4 Methods to Check the Collimation Error of a Total Station

A total station is an important surveying instrument used to determine positions on site by measuring angles and distances simultaneously. It is used for a variety of tasks such as routine layout marking, as-built verification, topographic surveying, and checking the positions of structures, but if the instrument’s condition or observation conditions are not properly set, small deviations may occur in the measurements. Among these, collimation (sighting axis) error can affect angular observations even when you believe you are aiming correctly, so it is something site personnel should be regularly aware of.

The sighting axis is the axis related to the direction of the line of sight when aiming at a target with a telescope. Generally, it is easiest to understand as the line that passes through the center of the telescope’s crosshairs toward the target. In optical surveying instruments, if the condition of this sighting axis is not appropriate, it can affect the consistency of direct and reverse observations and the reproducibility of angles. To avoid reducing on-site work efficiency, it is important to make a habit of checking it before work or whenever measurement results feel suspicious, rather than searching for the cause after an anomaly occurs.

Table of Contents

• Understand the impact of collimation error on field surveying

• Method 1 Check horizontal angle differences using direct and reverse observations

• Method 2 Verify reproducibility using clear distant targets

• Method 3 Check for deviations by comparing with known points or reference directions

• Method 4 Monitor error trends through routine inspections and record management

• Conditions to check when suspecting collimation error

• Operational points for maintaining stable surveying in the field

• Summary

Understand the Impact of Collimation Axis Error on Field Surveying

Before checking the collimation axis error of a total station, it is important first to understand in what situations the collimation axis error is likely to cause problems. The collimation axis relates to the direction of sight when the telescope is aimed at a target. If the instrument is properly adjusted, when observing the same target in the telescope's direct and reverse positions, the observations will tend to fall within the range assumed by the instrument's specifications and operational standards. However, if there is a misalignment in the collimation axis, differences may appear between the values observed in the direct and reverse positions, or repeated measurements in the same direction may become unstable.

Sight-axis error is, rather than an error in the distance measurement itself, mainly related to the consistency of angular observations. For example, when checking a structure’s centerline, verifying directions near boundaries, setting out positions using long lines of sight, or carrying out transfers from a reference direction, a small angular misalignment can easily appear as a positional difference at the far end. Even differences that are not noticeable over short distances tend to manifest as lateral offsets as distance increases, so particular care is required for tasks that involve long-range sighting.

However, the causes of apparent shifts seen in the field are not all necessarily collimation-axis errors. Tripod settlement, improper leveling, centering errors, incorrect setup of prisms or reflective sheets, input mistakes for instrument height or mirror height, mix-ups of reference-point coordinates, and the effects of temperature or heat shimmer are among the multiple factors that can disturb measurement results. Therefore, when checking for collimation-axis error, it is important not to immediately assume an internal equipment fault, but to verify by isolating the observation method and the surrounding conditions.

Optical distance measuring instruments used in surveying are precision devices, and on-site quick checks should be considered separately from professional inspections and adjustments. The checks performed by on-site personnel are intended to determine whether there is a suspected abnormality in the condition of the instrument, whether it falls within a range that does not interfere with normal operations, or whether re-measurement or verification under different conditions is necessary. If observed values are clearly unstable, if the difference between forward and backward observations is larger than usual, or if doubts remain in important control surveys, it is safer not to force a decision based solely on on-site judgment; follow internal standards and the judgment of supervisors, and, if necessary, consider inspections in accordance with the operating manual or adjustments by specialist contractors.

Method 1: Verify the difference in horizontal angles with direct and reverse observations

One basic method to check for suspected collimation axis errors is verification by direct-and-reverse observations. The telescope of an electro-optical surveying instrument has a normal (direct) position for sighting the target and a reversed position for observing with the telescope inverted. Observing the same target in both direct and reverse positions and checking the consistency of horizontal and vertical angles makes it easier to identify any misalignment related to the line-of-sight direction.

When checking on site, first set the instrument on a stable spot. Choose a location where the tripod feet are unlikely to sink and are less prone to vibration, and perform centering and leveling carefully. If the setup here is sloppy, it becomes difficult to determine whether differences between direct and reverse observations are due to instrument axis errors or simply poor installation. In particular, on paved surfaces, over crushed stone, on embankments, or near temporary scaffolding, slight sinking or movement can affect the readings, so prioritize a stable installation during checks.

