain centered after rotating 180°:
- Correct half of the error using the capstan screw.
- Correct the remaining half using the leveling screws.
- Repeat the process until the bubble remains centered throughout a full rotation of the compass.
Important Notes:
- For compasses without leveling screws, the ball-and-socket arrangement may be used for this adjustment.
- Plate level adjustment is often considered an unnecessary refinement due to the inherent limitations of compasses.
- Level tubes are generally not provided in standard surveying compasses.
Additional Considerations:
- This adjustment requires patience and precision to achieve accurate results.
- Perform this procedure in stable conditions to avoid external influences, such as wind or uneven ground, which can affect accuracy.
- Regular checks of this adjustment can help maintain the compass’s accuracy over time. If frequent adjustments are needed, this may indicate wear or damage to the instrument.
While this adjustment process is thorough, it’s important to note that many surveying compasses do not include plate levels. Therefore, this procedure applies mainly to more specialized or precision instruments.
2.Sight Vane Verticality (Adjustment of Sight Vanes)
Objective:
To ensure that both the sight vane and object vane are vertical when the compass is level.
Procedure:
- Level the compass carefully to ensure accurate readings.
- Suspend a plumb bob approximately 25 meters away from the compass.
- Check vertical alignment: Move your eye from the top to the bottom of the eye vane slit and observe whether the hair of the object vane remains aligned with the vertical plumb bob.
- If the sight vane is equipped with a simple hole instead of a slit, check the line of sight from top to bottom along the hair of the object vane.
- Rotate the compass 180° and test the sight vane again by sighting through the hole in the object vane to ensure it remains aligned with the plumb bob.
Adjustment Procedure:
- If the hair of the object vane does not remain aligned:
- File one side of the bottom of the vane where it rests on the plate, or
- Insert a paper packing to correct the misalignment.
- Repeat the test and adjustment until the error is fully eliminated.
This adjustment ensures that the sight vane and object vane maintain accurate vertical alignment for precise sighting during compass use. Regular checks and adjustments are important to uphold the accuracy of your instrument over time.
3.Needle and Pivot Alignment (Adjustment of the magnetic needle)
Objective:
To ensure that the magnetic needle is straight and functions accurately.
Procedure:
1. Test for Vertical Bending:
- Check for rocking: Lower the needle onto its pivot and observe if there is any rocking motion at the ends of the needle’s upper surface.
Adjustment:
- If rocking is detected, remove the needle and carefully bend it in a vertical plane until the rocking is eliminated when replaced.
2. Test for Horizontal Bending:
- Step 1: Level the compass and record the readings at both ends of the magnetic needle in any given position.
- Step 2: Rotate the compass so that the graduation which was originally opposite the north end of the needle now faces the south end.
- Step 3: Compare the new reading at the north end with the previous reading at the south end.
- If the readings match, the needle is straight.
- If there is a discrepancy, half of this difference represents the amount of deviation from straightness.
Adjustment:
- To correct the horizontal bending, remove the needle and bend the north end horizontally by half the measured angle. Adjust it in the direction that would bring the north end reading to match the south end reading when reversed.
- Replace the needle and repeat the process until the readings are consistent and the needle is properly straightened.
This adjustment process ensures that the magnetic needle remains straight and provides accurate readings. Regular testing and correction of the needle are important to maintain the precision of the compass.
4.Pivot and Graduated Ring Centering (Adjustment of the pivot)
Objective:
To ensure that the pivot point is positioned at the geometrical center of the graduated ring.
Test Procedure:
- Step 1: Read the values at both ends of the magnetic needle at various points around the graduated ring.
- Step 2: If the readings at both ends of the needle differ by exactly 180°, the pivot is correctly centered (assuming the needle is straight).
Adjustment Procedure:
- If the readings do not differ by 180°, follow these steps:
- Identify the maximum discrepancy: If the difference between the readings varies across positions, determine where the discrepancy is greatest.
- Adjust the pivot: Remove the needle and carefully bend the pivot at a right angle to the position where the discrepancy was most pronounced. Bend the pivot towards the larger segment of the graduated ring as determined by the needle’s position.
- Repeat the test: Continue testing and adjusting until the readings at both ends of the needle agree by 180° at all positions around the ring.
This procedure ensures that the pivot is centered, maintaining the accuracy of angle measurements on the compass. Regular testing and adjustment are necessary to sustain the precision of the instrument over time.
Conclusion
Surveying compasses are essential tools that require careful adjustments to ensure precision and accuracy in the field. By performing both temporary and permanent adjustments, surveyors can maintain the reliability and functionality of their instruments over time. Temporary adjustments, such as centering and leveling, are crucial at each station, while permanent adjustments—like aligning the vertical axis, ensuring sight vane verticality, and maintaining needle and pivot alignment—preserve the compass’s baseline accuracy.
