What Is AR Drift? A Comprehensive Guide to Causes and Countermeasures on Construction Sites｜The Latest Tool Usage Techniques Site Personnel Should Know

If you can hold up a smartphone or tablet camera on site and overlay drawings or 3D models onto the actual scene, it becomes extremely convenient to intuitively check positions and give construction instructions. It reduces the hassle of measuring with a tape or string line and enables visual information sharing that paper drawings cannot provide. However, when you actually try using AR (augmented reality) on construction sites, you often get the feeling, “The display is slightly misaligned with reality...?” The virtual lines and models you’ve displayed can appear shifted by tens of centimeters (several inches) from where they should be. This problem of AR drift (coordinate drift) has become a major barrier to making AR practical on site.

In recent years, the Ministry of Land, Infrastructure, Transport and Tourism’s *i-Construction* and other DX initiatives in the construction industry have accelerated, and the introduction of digital technologies to construction sites is progressing. AR is also expected as a labor-saving and intuitive verification tool on site, but traditionally, due to positioning accuracy issues, it has been difficult to use it for more than just “trying it out.” Enter the combination of high-precision GNSS (RTK) and smartphone AR. Smartphones can now achieve positioning on the order of several centimeters (a few in), dramatically reducing AR display misalignment (drift). In this article, we explain what AR drift is, and discuss its causes and countermeasures in detail. Furthermore, as practical tips on the latest tools that site personnel should know, we introduce how RTK technology can realize high-precision AR displays, and finally present an example application of LRTK for simple surveying.

Table of Contents

• What is AR drift?

• Main causes of AR drift

• Challenges caused by AR drift

• Traditional countermeasures and their limitations

• Eliminating drift with high-precision GNSS (RTK)

• Field deployment of centimeter-accuracy AR with LRTK (cm level accuracy (half-inch accuracy))

• Expanding use cases with LRTK: enabling simple surveying

• Frequently Asked Questions (FAQ)

What is AR drift?

When virtual objects are displayed in the real world through smartphones or tablets, the AR overlay can appear offset from their actual positions. For example, a design line that should be drawn on the ground may appear to float about 30 cm (11.8 in) above the actual ground, or a 3D model of equipment may be displayed slightly to the side of its intended installation position. This phenomenon, in which AR objects do not exactly align with real objects and gradually shift position, is referred to in the industry as "drift (漂移)". If the offset becomes large, the AR display can no longer be trusted and becomes unusable for on-site verification or work support.

Main causes of AR drift

The main causes of display misalignment in current smartphone AR include the following:

• GPS position error: The GPS built into a typical smartphone can have errors of several meters (several ft). This prevents placing a virtual model at an exact location, and if the location data is offset the AR display will appear shifted by the same amount.

• Heading and attitude sensor errors: A smartphone’s electronic compass (magnetic sensor) and gyroscope have slight errors and drift. Even a small error in heading can cause larger display offsets for more distant objects (for example, if the heading is off by 2°, at a distance of 10 m (32.8 ft) this can lead to a position offset of tens of centimeters (several in)).

• Limits of AR tracking accuracy: AR apps estimate device movement by tracking feature points in the camera feed, but if tracking is not perfect, virtual objects can drift gradually as you move. Large movement ranges or monotonous scenes with few distinctive features can cause unstable mapping accuracy and lead to gradual display drift.

• Environmental factors and data coordinate mismatches: Disturbances in the surrounding magnetic field or the influence of metal structures can skew the compass and shift heading. Also, if the coordinate system of the drawing data being used does not match the on-site positioning coordinates, large discrepancies can occur. For example, if the reference point in a drawing is set incorrectly, AR displays can be off by several meters (several ft).

• Small errors in initial alignment: When placing a model in AR, the initial alignment is sometimes done manually. If this initial position is even slightly off, it can result in large errors at distant points on a wide site. A single small angular or distance misalignment can have a large impact later on.

• Drift over time: Cumulative sensor errors and changes in the surrounding environment can cause AR displays to slowly drift over time. Something that was aligned correctly at the start can gradually diverge after using the system for a while.

As described above, AR displays inevitably experience "misalignment" due to device positioning accuracy, attitude sensor accuracy, and the characteristics of the AR engine.

