Causes and Countermeasures of AR Drift in AR Surveying

In recent years, AR surveying, which uses AR (augmented reality) technology, has been attracting attention at construction and civil engineering sites.

By displaying the site through a smartphone or tablet camera and overlaying design drawings or 3D models onto the real scene, the need to measure with a tape measure against plans or to drive stakes to check positions can be greatly reduced, allowing site supervisors and surveying technicians to intuitively grasp the situation and potentially prevent construction mistakes while improving work efficiency.

However, when you actually try AR surveying, you will often feel, "Is the display on the screen slightly misaligned with reality...?" The virtual lines and structural models that were carefully displayed can appear to be displaced by tens of centimeters (several in) from where they should be. Such discrepancies in AR display are a major obstacle to putting AR into practical use on-site. In the industry, the phenomenon in which AR displays gradually drift is also called AR drift (drift), but when the drift is large, AR information becomes unreliable, and it becomes difficult to use AR for more than a 'toy' in surveying and construction management.

In this article, we provide a detailed explanation of the causes of AR drift that occurs in AR surveying and the countermeasures. First, we explain what AR drift is and organize the main reasons why it occurs. Then, taking into account the challenges AR drift brings to the field, we review conventional countermeasures and their limitations. Finally, we introduce the latest solutions that use recently developed high-precision GNSS (RTK) to solve the AR drift problem and present examples of applying LRTK to simplified surveying.

Table of Contents

• What is AR drift (漂移)?

• Main causes of AR drift

• Problems caused by AR drift

• Conventional AR drift countermeasures and their limitations

• Eliminating AR drift with high-precision GNSS (RTK)

• Centimeter-precision AR surveying enabled by LRTK

• Expanded use cases with LRTK: enabling simple surveying

• Frequently Asked Questions (FAQ)

What is AR drift (drift)?

When displaying virtual objects in the real world through a smartphone or tablet, a misalignment called AR drift can occur. 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 virtual equipment model may be displayed shifted laterally from its intended installation position. It is a phenomenon in which AR objects that should align perfectly gradually shift position, and in the industry it is referred to as drift (drift).

If this AR drift is large, the information displayed on the screen will diverge from reality, making it difficult to use AR for on-site inspections and work. If, despite displaying data in AR, its lack of reliability means you end up re-measuring with a tape measure or surveying instruments, the benefits of introducing AR surveying are effectively halved.

Main causes of AR drift

In current smartphone-based AR, the primary causes that lead to display position errors (drift) are as follows.

• GPS（位置測位）の精度限界: The smartphone's built-in GPS can produce errors of several meters, so you cannot place a virtual model precisely at the intended position. If the location information is shifted, the AR display will appear at the correspondingly shifted location.

• 方位センサーの誤差: The smartphone's electronic compass (magnetic sensor) and gyroscope have slight errors and time-dependent drift. Even a small heading error causes larger display displacement for distant objects (for example, a 2° heading error produces a positional offset of several tens of centimeters at 10 m (32.8 ft)).

• ARマッピングの誤差: AR engines (ARKit or ARCore, etc.) estimate the device's movement based on feature points in the camera image, but if tracking is not perfect, the positions of virtual objects can slowly drift as you move. When the movement range is wide or the environment lacks distinctive textures or objects, mapping becomes unstable and the AR display can drift.

• 環境要因や座標の不整合: The compass (heading) can be disturbed by surrounding magnetism or large steel structures, and large discrepancies can also occur if the coordinate system of the drawings being used does not match the local positioning coordinate system. For example, if the coordinates of the reference point set in the drawings are misinterpreted, the AR display on site could differ by several meters.

As described above, due to limitations in smartphone positioning accuracy, orientation sensor accuracy, and AR mapping methods, some drift inevitably occurs in AR displays.

Challenges Caused by AR Drift

If AR display misalignments are left unaddressed, they create various obstacles for on-site use. First, because the trust in AR information is undermined, workers will not trust the display and will end up needing to double-check with traditional surveying. For example, even locations that were actually installed correctly may appear misaligned in AR, prompting doubts like "Was this a construction mistake?", or conversely there is a risk of overlooking actual positional errors due to AR display inaccuracies. Low-accuracy AR can lead to incorrect judgments and therefore be dangerous to use on site.

Because large AR drift makes it unusable in practice, even if it’s introduced, onsite workers ultimately end up having to rely on tape measures or total stations. If the display is off by several meters (several ft), that's out of the question, but even a deviation of several tens of centimeters (several tens of inches) is unacceptable on the exacting sites of civil engineering and surveying. If the perception spreads that "AR is only for play and cannot be used for serious surveying," the promising new technology will not see wider adoption. The AR drift problem is therefore a major issue that must be overcome to establish AR surveying in the field.

Conventional AR Drift Countermeasures and Their Limitations

So, before the introduction of high-precision GNSS, what measures were taken in the field to address AR display misalignment? The main possible countermeasures are as follows.

• Sensor calibration: Methods for calibrating a smartphone's electronic compass before use or resetting gyroscope drift by placing the device on a level surface. Procedures such as waving the device in a figure-eight pattern to calibrate the magnetic sensor, or pointing the phone at a reference straight line to fine-tune the heading, are used to reduce sensor errors as much as possible. However, this does not completely eliminate offsets, and prolonged use or changes in the environment will allow errors to accumulate again.

• Aligning with markers or reference points: A method of placing QR codes or marker boards at various locations on site and reading them with the camera to calibrate the virtual model's position. Alternatively, the virtual model can be manually aligned at points corresponding to known positions on site. This corrects initial positional offsets, but when moving over a wide area offsets can reappear in distant locations, and placing markers each time is time-consuming.

