5 Causes of Unstable Distance Measurements with Optical Distance Meters

When performing field work with an electronic distance meter (EDM), you may encounter situations where the measured distance to the same point varies slightly, distance measurement takes longer than expected, or repeated measurements fail to settle to a stable value. Variations in distance are not necessarily caused solely by a fault in the instrument itself. They can arise from a combination of factors such as the condition of the prism or reflective surface, line-of-sight conditions, aiming, meteorological conditions, stability of the instrument set-up, settings, and work procedures.

On site, what matters is not immediately assuming the instrument has malfunctioned, but isolating the cause by aligning measurement conditions one by one. A total station is a device that handles distance and angle, and the round trip of light to the target, reflections, the line of sight, and the instrument’s setup all affect the results. Therefore, the more unstable the distance measurements are, the more important it is to verify the basic conditions.

Table of Contents

• How to recognize unstable measurement distances in the field

• Cause 1: Conditions of the prism or reflective surface are not consistent

• Cause 2: The line of sight or aiming is slightly disturbed

• Cause 3: Affected by weather conditions or the measurement environment

• Cause 4: There are discrepancies in basic settings such as the instrument station or mirror height

• Cause 5: Measurement procedures and recording rules are not standardized on site

• How to troubleshoot when distances are unstable

• Summary: The more the field conditions are standardized, the more stable the measured distance becomes

How to evaluate unstable measurement distances in the field

There are several patterns of unstable distance measurements with an electronic distance measuring instrument. For example, even when the same prism is placed in the same position, the measured distance may vary slightly from one measurement to another. Also, there may be a delay between pressing the distance-measurement button and the result appearing, or measurements may sometimes succeed and sometimes fail. In non-prism measurements, the measured distance may correspond to a surface different from the one being targeted.

When this kind of phenomenon occurs, the first things to look at are the magnitude of the variability and its reproducibility. Check whether it consistently shifts in almost the same direction each time, whether it moves up and down with each measurement, or whether it only happens at certain distances or in certain directions. If it is unstable in the same way in all directions, suspect the instrument setup, its settings, weather conditions, or the instrument’s maintenance condition. On the other hand, if only a particular measurement point is unstable, the cause may lie in conditions on the measurement-point side, such as the prism, reflective surface, line of sight, obstacles, or the material of the target surface.

In practice, you can't assume everything is fine just because a measurement has been taken once. If you proceed with coordinate calculations or as-built verification while distances are still unstable, it can become difficult later to reconcile point clouds and coordinates, which may require re-measurement or rechecking. Reference points, backsight points, edges of structures, as-built control points, and restoration points in particular are locations where even slight measurement instability can more easily affect downstream processes. To shorten work time, it's important to identify and eliminate the causes while the measurements are still unsettled.

Also, when evaluating the measured distance of an electro-optical distance meter, it is easier to identify the cause by checking it together with changes in angles and coordinates rather than judging it alone. Whether only the distance is unstable or the horizontal and vertical angles are also fluctuating will change which factors you should suspect. If only the distance is erratic, reflection conditions or distance-measuring conditions are often involved; if angles are also affected, you should check sighting/aiming, the tripod, leveling, the instrument station, and the mirror’s mounting/holding condition.

On site, people tend to blame unstable measured distances with a single comment: "the instrument is acting up." However, in many cases the variation in measurement conditions is actually the cause. Below, we organize the main causes to check when an optical surveying instrument’s measured distance is unstable, in the order that makes them easiest to isolate in practice.

Cause 1 The conditions of prisms and reflective surfaces are not uniform

When the measured distance from an electro-optical surveying instrument is unstable, the first thing to check is the condition of the prism or reflecting surface. In measurements using a prism, the prism’s orientation, dirt, how securely it is fixed, the plumbness of the pole, and the prism constant setting affect the distance. In non-prism measurements, the material, color, angle, surface irregularities, wetness of the target surface, and the presence of nearby reflective objects affect the measurement results.

A prism receives light from an optical surveying instrument and reflects it back. Therefore, if the prism face is not correctly oriented toward the instrument, the reflection can weaken and distance measurements can become unstable. The farther the distance, the more a slight misalignment or wobble in holding will affect the measurement. When the prism is held on a pole, even if the operator appears to be standing still, the tip can sway due to wind or ground/footing conditions.

Contamination of the prism is another easily overlooked cause. Mud, dust, water droplets, or fingerprints on the surface change its reflectivity. On site, prisms and reflective sheets are prone to becoming dirty in light rain, after watering, near excavated soil, after pavement milling, or around concrete pours. If the measured distance suddenly becomes unstable, it is effective to first visually inspect the reflective surface before considering more complex causes.

