Six Conditions to Minimize Long-Distance Measurement Errors with an Electro-Optical Distance Meter

An electronic total station is a surveying instrument widely used on construction sites, civil engineering works, land development, rivers, roads, and for verifying the positions of structures. Even small deviations that are unlikely to cause major problems in short-range layout can, in long-distance measurements, combine with factors such as angle, distance, weather conditions, sighting conditions, instrument setup, and the placement accuracy of the reflector, and appear in the results as differences that cannot be ignored.

Especially for long distances, it is important not to judge based only on the instantaneous reading, but to check whether the measurement conditions themselves are stable. The accuracy of an electro-optical distance meter (EDM) is not determined solely by the machine’s performance; it is influenced by site conditions, work procedures, verification methods, and how records are kept. In this article, we organize six conditions that field practitioners should check to reduce errors in long-distance measurements, from a field-practitioner’s perspective.

Table of Contents

• In long-distance measurements, small deviations can lead to large errors.

• Condition 1 Ensure the stability of the instrument point and the backsight point

• Condition 2: Prepare the line of sight and the reflecting surface

• Condition 3: Be aware of weather conditions and atmospheric effects

• Condition 4: Carefully perform installation, leveling, and sighting.

• Condition 5: Verify values by reciprocal observations and multiple measurements

• Condition 6: Record measurement results and site conditions so they are available for evaluation.

• To stabilize long-distance measurements, a review of the entire workflow is necessary.

In long-distance measurements, small deviations can lead to large errors

In field measurements using a total station, position and height are often checked based on angle and distance measurements. At short distances, slight sighting errors or a tilt of the reflector may not be readily apparent. However, as the measurement distance increases, the same angular error produces a larger positional difference on site. When aiming at a distant point, even a tiny directional deviation tends to show up as a difference in the measured point’s position or height.

One thing to watch out for in long-distance measurements is that the causes of error are not limited to a single source. A slightly unstable instrument setup, insufficient verification of the backsight, errors in the entered reflector height, heat-haze-like shimmering in the line of sight, the reflector face not being properly oriented—many factors can accumulate. As a result, even if the measured values appear stable at first glance, they may actually contain errors biased in the same direction.

Also, at long distances it becomes difficult for workers to visually detect anomalies. At close range they can immediately check the reflector's tilt and installation condition, but when several hundred meters or more away it becomes hard to grasp the setup at the other end in detail. Even when work is being carried out using radio communication or signals, pre-work confirmation rules are essential to avoid overlooking the prism's orientation, the pole's verticality, the stability of the installation surface, or mix-ups of survey point names.

In long-distance measurements with a total station, you need to treat the work as a single sequence that includes not only setting up the instrument and taking measurements, but also preparing beforehand, checking during measurement, and verifying afterward. Rather than simply trying to suppress errors, it is important to reduce conditions that are likely to introduce errors in advance and to create a situation in which anomalous values can be detected. When measurement results are used for setting out, as-built verification, control-point transfer, or checking the alignment of structures, the impact on subsequent processes is significant, so the more distant the measurement, the more careful management is required.

Condition 1 Ensure stability of the instrument tip and the rear view

The first conditions to check in long-distance measurements are the stability of the instrument station and the backsight. A total station performs measurements using the instrument station and the backsight that establishes direction as references. If these references remain unstable, no matter how carefully the measurements are taken, the overall results may be offset. In long-distance measurements, even slight errors in the instrument station or the backsight can greatly affect later measured points, so it is important not to simplify verification of the reference side.

At the instrument station, first check that the tripod legs are firmly in contact with the ground and that sinking or slipping is unlikely. Stability varies greatly depending on the installation surface—paved surfaces, crushed stone, embankments, near the shoulder, on temporary scaffolding, etc. Even if there appears to be no problem, slight movement can occur when a worker walks nearby, when heavy machinery passes, or when the tripod is exposed to wind. Because this slight movement can affect results in long-distance measurements, it is important to check the tripod spread angle, the anchoring of the leg tips, and the tightness of the instrument, and to recheck for any shift in leveling during measurements.