Challenges Caused by AR Drift

If AR overlays remain misaligned, various problems arise for on-site use. First, because the display's reliability is undermined, workers cannot trust the AR information, making it difficult to apply in practice. For example, even areas that are actually installed correctly may appear misaligned in AR, causing workers to worry, "Could this be an installation error?", or conversely, there is a risk that real misalignments will be overlooked because of erroneous AR displays.

Also, because low-accuracy AR cannot be used in practical work, even if it is introduced, measurements on site will end up being rechecked with tape measures and surveying instruments, and the much-vaunted digital technology becomes nothing more than a pipe dream. If the display is off by several meters it's out of the question, but even an offset of a few tens of centimeters cannot be used for important positioning tasks. In the end, conventional surveying checks are required at key points, which diminishes the benefits of using AR.

Conventional countermeasures and their limitations

To reduce AR display misalignment, various techniques have been tried in the field. Let's look at representative countermeasures and their challenges.

• Placing markers or QR codes: This method involves affixing AR markers (image markers) or QR codes on-site in advance and using the camera to read them as reference positions. It is convenient, but applying it over a wide area requires placing markers everywhere, which is impractical. Especially outdoors, markers may peel off or become dirty due to wind and rain, making stable operation difficult.

• Manual alignment using landmarks: This method fine-tunes the position manually by comparing clear on-site landmarks—such as corners of structures or reference points—with the AR model. Some correction is possible, but much depends on the operator’s judgment, so accuracy is limited. It is also difficult to achieve the same precision each time, and results can vary between operators.

• Readjusting by resetting each time: This method redoes (resets) the model alignment on the spot whenever the model display feels shifted. While it temporarily fixes the issue, each reset interrupts work and is inefficient. Since the root cause is not resolved, you end up resetting repeatedly, which can be counterproductive.

• Cross-checking with pre-survey data: This method acquires reference point coordinates on-site with surveying instruments and aligns the digital model to those coordinates. Accuracy improves, but it requires specialized surveying work and is labor-intensive. Coordinate transformations and alignment require expert knowledge and incur personnel costs. There is also a risk that conversion errors leave residual misalignment.

While each of these countermeasures is somewhat effective, they often compromise the original benefit of being able to use AR easily in real time. Because the measures require too much effort and cost, many on-site personnel may have half resigned themselves to thinking, "Some degree of AR misalignment is inevitable."

Eliminate Drift with High-Precision GNSS (RTK)

The trump card for fundamentally solving these AR misalignment issues is the use of high-precision GNSS positioning. Among them, the RTK (Real Time Kinematic, real-time kinematic) method achieves positioning accuracy of a few centimeters (a few in) by correcting satellite positioning error information in real time. While ordinary smartphone GPS has errors of several meters (several ft), using RTK can measure the current position with extremely high accuracy: ±1～2 cm (±0.4～0.8 in) in horizontal position and within a few centimeters (within a few in) in the vertical direction.

If RTK can provide centimeter-level positioning information (cm level accuracy, half-inch accuracy), the task of aligning virtual objects to real-world coordinates becomes dramatically easier. Because the smartphone can determine its absolute position almost perfectly, if you simply provide coordinate data from the blueprints or models, reality and the virtual will match almost exactly in AR. What used to be difficult—superimposing a model over a real-world scene exactly according to the drawings—becomes practically possible.

Furthermore, by continuously feeding the high-precision absolute coordinates obtained by RTK back to the AR system, the AR engine's drift (the gradual shifting phenomenon) is automatically corrected. As a result, a stable display is achieved so that, without even noticing, "the model won't float off into mid-air even if you walk around." You also won't have the hassle of a virtual model shifting on its own after being placed and having to be readjusted repeatedly.

In recent years, "smartphone RTK" solutions that make this RTK technology easy to use on smartphones have also emerged. For example, LRTK (L R T K) is one such solution, and with a small RTK-compatible GNSS receiver that attaches to a smartphone and a dedicated app, anyone can easily achieve centimeter-level accuracy (half-inch accuracy) positioning. In the next chapter, let's look at how this LRTK can be used to bring high-precision AR to the field.