Setting the prism constant is also important. If the settings on the total station (EDM) do not match the type of prism used or its mounting conditions, a consistent difference in distance may occur. This usually appears as a systematic shift in the measurement results rather than random variation each time. However, if multiple prisms are used interchangeably on site or a prism is replaced during work, a mixture of settings can make distance checks appear unstable.

In non-prism measurements, the condition of the reflecting surface has an even greater influence. Measurements can become unstable on dark surfaces, wet surfaces, slanted surfaces, rough surfaces, slender members, or objects with depth. If there is grass, a net, temporary structures, rebar, signs, or glass in front of the targeted point, the light-wave surveying instrument may pick up unintended reflections. Especially when aiming at a wall or the corner of a structure from an oblique direction, the measurement surface may not be consistent and the measured distance may not stabilize.

When checking reflection conditions, methods such as measuring the same measurement point from a slightly different angle, switching to prism measurement, or establishing a stable target on the measured surface can be useful. If measurements are unstable without a prism but become stable after placing a prism, it is easier to conclude that the cause lies on the reflective surface side rather than in the instrument itself. Conversely, if instability persists even with a prism, expand the scope of checks to include line of sight, aiming, setup, and settings.

On sites where measured distances are unstable, it is important not to take the condition of the reflecting surface lightly. Electronic distance measurement (EDM) instruments are precision devices, but if the target is not stable the results will also tend to be unstable. First, checking whether the surface is reflecting correctly and whether measurements are being taken under the same conditions will help you isolate the cause more quickly.

Cause 2: The sightline or aiming is slightly off

Measurements with an optical distance meter assume a clear line of sight between the instrument and the target. When the line of sight is poor, distance measurements tend to become unstable, and even if you believe you are measuring the same point the measured distance can fluctuate. If the target is completely out of sight you cannot measure it, so the cause is easy to notice, but the situation that often causes problems is when the target appears to be visible while an obstacle is actually on the measurement line.

Things that obstruct the line of sight include vegetation, temporary enclosures, heavy equipment, scaffolding, materials, workers, vehicles, power lines, fences, and protective sheets. At a site, people or vehicles may pass through for only an instant, and measured distances can vary depending on the timing of the measurement. Also, summer vegetation and roadside plantings may appear clear to the eye, but thin branches or leaves can intrude into the path of the ranging beam.

Instability in aiming can also cause distance instability. When aiming at a prism or a survey point through the telescope of an optical surveying instrument, if the way the crosshairs are aligned shifts slightly each time, it can affect not only the angle but also the distance measurement. In particular, when targeting slopes, corners of structures, narrow stakes, or the edges of reflective sheets, even a small change in the aiming point can result in measuring a different surface. In non-prism measurements, if the aiming center is off, the instrument is more likely to pick up a nearer or farther surface.

Keeping the prism pole vertical is also closely related to sighting. If the pole is tilted, the prism’s position will be displaced from directly above the survey point. This often appears as a shift in coordinates rather than instability in the measured distance, but if the pole is swaying the measurement distance itself can also become unstable. On windy days, at height, on slopes, on crushed stone, in mud, or on narrow footings it is difficult to keep the pole straight.

Problems with line of sight or aiming are easily overlooked when the measurer and the worker at the mirror make separate judgments. Even if something seems visible from the instrument side, the mirror-side worker may have unstable footing and the pole may be moving. Conversely, the mirror-side worker may think they are standing without issue, while temporary materials or grass may be in the instrument’s line of sight. When the measurement distance is unstable, it is important for the instrument side and the mirror side to share the situation and not proceed based on the judgment of only one party.

To verify the stability of the aim, don’t just measure the same point multiple times; also check how the target appears through the telescope. Make sure you are capturing the center of the prism, aiming at the center rather than the edge of the reflective sheet, and for non-prism targets check whether the target surface has any depth or tilt. When the measurement distance is unstable, simply readjusting the aim can sometimes make the readings settle.

Also, there are cases where measurements are being forced from the instrument point's location. In situations where you have to measure through a narrow gap while avoiding obstacles, even if you can obtain a measurement, its stability tends to be low. In such cases, it is necessary to review the work plan, for example by moving the instrument point, setting up a relay point, changing the measurement sequence, or avoiding times when obstacles are moving.

For an electronic distance meter (EDM), the measured distance depends not only on whether the target is visible but also on whether you can consistently and stably aim at the same target. Ensuring a clear line of sight, steady aiming, and a stable reflector (mirror) side is fundamental to reducing distance variability.