Checking the backsight is equally important. Confirm that the coordinates of the backsight are correct, that the point on site matches the point on the drawings, and that point names have not been confused. On site, there may be multiple similar pins, stakes, markings, or temporary points nearby. Especially on large work areas, similar control point names and auxiliary point names can appear together, creating a risk of setting the backsight based on assumptions. Before long-distance measurements, checking not only the coordinate values but also the relative positions of points and the sense of distance between known points makes it easier to prevent mix-ups.

You should also check whether the combination of instrument station and backsight point is appropriate. If the backsight distance is extremely short while the measurement target is far away, a small error in the direction setting can be magnified at long range. Even when site conditions make ideal placement difficult, you need to consider backsight distance, measurement direction, line of sight, and the reliability of known points comprehensively to ensure the reference setup is not unreasonable. If the reference direction is unstable for distant targets, the reproducibility of the results will also decrease.

Also, entering the instrument height and reflector height is a basic task, yet it is one of the parts most likely to cause errors. Because long-distance measurements can affect not only the horizontal position but also the elevation, it is effective to adopt procedures that prevent simple mistakes, such as reading the input values aloud to confirm them, cross-checking them against field notebook or device records, and rechecking them before and after measurements. In particular, when multiple people are working, it is important not to leave unclear who measures, who enters the data, and who verifies it.

If the stability of a reference point is in doubt, performing checks by comparing with another known point or conducting verification measurements between known points increases the likelihood of detecting anomalies before measurement. For long-distance measurements, it is important not to complete the work with a single measurement, but to confirm that the reference is correct before proceeding with the main measurement. Stabilizing the instrument point and the backsight is a fundamental condition that supports the accuracy of long-distance measurements.

Condition 2: Ensure a clear line of sight and proper condition of reflective surfaces

Stable line of sight from the instrument to the reflector is indispensable for long-distance measurements. Optical distance-measuring instruments determine distance by the transmission and reflection of light to the target, so any obstruction or disturbance in the line of sight makes the measured values unstable. Even when the line of sight appears to be clear, at long distances branches, grass, temporary materials, vehicles, workers, excavator booms, safety installations, and the like can partially intrude. Even slight blockage can cause ranging to become unstable or make sighting difficult, so caution is required.

On site, it is important to confirm line of sight from both the instrument side and the reflector side before measurement. Just because the instrument side can see it does not mean the surroundings of the reflector are fully understood. The person on the reflector side also needs to check that there are no swaying grass or temporary structures around the survey point, that the position where the pole is placed is stable, and that the reflector is facing the instrument. At long distances, voices are hard to hear, so the means of communication and agreed signals also affect measurement accuracy.

The condition of the reflective surface is also important. If the reflector is dirty or has water droplets or mud on it, it can cause unstable distance measurements. In long-distance measurements the received signal tends to be weaker, making the measurement more susceptible to the reflector’s dirt and orientation. Before measuring, inspect the reflector’s surface and clean it if necessary before use. Also confirm that the reflector is properly oriented with respect to the instrument, and avoid taking measurements while it is tilted.

The verticality of the pole must not be overlooked. In long-distance measurements, it becomes difficult for the instrument side to detect small tilts of the reflector. If the reflector-side operator holds the pole tilted, discrepancies can occur in the measured point’s position and height. It is especially difficult to keep the pole stable on slopes, road shoulders, crushed stone, mud, or narrow footing. Use a bubble vial and support devices, and, when necessary, consider a tripod-mounted reflector—take such measures to improve stability on the reflector side.

The background within the line of sight also affects how easy it is to sight the target. If there is a bright sky, strong sunlight, highly reflective structures, moving vehicles, or similar behind the reflector, it can make aiming difficult. At long distances the reflector appears smaller, so even the increased difficulty of distinguishing it from the background makes targeting harder. If possible, adjust the measurement time or the installation position so that measurements are taken when the reflector can be stably and easily captured.

The longer the measurement distance, the more difficult it becomes to completely control the environment along the path. For that reason, instead of checking the line of sight once and calling it done, you need to verify during measurement that the readings are not suddenly fluctuating, that the alignment remains stable, and that there is no movement on the reflector side. In long-range measurements, properly setting up the line of sight and the reflecting surface has a major impact on the stability of the measured values.