Next, choose a clear target at a reasonable distance. If the target is too close, small deviations in aiming are hard to detect; conversely, targets that are too far away, where the image wavers, are more susceptible to atmospheric effects. On site, it is important to select targets whose aiming position can be easily reproduced, such as survey marks with clear contours, reflective targets, the center of a thin pole, or a distinct corner of a structure. Avoid tips of vegetation, swaying temporary structures, vehicles, and surfaces with unstable reflections.

In the direct (positive) position, precisely sight the target and record the horizontal angle. Then reverse the telescope and sight the same target, recording the value in the reverse position. In direct-reverse observations, rather than simply comparing the observed values themselves, check whether the relationship between the direct and reverse readings falls within the normal range. Specific tolerances vary depending on the instrument specifications, the instruction manual, internal standards, the accuracy class of the work, and site management criteria, so they should not be treated as a single universal value. What is important is to confirm that repeated observations under the same conditions do not show large, scattered differences and that there are no changes compared with past inspection records.

Even if a difference is observed between direct and reverse observations, it is risky to immediately conclude that it is a collimation error. Differences can also occur when the aiming point shifts slightly each time, when focus is inadequate, when it is difficult to center on the target, or when the instrument is moving subtly. Therefore, repeating the measurement several times on the same target and performing the same checks on another target makes it easier to judge. If the same trend appears across multiple targets, an inspection of the instrument is more likely to be necessary.

The advantage of direct and reverse observations is that they are relatively easy to perform in the field and allow a rough assessment of the instrument’s condition. They can be carried out quickly before work as part of daily inspections, and can be incorporated as checklist items before important surveys. In particular, performing direct and reverse observations before setting a reference direction over long distances or before control point surveys that affect subsequent processes can reduce concerns about the measured values.

Method 2: Verify reproducibility using a clearly defined distant target

The second method for checking the sighting-axis error is to use a distant, clearly defined target and verify the repeatability when repeatedly sighting in the same direction. When checking errors of an optical surveying instrument, the basic approach is to examine the instrument’s axis relationships, such as by direct and reverse observations, but in field work it is also an important criterion whether the angle remains stable when aiming at the same target.

In this method, it is important first to choose a target that is easy to sight and does not move. Even for a distant target, longer distance is not necessarily better. If the target is so far that it becomes blurred, or if it is affected by heat shimmer, the observations become unstable due to atmospheric conditions rather than the state of the sighting axis. In practice, it is important to select a distance and subject that the instrument’s telescope can clearly center on. For example, fixed reflective targets, structures with distinct corners, and poles installed stably are suitable candidates.

As a checking procedure, sight the same target several times and record the horizontal angle value each time. If you first swing the telescope to a different direction and then return it to the target, it becomes easier to confirm the variation on re-sighting than simply reading the values without releasing the instrument. Repeating the same steps not only in the direct (face) position but also in the reverse position makes it easier to verify both the repeatability of the sighting and the face–reverse consistency.

When assessing reproducibility, it's important not to focus only on single deviations but to look at how the observed values behave. If measurements scatter randomly each time, factors such as sighting operation, how the target appears, tripod stability, and atmospheric turbulence may be influencing the results. On the other hand, if differences repeatedly show a bias in the same direction, that provides grounds to suspect misadjustment of the instrument or shifts related to the sighting axis. In the field, considering variability and bias separately makes it easier to sort out the causes.

When checking with a distant target, focusing is also important. If the scope is not properly focused, it becomes difficult to accurately capture the center of the target. Also, if parallax remains, the perceived alignment between the crosshairs and the target can shift with even a slight change in the observer’s eye position. You need to carefully adjust the focus of the eyepiece and the objective lens, and observe only after ensuring the crosshairs and the target appear stably superimposed.

Observer habits also affect reproducibility. If the direction taken to approach the target differs each time, the way clamps or fine-adjustment screws are used is inconsistent, or the observer unconsciously aims at the left or right edge of the target, such variations in operation prevent a correct assessment of the instrument’s condition. In verification work, decide how to define or capture the center of the target and make observations using the same procedure. When multiple people carry out the check, observer differences will become apparent, which helps to distinguish operational problems from equipment issues.