Cause 3: Influenced by weather conditions and the measurement environment

The instability of measurement distances in electro-optical distance meters (EDMs) can also be caused by meteorological conditions and the surrounding environment. In outdoor surveying, factors such as air temperature, air pressure, humidity, wind, rain, fog, heat haze, strong sunlight, heat radiating from the ground surface, and dust can affect the measurement environment. Depending on site conditions, even if there are no obvious problems with the instrument or the reflector, the distance readings may have difficulty stabilizing.

Particular attention should be paid to fluctuations near the line of sight caused by temperature differences and heat. On paved surfaces, concrete, steel plates, slopes, reclaimed land, and roads in summer, heat rising from the ground can make the air shimmer and cause the image seen through the telescope to appear to waver. In this state the sighting itself can become unstable, and distance measurements may also become unstable. Because long-distance measurements are more susceptible to the condition of the air, measurements can be fine at short range yet become unstable only at longer distances.

Rain and fog also affect distance measurement. Even light rain can change the reflective condition if droplets adhere to prisms or reflective surfaces. When fog or fine raindrops are present in the measurement path, the light can be attenuated or unintended reflections may be introduced. On-site, it is common for the measured surface to be wet immediately after rain, for water droplets to remain on reflective sheets, or for prisms to be used without being wiped.

The effect of wind is also important. When the wind is strong, tripods, prism poles, temporary structures with reflective sheets, and protective coverings on the object being measured can move slightly. Even if the worker on the mirror side holds the pole, the center of the prism will sway slightly in strong winds. When using a long pole or working on unstable footing, that sway becomes even larger. If you feel the measurements are not consistent, it is important to observe what the wind is moving.

Check meteorological corrections and environmental settings as well. With optical distance-measuring instruments, depending on the model and how they are operated, you may input correction values such as temperature and atmospheric pressure. If the entered values differ significantly from the on-site conditions, they can affect how distances are handled. While in routine work this may not appear as a large discrepancy, for long-distance measurements or tasks requiring high accuracy, it is safer not to skip verifying environmental conditions.

Dust and smoke are also site-specific factors. Near demolition, cutting, excavation, paving, concrete placement, and heavy equipment operations, fine particles can become airborne. If there is a high concentration of dust along the measurement path, distance measurements can become unstable. In addition, exhaust from heavy equipment or heat from generators passing near the measurement line can cause image shimmering and measurement instability.

If environmental factors are suspected, possible responses include measuring the same point at different times, shortening the distance to check, changing the instrument setup point, moving to a location less affected by wind, and cleaning the prism or reflective surface. If measurements are stable in the morning but become unstable in the afternoon, suspect the effects of solar radiation or heat. If instability occurs only after rain, suspect water droplets or wet reflective surfaces.

Measurements with an optical surveying instrument may appear to be completed solely by the instrument and the survey point, but in reality the air and the site environment between them are also part of the measurement conditions. When the measured distance is unstable, checking not only the device settings in front of you but also the environment along the measurement line is the quickest way to find the cause.

Cause 4: Discrepancies in basic settings such as instrument station and mirror height

When distance measurements from an electro-optical distance meter are unstable, attention must also be paid to the instrument station setup and basic settings. Basic items such as poor instrument leveling, a sinking tripod, inadequate securing of the instrument point, unstable backsight verification, incorrect mirror-height input, or an incorrect prism constant or distance-measurement mode directly affect the reliability of distances and coordinates.

The condition of the tripod setup greatly affects measurement stability. Even when it appears stable on paved surfaces, on crushed stone, embankments, soft ground, slopes, temporary scaffolding, or deck slabs the tripod legs can gradually sink or slip. Even if there is no problem immediately after setting up the instrument, it may shift slightly due to vibrations during work, foot traffic, or the passage of heavy equipment. If not only distances but also angles and coordinates are changing, you should first suspect that the instrument’s position is unstable.

Checking the leveling is also indispensable. A total station assumes a horizontal condition during observations. If the leveling is disturbed, it affects the measurement direction and the handling of vertical angles, making it difficult to maintain consistency in the results. Just because you leveled the instrument at the start of work doesn't mean it will remain so until the end. When performing long-duration work, or when there are temperature changes, vibration, or settling of the tripod legs, make it a habit to check the bubble level and the electronic leveling display periodically.

An incorrect mirror height input often affects heights and coordinates more than the distances themselves, but on site it can cause the entire set of measurement results to appear unstable. In particular, when slant distance, horizontal distance, elevation difference, and coordinate values are being confused or viewed interchangeably, an error in mirror height can make it feel like "the distances don't match." You should also check factors such as pole extension, the mounting position of the prism, the height when using a reflective sheet, and differences between the ground surface at the survey point and the height of the control point.