On-site deployment of centimeter-level accuracy AR with LRTK

LRTK is a cutting-edge solution that combines an RTK positioning device for smartphones with an AR cloud service to achieve on-site "no-drift AR". By attaching an LRTK receiver to a smartphone and receiving correction information from satellites, immediate RTK positioning is possible on site (in locations with internet access, correction data can be obtained from a VRS (virtual reference station) service using the Geospatial Information Authority of Japan’s Continuously Operating Reference Stations; in mountainous or other areas where communication is difficult, it is also possible to operate by installing a private simple base station and sending correction data wirelessly. Also, within Japan, you can obtain correction information without a network connection by using CLAS [centimeter-class positioning augmentation service] provided by the Quasi-Zenith Satellite System Michibiki). In this way, correction methods can be flexibly selected according to on-site conditions, allowing the smartphone to always determine a high-precision current position. Then, by displaying the design data prepared in the cloud as AR based on that high-precision positioning, the model can be projected at full scale into its designated position in the real world without time-consuming pre-alignment work.

For example, if you prepare the site's boundary line data, you can use LRTK to recreate that line on the ground almost perfectly through your smartphone. Without having to draw temporary lines on the ground, you can check at a glance on your smartphone screen whether the design position and the actual site are misaligned. Even when walking around a large area, the model is always displayed fixed to the correct coordinates, so there's no worry about significant positional drift at the far edges. It literally lets you say goodbye to the problem of AR drift.

Furthermore, LRTK is designed with ease of use on site in mind. As a compact device integrated with a smartphone, there is no need to carry a tripod and heavy surveying equipment as before. With just a smartphone, site supervisors and construction management engineers can perform on-the-spot positioning + AR display themselves, significantly increasing the number of tasks they can complete without relying on a specialist surveying team. For example, batter boards (chouhari) and as-built verification tasks that were previously outsourced to external surveying companies can be checked immediately by the team using LRTK. This reduces losses from waiting for outsourced work and schedule coordination, enabling the on-site PDCA cycle to run more swiftly.

Thus, by enabling high-precision AR with a single smartphone, AR technology is, for the first time, beginning to take hold on-site as a practical tool for professional use. "Drift-free AR," supported by high-precision positioning, has the potential to greatly transform construction management and meeting practices.

Use cases expanding with LRTK: simple surveying becomes possible

High-precision AR enabled by LRTK does more than simply overlay design data in situ. It is also expected to be used as a simple surveying tool on site. A new surveying workflow using smartphones is emerging, allowing surveying and inspection tasks that previously required multiple people to be performed intuitively by a single person.

For example, during pre-construction setting out pile positions, the LRTK coordinate navigation function displays the distance and direction to the specified survey point on a smartphone screen, allowing a single worker to accurately identify the point. By simply following the arrows and virtual stakes (AR markers) shown on the screen, they can reach the designated position with an accuracy within a few centimeters (cm-level accuracy (half-inch accuracy)). Even those with little surveying experience can perform pile-driving work with confidence.

Also, even in post-construction as-built management, if the design 3D model is projected in AR beforehand, you can instantly confirm on-site whether the finished work matches the drawings. Whether embankments or structures have reached the design elevation is obvious at a glance on a smartphone screen. If necessary, you can perform an LRTK point cloud scan on-site and, by comparing the design data with the as-built in the cloud, carry out quantitative verification.

Furthermore, LRTK also has a unique feature called positioning photos. It is a function that automatically tags site photos taken with a smartphone with high-precision capture position coordinates and camera pose information, and allows them to be shared on the cloud. If you visit the same location later, the places where photos were taken in the past will be displayed as icons in AR, making it easy to check changes over time and identify areas requiring repair. On-site recordkeeping and maintenance management, which until now relied on manual labor and intuition, can also be greatly streamlined.

Thus, LRTK is an all-in-one on-site DX tool that combines AR-based visualization with positioning and recording functions. Not only does its high-precision AR eliminate the stress of "misalignment," but because it can seamlessly handle surveying, inspection, and recording, it is expected to dramatically improve on-site productivity and accuracy. If you are interested, please consider exploring new surveying and construction management methods that utilize LRTK.