This method is useful on site for checking the stability of measurements rather than for strictly calculating the collimation-axis error itself. It is particularly helpful when you feel something is off—when angles are harder to align than usual, when the coordinates of the same point do not remain stable, or when the direction shifts slightly each time you re-take a backsight. If performed regularly as a pre-work check, it can also lead to early detection of anomalies.

Method 3: Verify deviations by comparing with known points or reference directions

The third method is to verify, by comparing with known points and reference directions, whether there are any unnatural discrepancies in the observation results. In practical work using a total station, it is common to set the instrument station and a backsight, establish the reference direction, and then observe the survey points. By comparing the relationships among known points and past observation results in this way, it becomes easier to notice abnormalities in the instrument condition, including collimation axis errors.

In verification tasks, it is important to first select a reliable known point. If the known point itself may have been moved or damaged, you cannot judge instrument error based on that point. Verify the condition of the control point marker, the effects of nearby excavations or paving, past observation records, how the coordinate system is handled, and the consistency of point names to determine whether the point can be used as a reference. In particular, temporary control points on site can be displaced by contact with heavy machinery or changes in the ground, so it is safer not to treat them lightly as absolute references.

After setting up the instrument at the station and sighting the backsight to establish the orientation, observe another known point. Confirm whether the observation results are consistent with the direction and distance calculated from the known coordinates. If discrepancies arise, it is necessary to check multiple factors, not only collimation axis error but also instrument centering error, using the wrong backsight, coordinate input mistakes, prism constant settings, mirror height, instrument height, the selection of the observation target, and so on. Suspect a collimation axis error when, even after verifying these basic conditions, a consistent trend in the angular direction is still observed.

When verifying against reference directions, it is important not to judge based on a single point alone. Relying on only one known point makes it difficult to rule out problems with that point’s coordinates or on-site conditions. If possible, observe multiple known points and compare how deviations vary by direction. If only one point disagrees, there may be an issue with that point’s condition or input values. If the same trend appears across multiple directions, it is more likely that the cause lies in the instrument setup or the observation system.

Also, when verifying known points, it is important to treat distance consistency and angular consistency separately. If distances are generally correct but directions are unstable, factors related to sighting or angle observation are the likeliest suspects. Conversely, if neither distances nor angles agree, the cause may be a more fundamental setup error, such as a mix-up of the coordinate system, misidentification of point names, or incorrect instrument-station setup. When checking an electronic total station, organizing which values agree and which do not is the quickest way to determine the cause.

Comparing with past observation records is also useful. If the reference directions or inter-point relationships previously measured at the same site remain, you can detect changes by comparing them with the current observations. However, when using past records you must also verify the conditions of those records. If the coordinate system used, observation date, instrument station, back sight, measurement method, or the condition of the points differ, a simple comparison may not be valid. Do not be reassured merely because records exist; it is important to confirm that the conditions for comparison are met.

Comparing with known points or reference directions has the major advantage of allowing you to check the instrument’s condition in a way that closely matches actual fieldwork. While direct and reverse observations and repeatability checks are closer to verifying the instrument itself, this method makes it easier to detect offsets that will affect results in real surveying operations. By taking the extra step of checking against known points before starting important stakeout or as-built verification, you can reduce the risk of rework or the need to re-survey after the work.

Method 4: Confirm error trends through daily inspections and record management

The fourth method is to identify trends in sighting-axis errors through routine inspections and record management. Checking errors in optical surveying instruments is not something that is sufficient to perform only once. Precision instruments are affected by impacts during transport, storage conditions, frequency of use, site conditions, and changes from long-term use. Therefore, it is important to understand the instrument’s normal condition through regular inspections and the accumulation of records.

During daily inspections, first check the appearance and basic operation. Check for dirt or fogging on the telescope or objective lens, any abnormal feel in the clamps or fine-motion screws, looseness at the connection to the tripod, and any irregular movement of the alignment device. These are not sighting-axis errors themselves, but they affect the stability of sighting operations. If you check angles while the instrument feels off to operate, you may not be able to make correct judgments.

Next, preparing predetermined inspection positions and methods makes it easier to detect changes. For example, set up a stable reference target for inspection near the office or material yard, and perform direct-and-reverse observations and reproducibility checks at a fixed distance and direction. If you carry out checks at different sites, distances, and targets each time, differences due to varying observation conditions become larger and it becomes harder to read changes in equipment condition. Bringing the conditions as close to the same as possible, within practical limits, improves the accuracy of record management.