Selection of the distance measurement mode is another item that is easily overlooked. You need to switch between modes such as prism measurement, non-prism measurement, and reflective sheet measurement according to the target being measured. If you measure in the field with the previous settings still in place, you may end up measuring under conditions different from what you intended. If the distance is unstable, check the distance measurement mode, the number of measurements, how averaging is handled, the units, and the type of distance being displayed.

Checking the backsight is also important. What may appear to be unstable distances can actually be caused by an unstable instrument point or backsight orientation. If you proceed with measurements while the angle or distance is wrong during the backsight check, the results at individual survey points will seem inconsistent. Basic mistakes can also occur, such as the reference point or backsight point moving, stakes becoming loose, confusing the positions of survey bolts, or the reflective target being on a different point.

Also, confusing instrument point names, backsight point names, coordinate systems, site coordinates, and local coordinates can be misinterpreted as measurement instability. Even if the measured distances themselves are stable, the results will not match if the design values or known point coordinates being compared are different. On site, it is necessary to separate considerations of distance variation and coordinate inconsistency. It is important to determine whether the distance display itself is stable or whether deviations occur after coordinate calculations.

Causes related to basic settings are often preventable if checked, yet they are items that tend to be overlooked due to assumptions. When measured distances are unstable, calmly checking not only the prism and line of sight but also the instrument station, centering and leveling, backsight, mirror height, distance-measurement mode, and coordinate system alignment can reduce rework on site.

Cause 5 Measurement procedures and recording rules are not standardized on-site

The causes of unstable measured distances with an optical distance measuring instrument are not limited to the equipment or the environment. Lack of standardization of work procedures and record-keeping rules on site can also lead to variability in results and insufficient verification. If different operators aim at different target positions, take different numbers of measurements, use different criteria for deciding to re-measure, apply different point-naming conventions, or keep records in different ways, distances can appear to be unstable when reviewed later.

For example, when measuring the corner of a structure, one worker may aim at the near face of the corner while another may aim at the far face. If the position where the reflective sheet is attached differs slightly between people, the actual measurement location will change even if the point name is the same. If it is not standardized whether to measure the center of a pile, the mark on the pile head, or a point on the ground surface, the measured distances and coordinates will not match.

How you handle the number of measurements is also important. If it’s unclear whether you take a single measurement and record it, take multiple measurements to confirm stability, or which value to adopt when the results vary, you won’t be able to make a judgment when reviewing the data later. In situations where distance measurements are unstable, it’s important not just to repeat measurements, but to record why you remeasured and under what conditions you did so.

Management of point names and station numbers is also related to distance verification. Even if the measured values themselves are correct, mistaking a point name can cause you to compare distances or coordinates against a different point. On site, temporary points, auxiliary points, known points, as-built control points, and restoration points are mixed together. If work is carried out with ambiguous point-naming rules, confusion such as "the distance doesn't match" or "it's different from last time" is likely to occur when checking measurement results.

Also, if you don’t record the measurement conditions, it becomes difficult to trace the cause later. If information such as whether it was a prism or non‑prism measurement, which instrument station the measurement was taken from, which backsight was used, what the mirror height was, and whether there were problems with the weather or visibility is not retained, the effort required for rechecking increases. Especially on multi‑day or multi‑person jobs, insufficient records are a major cause of rework.

For standardizing procedures, a verification flow that is easy to carry out on site is more effective than a complicated mechanism. Before measurement, check the instrument point and backsight, confirm the ranging mode and mirror height, verify line of sight and the reflecting surface, and decide on a workflow to re-measure or cross-check at representative points after measurement to reduce differences between operators. Furthermore, when an abnormal value appears, share a rule to not accept it immediately but to step back in the order of re-aiming, checking the reflecting surface, confirming mirror holding, and checking the instrument point; this speeds up isolating the cause.

An optical surveying instrument is a device whose measurement results can be made more reliable by using it under appropriate conditions. However, if on-site use is inconsistent, you cannot fully leverage the instrument’s performance. At sites where measured distances seem unstable, it is necessary to check not only the equipment and environmental conditions but also the operators’ procedures and recording rules as potential causes.

How to isolate the cause when distance is unstable

When the measured distance from a total station is unstable, it is more efficient to isolate causes in order of their impact rather than checking them in the order they occur to you. First, confirm whether the variability in measurements is really a distance issue. Be clear about whether you are looking at slope distance, horizontal distance, height difference, or coordinate values, and check that you have not misread the displayed item. If only the coordinate values are off, the problem may not be with distance measurement but with the instrument station, the backsight, the coordinate system, or the prism height.