Condition 3 Be aware of weather conditions and atmospheric effects

In long-distance measurements with optical surveying instruments, the influence of weather conditions cannot be ignored. Air temperature, air pressure, humidity, wind, solar radiation, and heat rising from the ground surface all affect the stability of distance measurement and sighting. Especially during periods of strong sunlight, or when measuring long distances over paved surfaces, concrete surfaces, or dry development sites, the line of sight can shimmer. If, when looking through the instrument, the target appears to sway or wobble, it becomes difficult to accurately aim at the exact same point.

A common problem in long-distance measurements is that, although the target is visible, the sighting is unstable. Even if the reflector itself can be confirmed, air turbulence can blur the image or make it appear to sway up and down or side to side. If measurements are forced to continue in this condition, not only will the measurement values scatter, but the results may vary depending on where the operator aimed. Atmospheric effects that are hard to notice at short range become more noticeable at long range, so care must be taken with the time of measurement and the way survey lines are run.

Weather conditions affect work planning. Early morning and evening can sometimes have relatively little solar-induced shimmer, but at some sites other problems may arise, such as morning dew, backlighting, dimness, or time constraints on work. Around midday it is bright and easier to work, but thermal shimmer caused by heating of the ground surface can become stronger. On windy days the stability of the instrument body, tripod, reflector, and pole is affected. When conducting long-distance measurements, it is important to judge not simply whether the weather is good or bad, but to take into account the state of the air along the measurement line and any movement of the target.

When correction settings such as air temperature and atmospheric pressure are required, confirm that they have been entered appropriately according to site conditions. Measurement instruments may provide settings for atmospheric correction, but if they remain at their defaults or retain settings from the previous site, the impact can be significant at long distances. Because configuration methods and the required input items vary depending on the equipment and the site management standards, it is important to check them before measurement in accordance with the user manual and site rules.

Rain, fog, dust, and strong backlight are also conditions to be aware of. Even light rain can affect sighting and distance measurement if water droplets adhere to reflective surfaces or lenses. Fog or dust reduces visibility and makes it difficult to acquire distant reflectors. In backlight, the target becomes hard to see and sighting errors are more likely to occur. You must judge not only whether measurement is possible, but also whether the results can be used reliably.

The influence of weather conditions cannot be completely eliminated by operator vigilance alone. However, measures such as avoiding time periods when effects are likely to occur, dividing survey lines into shorter segments, taking multiple measurements to check for variability, and recording weather and visibility conditions with the results will make later interpretation easier. In long-distance measurements, weather conditions should be considered not as external factors but as part of accuracy management.

Condition 4 Carefully perform installation, leveling, and sighting

In measurements with optical surveying instruments, the care taken in the basic tasks of setup, leveling, and sighting determines the results. In long-distance measurements, even slight carelessness in these basic procedures can lead to significant discrepancies. No matter how capable the instrument is, if the setup is unstable, the leveling inadequate, or the sighting off each time, you will not obtain stable measurement results. The farther the distance being measured, the more important it is to return to the basics.

During setup, place the tripod in a stable position to create conditions that reduce leg sinking or slipping. On soft ground, the tripod feet may sink during measurement. On paved surfaces or steel plates, the feet may slip. If you erect the tripod in a cramped space with an awkward posture, forces applied each time the instrument is operated can cause it to move slightly. For long-distance measurements, setup is not a one-time action: check the centering and leveling before and after measurement, and if any displacement is found, determine whether a remeasurement is necessary.

Leveling is the basic procedure for keeping the instrument’s vertical axis correctly aligned. If leveling is inadequate, it can affect horizontal and vertical angles. Even if the instrument is properly leveled at the start of measurements, it may gradually shift due to tripod settlement, wind, or contact during operation. For long-distance measurements, it is desirable to check the leveling condition intermittently after measuring multiple points or when measurement times become long. In particular, making it a habit to check leveling before and after measuring critical points makes it easier to notice any abnormalities.

During sighting, you must steadily aim at the center of the target. At long distances the reflector appears small, and if focusing is inadequate it becomes difficult to capture the target accurately. Neglecting diopter adjustment or focus can cause different operators to aim differently, or even cause the same operator to shift their aim slightly between measurements. Ensure that the crosshairs and the target image in the telescope are clearly visible, and avoid peering in from an awkward posture.

