5.5.2. Calibration of Total Station

• Collimation Error Check: Verifying the alignment of optical axis with the mechanical axis.

The collimation error check ensures that the optical and mechanical axes of the Total Station are correctly aligned. This is essential because if these two axes are not aligned, measurements taken from the instrument can be inaccurate. The optical axis refers to the line of sight through the instrument, while the mechanical axis is the physical axis around which the instrument rotates. Verifying this alignment often involves using a distant target (often a known point) and checking whether the sighted line aligns with the target in both horizontal and vertical measurements.

Imagine trying to aim a camera at a subject but finding that the lens is not lined up with the body of the camera. If your camera isn't focused correctly, the pictures you take will be blurry or misaligned. Similarly, if the optical and mechanical axes of the Total Station are not aligned, the data collected will not be accurate.

• Horizontal and Vertical Angle Calibration: Comparing observed angles with known angular references.

This step involves checking the accuracy of the Total Station's measurement of horizontal and vertical angles against established references. Typical references include known angles or benchmarks that have been measured previously. By comparing the measurements from the Total Station with these known values, a surveyor can assess whether any calibration adjustments are necessary. If the angles do not match, it indicates a need for recalibration of the instrument to ensure precise angle measurements.

Consider a carpenter who uses a square to check right angles in a frame. If the square is off, all the angles in the frame will be incorrect. Just like the carpenter needs to verify that the square is accurate, a surveyor needs to confirm that the Total Station measures angles accurately against known standards.

• EDM Calibration: Checking distance measurement against a standard baseline.

Electronic Distance Measurement (EDM) calibration verifies the accuracy of the distance measurements taken by the Total Station. This is typically done by measuring a known distance along a baseline—usually a long, straight line of precisely measured distance. The distances measured by the Total Station are then compared to this known distance. If there are discrepancies, adjustments to the EDM can be made to improve accuracy.

Think of an athlete training with a measuring tape to gauge their sprint distance. If the tape is not accurate, every time they measure their performance, they may believe they are faster or slower than they really are. Similarly, if the Total Station's EDM is not calibrated to a precise standard, the measurements of distance captured during a survey can lead to significant errors.

• Plummet Check: Ensuring optical or laser plummet is correctly aligned with the vertical axis.

The plummet check involves ensuring that the plummet (either optical or laser) of the Total Station is precisely aligned with the instrument's vertical axis. This is crucial because the alignment affects the accuracy of vertical measurements. An improperly aligned plummet can cause errors in establishing the true vertical point of the instrument. Surveyors typically perform this check by observing a known point directly below the instrument and adjusting the total station until the plummet aligns exactly with that point.

Think of a heavy pendulum hanging from a ceiling. If the pendulum is already swaying even slightly off-center, every time it swings, it may indicate a different angle, giving you an inaccurate reading of where the center is. In the same way, an aligned plummet ensures that the Total Station accurately reflects vertical positions, ensuring that measurements taken are based on true vertical lines.

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