Next, measure the same point multiple times under the same conditions. Comparing measurements taken consecutively without re-aiming with those where you intentionally break the sight and then re-aim makes it easier to find the cause. If measurements are stable in consecutive runs but change after re-aiming, the cause may lie in the aiming position or the choice of target surface. If measurements are unstable even when taken consecutively, suspect reflective surfaces, line-of-sight issues, wind, instrument setup, or environmental conditions.

It is also effective to check by switching between prism and non-prism measurements. If a measurement is unstable with non-prism but becomes stable when using a prism, the cause can be attributed to the target surface’s reflection conditions or the aiming position. If it is unstable even with a prism, check the prism’s orientation and cleanliness, how the pole is held, the line of sight, and the instrument-side conditions. By seeing whether measurements stabilize when you change the reflective surface, you can narrow the cause down to the measurement target side.

You can also check by changing the distance. If it is stable at short range but unstable at long range, line of sight, aiming, weather conditions, and reflectivity may have a large influence. Conversely, if it is unstable even at short range, you should more strongly suspect problems with settings, instrument setup, the prism, the measurement target, or the procedure. As distance increases, small vibrations or reduced visibility are more likely to affect the results, so for long-distance measurements you need to ensure conditions are matched more carefully.

Changing the instrument station can also help isolate the cause. If measuring the same survey point from a different instrument station yields stable results, there may have been a problem with the line of sight or installation conditions at the original instrument station. If measurements are unstable from any instrument station, the cause may lie with the reflection conditions at the survey point, the mirror mounting, or the condition of the target surface. If instability is present across the entire site, it is necessary to check weather conditions, equipment inspection, and settings.

Do not forget backsight checks and known-point verification. It is especially important to return to a reference point and confirm it when you feel the measured distance is unstable. If the distance and angle to a known point are stable, it becomes easier to confirm that there is likely no major problem with the instrument body or the instrument point. If the reference point is unstable, continuing to measure subsequent points will not increase reliability. Before proceeding, you need to recheck the instrument point, leveling, backsight, prism, and the environment.

When isolating causes, the important thing is not to change them all at once. If you wipe the prism, change the ranging mode, move the instrument station, adjust the mirror height, and change the measurement target all at the same time, you won’t know which action was effective. Although you’re pressed for time in the field, for root-cause tracking it can actually be faster overall to change one condition at a time and record the results.

If unstable measurement distances are left unaddressed, corrections in later stages will become larger. In tasks such as as-built verification, boundary restoration, setting out, existing-condition surveys, and quantity confirmation, the reliability of measurements underpins the entire operation. If you sense even a slight anomaly, it is important to check on the spot using control points or representative points, isolate the cause, and only then proceed to the next task.

Summary The more consistent the site conditions are, the more stable the measured distance becomes

The causes of unstable measurement distances of electronic distance measuring instruments lie in various on-site conditions such as prisms or reflective surfaces, line of sight and aiming, meteorological conditions, instrument station and basic settings, and work procedures and recording rules. It is important not to suspect only a mechanical fault, but to check everything including the measurement target, the measurement path, the installation condition, the settings, and the operator’s movements.

In particular, even when you think you are measuring the same point, it is common on site for the reflective surface to be different, the aiming position to be off, the prism to be shaking, obstructions to be in the line of sight, or the ranging mode to be incorrect. Each of these is a small factor on its own, but when they overlap they manifest as instability in the measured distances. That is precisely why it is important to make a habit of performing checks before measurement and verifications after measurement.

If the distance is unstable, first check the type of value being displayed, measure the same point several times, confirm the reflective surface and line of sight, and review the instrument station and settings. Then, as needed, narrow down the cause step by step by changing the instrument station, switching to prism measurement, changing the time of day, rechecking at a reference point, etc. By isolating causes one by one, you can reduce unnecessary re-measurements and rework.

Total stations are important instruments that establish site control, verify positions, and support the accuracy of construction and inspection. Rather than dismissing situations where measured distances are unstable as mere malfunctions, responding from the perspective of standardizing site conditions makes it easier to enhance the reliability of surveying results. In daily operations, it is necessary to be aware that accuracy is supported not only by the instrument’s performance but also by the cumulative practices of setup, sighting, reflection, recording, and verification.

On site, efficiency of measurement work and ease of record management are also required. For this reason, it is effective to link and save the verification results from the total station with point names, instrument stations, backsights, distance measurement conditions, photos, work notes, and so on. In situations where distances are unstable, recording the measurement conditions and the process of judgment makes rechecking and sharing with stakeholders easier.