Frequently Asked Questions (FAQ)

Q: *What equipment is required for high-precision AR display?* A: *Basically, you need a set that includes an AR-capable smartphone and a high-precision GNSS receiver (a device that supports RTK), along with a compatible AR display app. For example, a typical case is using an RTK antenna that attaches to the smartphone—like LRTK—and a dedicated app. The smartphone itself does not have to be the latest high-end model, but a device with sufficient performance that supports AR functionality (ARKit or ARCore) is preferable. Also, when operating for long periods, it's advisable to prepare a portable power bank or similar.*

Q: *If you can use a smartphone, can anyone operate it? Is special training required?* A: *Compared to traditional surveying equipment, it is intuitive to operate, but we recommend receiving basic training in advance. Learning how to use the app, basic knowledge about RTK, and operational precautions beforehand will reduce the risk of confusion on site. Nonetheless, it is designed so that even those who are not professional surveyors can handle it adequately. In fact, there are cases where site supervisors and construction management engineers mastered it after a few hours of instruction and trial use and are now using it on site. At first, try it experimentally while cross-checking with measurements from experienced surveyors, and gradually expand its scope of application so it will be adopted smoothly.*

Q: *Do you need to provide an RTK base station every time? Can it be used at sites without internet?* A: *It depends on how RTK correction information is obtained. If there is a nearby permanent reference station and internet connectivity is available, you can receive correction data from services such as the VRS (Virtual Reference Station) service provided by the Geospatial Information Authority of Japan, so you can achieve centimeter-level accuracy (half-inch accuracy) without installing a dedicated base station. In this case, a smartphone must be able to connect to the internet via a mobile network or similar. On the other hand, in remote mountainous areas where communications are unreliable, it is possible to set up your own mobile base station and transmit correction data wirelessly. Also, within Japan, if you use a receiver compatible with Michibiki’s CLAS (centimeter-level positioning augmentation service; cm level accuracy (half-inch accuracy)), you can obtain correction information without internet. LRTK supports these multiple methods, so positioning can be carried out flexibly according to site conditions.*

Q: *How accurately do AR displays actually align with the real world? I'm worried about errors.* A: *Under good conditions, horizontal position errors are on the order of 1-2 cm (0.4-0.8 in), and height errors are within a few cm (a few in). Visually, you hardly notice any offset. However, this assumes RTK provides a stable "Fix" solution and the device's attitude sensors are properly calibrated. Accuracy degrades in environments with poor satellite reception, and if the device's compass is misaligned it will affect the display. It cannot be said to always align perfectly, but in normal outdoor environments you can achieve accuracy that is practically sufficient. The important thing is to verify AR display accuracy on site using known points or landmarks as needed. Even with some error, comparing against local references and making corrective judgments on site makes it fully usable in practice and improves reliability.*

Q: *Is alignment with AR possible in dark places or at night?* A: *The accuracy of GNSS positioning itself does not change at night. However, AR overlays visible in the camera feed become harder to see if the surroundings are too dark, and the device’s AR functions (visual tracking) also find it difficult to maintain accuracy. When using it at night or in dark locations, illuminate the target with work lights or otherwise ensure sufficient brightness, or use a device equipped with LiDAR, which can be somewhat more stable in low light. That said, for safety reasons it is preferable to conduct surveying and AR checks during daylight whenever possible. In dark sites, instead of relying on AR in the camera view, switching to coordinate guidance based on high-precision positioning (numeric guidance) is also an effective approach.*

Q: *What should be done in places where GNSS cannot be used, such as indoors or underground?* A: *In environments where satellite positioning does not reach, such as indoors or inside tunnels, absolute positions from RTK cannot be obtained. In that case, as an alternative, there is a method to perform AR in local coordinates using known control points. For example, if reference points are set on the building floor in advance with a total station and those coordinate values are manually entered into the app and used instead of the smartphone position, it is possible, in a simple way, to display AR. Recently, research has also advanced on AR systems using indoor positioning technologies such as UWB (ultra-wideband) and Visual SLAM. At present, achieving centimeter-level (half-inch accuracy) alignment indoors is not easy, but depending on the application, simple AR displays using plane detection or marker-based AR can be substituted in some cases. The point is that in places where GNSS cannot be used, combining other positioning methods can broaden the possibilities for AR use.*

Q: *I'm worried about the upfront cost; after all, is the equipment expensive?* A: *Compared with conventional surveying instruments and 3D scanners, a smartphone plus an RTK receiver can be introduced at a relatively affordable cost. Exact prices vary depending on the model and service format, but in many cases it costs only a fraction of a dedicated surveying GNSS set. Furthermore, you can expect cost reductions from reducing outsourced surveying work and from fewer rechecks and rework. Depending on the scale of the site, and considering savings in labor and time, there are reports that the payback period can be relatively short. Also, some products are offered as subscriptions (monthly services), such as LRTK, allowing you to keep initial costs low and trial them. We recommend starting with a small-scale site, assessing the effects, and expanding gradually.*