In the record, note the observation date, instruments used, observer, weather, an overview of the temperature, the inspection target, results of direct and reverse observations, variability during re-aiming, and any observations you noticed. Not only numbers, but memos such as “it was hard to focus,” “the image was shaking,” “the tripod base was unstable,” or “checked after moving” are also useful. This makes it easier to determine later whether changes in the numbers are due to the condition of the equipment or to the observation conditions.

In record management, what matters is looking at trends rather than overreacting to a single outlier. If there is a large difference on only one day, it may be due to the observation environment or the way the operation was performed. However, if multiple inspections show the difference increasing in the same direction, or if the positive/negative discrepancy has clearly grown compared with before, these are reasons to consider equipment inspection. On-site, inspection records tend to be omitted because of busyness, but without records you lack the information needed to judge when an abnormality occurs.

Also, at sites where equipment is shared by multiple people, the importance of record management increases. Even if one person senses something unusual, if that information is not communicated to others, the same equipment may continue to be used in important surveying tasks. Keeping inspection results in a shareable form makes it easier for the entire team to grasp the equipment’s condition. In particular, it is important to thoroughly record and share information when the equipment may have been dropped or subjected to a strong shock, when it has been used after rain, or when unstable values appear during long-distance observations.

Daily inspections and record-keeping are not just for making a strict, on-the-spot determination of whether a sighting-axis error exists. Rather, they create a baseline for normal conditions and provide a mechanism to notice changes quickly. The reliability of measurements from optical surveying instruments is supported not only by the performance of the equipment but also by the daily management practices.

Peripheral conditions to check when suspecting a line-of-sight axis error

When collimation axis error is suspected, it is essential to check the surrounding conditions at the same time. When surveying results show discrepancies, it is easy to want to blame errors in the instrument itself, but on site the cause is often shortcomings in more basic conditions. Verifying collimation axis error is more accurate when you methodically address each surrounding condition.

First, check the instrument's setup. If the centering is off, it will affect the entire observation centered on the instrument point. If leveling is insufficient, the stability of angular observations will also decrease. In situations where the tripod legs are not firmly secured, the footings sink into soft ground, or the instrument is subjected to vibration from workers or vehicles, the instrument's orientation may change during observations. Before checking the collimation error of the line-of-sight axis, it is necessary to review whether the setup is stable.

Next, check the condition of the target. When using a prism or reflective sheet, verify that the center position is correctly sighted and that there are no issues with tilt or installation height. If the pole is not kept vertical or the reflective surface is not oriented properly, measurement readings can differ. In non-prism measurements, distance measurement results can also become unstable due to the material, angle, color of the reflective surface, or nearby reflective objects. Even if you intend only to check the angle, if the appearance of the target is unstable you will not be able to sight it correctly.

Focus and parallax during sighting are also important. If the crosshair is not clearly visible, the target image is out of focus, or the way the crosshair overlaps the target changes with the observer’s eye position, the reproducibility of sighting decreases. Be especially careful during long work periods or in dim evening conditions, when sighting tends to become lax. Before any verification work, make the crosshair clearly visible at the eyepiece, focus on the target with the objective lens, and confirm that no parallax remains.

Weather conditions should not be overlooked. When strong sunlight causes heat shimmer near the ground, distant targets can appear to shimmer. Rain, fog, dust, backlighting, and nighttime lighting conditions also affect the ease of sighting. Checking sighting-axis errors is easier to judge under as stable visibility conditions as possible. If discrepancies appear under poor conditions, it is advisable to recheck at a different time of day or with different targets.

Check the input values and settings as well. Although this is a separate issue from collimation error, mistakes in instrument height, mirror height, prism constant, distance-measurement mode, coordinate system, instrument point name, or backsight point name can introduce deviations in the results. Because multiple factors can interact simultaneously on site, it is important not to judge based only on angular deviations but to verify the settings and observation conditions together. In particular, be careful immediately after bringing equipment in from another site or when the previous day's settings remain.