In long-range measurements, even if you think you are correctly aiming at the center of the reflector, its appearance can change depending on the background and lighting conditions. If the reflector’s outline is blurred or there are similar reflective objects nearby, you can become uncertain about where to aim. In such situations, you should include not only the measured values but also the ease of sighting as part of your records and decision-making. Don’t assume there’s no problem just because a number was produced; you need to verify that the operator could stably aim at the same point.

Care is also necessary when operating the instrument. If the fine-adjustment or locking screws are handled roughly, you may overshoot the target or cause slight shifts when tightening. At long distances those tiny shifts will have an impact, so take your time aligning the target and avoid applying unnecessary force to the instrument while measuring. You should also avoid rushing and pressing the measurement button before the sighting has fully settled.

Setup, leveling, and sighting are the basics of the basics, but these fundamentals become especially important in long-distance measurements. Don't leave it to the instrument alone; by having the operator create a stable condition, the reliability of the measurements is improved. The farther the distance, the more necessary it is not to try to rush; you must thoroughly ensure the instrument is set up correctly, aligned properly, and aimed accurately.

Condition 5 Verify values through round-trip observations and multiple measurements

In long-distance measurements, it is important not to rely on a single measurement. Even if a measurement value is displayed, it is not necessarily the correct value. If there is misalignment of the line of sight, a tilt of the reflector, meteorological conditions, the instrument's leveling or setup, or errors in input values, an apparently plausible value may still contain errors. For particularly important survey points and reference survey lines, it is indispensable to take multiple measurements to confirm reproducibility.

When you perform multiple measurements, you are more likely to notice variability in the values. If you get nearly the same value each time, it is easier to judge that the measurement conditions are stable. Conversely, if the distance or angle changes inconsistently with each measurement, there may be problems with line of sight, the reflector, meteorological conditions, or the instrument installation. For long-distance measurements, you must consciously trace the causes of variability back from the numbers. You should not simply take an average; you need to check why the values are varying.

Reciprocal observations are also an effective verification method. When possible, measure not only from one side but also from the opposite side and compare the results. Depending on site conditions, reciprocal observations can be difficult, but for checking important control points or long survey lines it is desirable not to rely too heavily on measurements from only one side. If the forward and reverse measurements do not agree, consider the possibility of problems with one of the instrument stations, the backsight, the reflector setup, input values, or line-of-sight conditions.

In angular observations, performing direct and reverse observations can sometimes make it easier to detect errors related to the instrument and sighting. The observation method required varies depending on the accuracy demanded on site and the nature of the work, but when producing important results over long distances, it is necessary to decide not to rely solely on a single simple measurement. Overemphasizing work efficiency can lead to inconsistencies in as-built measurements or positions being discovered later, which may result in re-surveying and rework.

Verification measurements of known points are also important. By measuring known or check points before and after measuring a new survey point and confirming that the values agree, you can more easily detect anomalies such as the instrument moving, an incorrect backsight setting, or a disturbed setup. Especially for long-duration work or when measuring multiple survey points in succession, performing start-point and end-point checks can enhance the overall reliability of the measurements.

When checking measurements, it is also important to decide in advance the allowable differences on-site. If it is unclear what degree of difference warrants a remeasurement or what degree of variation requires revising the conditions, workers will end up making judgments based on their own instincts. Because the required level of precision varies depending on the type of construction work, the measurement purpose, and the management standards, it is advisable to organize confirmation criteria according to how the results will be used before beginning long-distance measurements.

Taking repeated measurements or making reciprocal observations may seem to increase the workload. However, considering the rework required if an error is overlooked in long-distance measurements, they are a necessary check in critical situations. To make measurement results trustworthy, what matters is not that a number appears but that it can be reproduced under the same conditions and is not contradicted by other methods of verification.

Condition 6 Record measurement results and on-site conditions so they can be assessed

In long-distance measurements, it is important to record not only the measured values themselves but also the conditions under which they were taken. When reviewing the results later, if you cannot determine from which instrument station, which backsight was used, at what reflector height, and under what weather or visibility conditions the measurements were made, it becomes difficult to assess the validity of the figures. Even if there seemed to be no problem at the time of measurement, if discrepancies are found in later stages, insufficient records make it impossible to trace the cause.