Operational points for maintaining stable surveying on site

The purpose of checking the sighting-axis error of an optical surveying instrument is not merely to look for equipment faults. The ultimate goal is to obtain stable survey results on site and to prevent rework and construction mistakes. To achieve this, it is important not only to know the checking methods but also to integrate error checks into daily operations in a way that does not impose extra burden.

First, it is effective to establish a rule to carry out verification work before important surveys. It is not realistic to perform lengthy inspections for every task, but for work that has a large impact on subsequent processes—such as establishing control points, checking layout lines and centerlines, staking out structure positions, as-built management, and surveys related to boundaries—the value of prior verification is high. Even short direct-and-reverse observations or known-point checks will reduce uncertainty once work begins.

Next, it is important to standardize observation procedures. If each observer differs in instrument setup, how they take backsights, whether they perform direct and reverse observations, and how they keep records, the quality of the results will not be stable. Although there are site-specific circumstances, defining a minimum set of checklist items makes it easier to maintain the same level of work even when the person in charge changes. For newcomers and less experienced staff, having clear verification procedures also makes it easier to detect anomalies in surveying results.

You should also decide on procedures to follow when you detect an anomaly. If you continue working based only on a subjective feeling when observed values don’t match, it can lead to major rework later. Prepare a stepwise set of responses—such as realigning the instrument, verifying with a different target, adding reciprocal (forward/reverse) observations, measuring one more known control point, cross-checking with another instrument, or consulting a supervisor—so that decisions remain consistent. For tasks involving especially important control values, it is safer not to make the judgment alone.

Care must also be taken in the storage and transportation of the instrument. An optical surveying instrument is equipment used on site, but it is also a precision optical device. Basic handling—avoiding strong shocks during movement, securing it firmly in its case, preventing it from tipping over inside vehicles, drying it properly after rainy conditions, and checking its condition even when it will not be used for long periods—supports surveying accuracy. It is important to detect abnormalities by checking for collimation (sighting-axis) errors, but managing the instrument in a way that reduces the likelihood of such abnormalities is equally important.

In recent years, the importance of data management and record sharing has grown even for on-site surveying work. Rather than treating the observation values obtained with a total station as one-off, organizing them together with inspection records, observation conditions, photographs, and work notes makes it easier to verify results later. When questions arise about survey results, having records makes it easier to trace the cause. Equipment checks and data management may seem like separate tasks, but in terms of overall on-site quality control they are connected.

Checking for collimation errors is not limited to difficult specialist tasks. In everyday practice, by consistently performing the basics—taking direct and reverse observations, re-sighting the same target, checking against known points, and recording the results—you increase your sensitivity to the instrument’s condition. Surveying tends to make numbers on a display appear correct, but it is essential to be conscious of verifying the conditions under which those values were obtained.

Summary

The collimation error of a total station can affect on-site angle observations and position verifications, so field personnel should check it regularly. In particular, small angular deviations are likely to appear in results when sighting over long distances, checking centerlines, setting out positions from a reference direction, and performing as-built verification. To avoid overlooking collimation errors, it is effective to combine direct-and-reverse observations, reproducibility checks with distant targets, cross-checks with known points and reference directions, and routine inspections with record keeping.

However, it is not appropriate to attribute all surveying discrepancies to collimation axis error. There are many error sources in the field, such as instrument setup, centering and leveling, target setup, focus and parallax, meteorological conditions, input of instrument height and mirror height, the prism constant, and the handling of coordinate systems. When checking for collimation axis error, it is important to isolate each of these conditions one by one and judge based on the variability and bias of the observations.

What matters on site is not rushing to check after an anomaly occurs, but establishing a workflow that makes routine checks easy. By continuing basic practices—performing short direct and reverse observations before important surveys, making verification against known points a habit, recording inspection results, and rechecking equipment before work if it has suffered any shock—you increase the reliability of surveying results. Total stations are convenient instruments that support high-precision work, but to obtain correct results you must ensure the instrument condition, observation procedures, and record management are all in order.

Furthermore, when considering on-site efficiency, it is important to review the entire workflow, including how measured data is utilized after measurement. If verification tasks, observation records, photo management, and the organization of coordinate data are fragmented, it becomes easy for having to search again and oversights to occur in later stages. In addition to reliable observations using a total station, keeping inspection records and observation conditions in a format that is easy to share makes it easier to stabilize quality management for the entire surveying operation.