Items to record include the instrument point, backsight point, station/point name, instrument height, reflector height, measurement time, operator, weather, visibility conditions, number of measurements, and whether remeasurement was performed. You do not need to write down everything in excessive detail, but you should preserve conditions likely to affect judgment in long-distance measurements. In particular, information such as atmospheric shimmering, strong wind, an unstable reflector setup, having checked the leveling during the process, or having performed a remeasurement will be useful when evaluating the results later.

When recording measured values, it is advisable to retain not only the adopted value but also the results of confirmatory measurements. If only the final adopted value is kept, it is impossible to tell whether that value was obtained consistently or selected from among variations. When measurements are taken multiple times, presenting them in a form that shows how much they differed makes it easier to explain the reliability of the results. In long-distance measurements, it is important to record not only the correctness of the result but also the basis on which you judged it to be correct.

At construction sites, surveying work and construction management are sometimes handed over to different personnel. Even if the person who performed the measurements remembers the situation, the details tend to become unclear as time passes. When another person uses the results, insufficient records make it impossible to judge how much trust to place in them. Because the results of long-distance measurements using a total station may be used for setting out positions, as-built verification, modification of construction plans, and explanations to stakeholders, they need to be recorded in a way that remains understandable later.

Also, when incorporating measurement results into drawings or coordinate data, verify consistency with on-site conditions. Even if the numbers are correct, if the positional relationships with on-site structures, existing installations, or known points feel off, a reconfirmation is necessary. In long-distance measurements, input mistakes or mix-ups of point names may not be immediately apparent visually. Rather than viewing measurement results in isolation, checking their relationships with surrounding points, comparing them with existing outputs, and confirming consistency with site conditions makes it easier to detect errors early.

Record-keeping is not mere paperwork. It is part of accuracy management in long-distance measurements. By recording the conditions during work, preserving verification results, and clarifying the basis for acceptance decisions, the reliability of measurement results is enhanced. In particular, for measurements that may later require accountability, the presence or absence of records can make a significant difference.

Stabilizing long-distance measurements requires a review of the entire workflow

To reduce errors in long-distance measurements with an electro-optical distance meter, it is necessary not to rely solely on the instrument’s performance but to review the entire measurement process. It is important to stabilize the instrument station and backsight point, maintain clear sightlines and proper reflecting surfaces, be aware of meteorological conditions, perform careful setup, leveling, and aiming, check reproducibility by repeating measurements several times, and record site conditions so decisions can be made from them. These measures are not effective individually; they only produce results when used in combination.

In long-distance surveying, getting a measurement reading is not the same as obtaining a usable result. Even if a total station displays distance and angle, mistakes such as mixing up reference points, a tilted reflector, fluctuations in the line of sight, or incorrect settings can leave the results uncertain. In practice, it is important to check not whether a measurement could be taken, but whether it can be used for construction and management without problem.

Work on site constantly changes due to weather, the schedule, movements of heavy machinery, temporary structures, and the placement of workers. A survey line that could be measured without issue in the morning may become unstable by noon because of heat haze. A line of sight that was clear in the morning can be blocked in the afternoon by materials or vehicles. To stabilize long-distance measurements, you need to check the conditions each time measurements are taken and be flexible in choosing measurement methods to suit the situation on site.

To reduce the burden of long-distance measurements, a site-wide surveying plan is also important. Consider whether it is necessary to measure a long distance all at once, whether intermediate points can be established to make checking easier, whether the arrangement of known points can be revised, and whether the work schedule can be adjusted. Rather than remeasuring multiple times along impractical survey lines, preparing reference points that are easier to measure can, in the end, stabilize both accuracy and efficiency.

When used properly, a total station is a reliable instrument for long-distance positioning and construction management. However, the farther the distance, the more it is affected by site conditions and work procedures. To minimize errors, it is important to carefully carry out pre-measurement preparation, checks during measurement, and post-measurement verification, and to have systems in place to detect anomalies.

In recent years, the number of methods to assist on-site verification has increased, such as surveying instruments, recording terminals, and construction management using three-dimensional data. However, regardless of what equipment or systems are used, the basics—confirming reference points, ensuring line of sight, understanding weather conditions, performing multiple cross-checks, and organizing records—do not change. To stabilize long-distance measurements using electro-optical distance meters, it is important not to overestimate the machine’s performance and to carefully verify site conditions and work procedures one by one.