• Combining with simple surveying: Rather than relying solely on AR, another approach is to measure coordinates at key points with a total station or a GNSS survey instrument and then offset the AR model position to match those results. For example, survey a particular point on site that corresponds to a point on the drawings, enter the difference into the app, and use it to correct the entire model position. This can achieve a certain level of accuracy, but the fact that surveying work is required in the first place means it can no longer be called "easy AR usage."

All of the above measures have some effect, but none provide a fundamental solution. Manual adjustments and marker placement require effort and time, and it is difficult for device sensors alone to maintain stable accuracy over long periods and across wide areas. They remain stopgap measures aimed at "minimizing drift as much as possible" and are insufficient to completely eliminate the AR drift problem.

Eliminating AR Drift with High-Precision GNSS (RTK)

A decisive solution to fundamentally resolve AR drift problems is the utilization of high-precision satellite positioning (GNSS). Among these, RTK (Real Time Kinematic) enables positioning at the centimeter level (several cm (about 0.4–1.2 in)) by correcting satellite-derived positioning errors in real time. While a typical smartphone GPS has errors of several meters, using RTK allows you to measure your current position with very high accuracy—±1–2 cm (±0.4–0.8 in) horizontally and within a few centimeters (within a few cm (about 0.4–1.2 in))) vertically.

If a smartphone can accurately determine its own absolute position to the centimeter (cm level accuracy, half-inch accuracy), it becomes dramatically easier to align the positions of virtual objects to actual coordinates. As long as you have coordinate data from blueprints or models, it's not a dream to overlay virtual models onto real-world scenes with perfect, exact alignment. What used to be difficult — "displaying drawings perfectly matched to real-world scenery" — becomes practically possible.

Furthermore, the high-precision positioning provided by RTK also suppresses the AR engine's drift (the phenomenon of gradually shifting). By constantly feeding the absolute coordinates obtained by RTK back to the AR system, slight tracking deviations are automatically corrected. As a result, a stable display is possible in which, without the user even noticing, "the model does not float or shift while walking around with the device." The hassle of a virtual model you placed drifting on its own and having to be readjusted will also be eliminated…

Recently, easy-to-use smartphone RTK solutions have also emerged. By combining a compact RTK-capable GNSS receiver with a dedicated app, anyone can now easily achieve centimeter-level positioning (cm level accuracy, half-inch accuracy) without specialized knowledge. One representative example is LRTK, which enables high-precision AR in the field through an antenna-integrated receiver device that attaches to a smartphone and cloud services. In the next chapter, we'll examine how to use this LRTK to specifically eliminate AR drift and introduce high-precision AR surveying on-site.

AR Surveying Realized by LRTK with Centimeter-Level Accuracy (cm level accuracy (half-inch accuracy))

LRTK is a cutting-edge solution that combines an RTK positioning device for smartphones with an AR cloud service to deliver on-site non-drifting AR. By using a compact antenna-integrated LRTK receiver that attaches to a smartphone’s charging port and receiving satellite correction information via a dedicated app, anyone can perform RTK positioning on-site. At sites with network connectivity, correction data can be obtained from services such as VRS (virtual reference station) services that utilize the Geospatial Information Authority of Japan’s electronic reference points, while in mountainous or other areas where communication is difficult, operations can adapt by setting up a private temporary base station or by using Japan’s quasi-zenith satellite みちびき’s centimeter-class augmentation service (CLAS). Using this high-precision current position information to project design drawings and 3D models prepared in the cloud via smartphone AR means the models appear at their true-to-scale positions without tedious alignment work.

For example, if you simply prepare the site boundary line data, you can use LRTK to reproduce that line on the ground almost perfectly through your smartphone. Without drawing temporary boundary lines on the ground with chalk or rope, you can check at a glance on your phone screen whether there is any discrepancy between the design position and the actual site. Even if you walk around a large site, the displayed model remains fixed at the correct coordinates and does not move, so there's no worry that the position will be off by tens of centimeters (several inches) at the edges. It literally means you can say goodbye to the "AR drift problem."

Furthermore, LRTK has been designed with ease of use on site in mind. Because it is a palm-sized device that attaches to a smartphone, there is no longer any need to carry tripods or bulky stationary equipment. With just a smartphone, site supervisors and technicians can perform positioning and AR display on the spot, so cases in which layout work and verification of as-built conditions that used to be outsourced to external surveying teams can be carried out immediately by the teams themselves are increasing. Unnecessary outsourcing and waiting for schedule coordination are reduced, significantly speeding up the site PDCA cycle. In this way, by enabling high-precision AR surveying with a single smartphone through LRTK, AR technology is for the first time beginning to take hold on sites as a practical, on-the-job tool.

Expanding Use Cases with LRTK: Making Simple Surveying Possible

The high-precision AR made possible by LRTK is expected not only to overlay design data but also to be utilized as a simple surveying tool. A new surveying workflow using smartphones is emerging, because it will allow a single person to intuitively perform surveying and inspection tasks that previously required multiple people.

For example, in pre-construction staking out of pile positions, the LRTK coordinate navigation function can display the distance and direction to the specified coordinates on a smartphone screen. Workers need only follow the on-screen arrows and virtual stake markers (AR markers) to walk to the designated position with an accuracy of several centimeters (cm level accuracy (half-inch accuracy)). Even inexperienced workers can pinpoint pile positions accurately without hesitation, so there is no need for surveying expertise.

Also, in post-construction as-built management (as-built quantity measurement) situations, if you display the planned BIM model and design lines in AR in advance, you can immediately check on site whether the finish matches the drawings. Whether embankments and structures are at the designed heights and shapes is readily apparent through a smartphone screen. If necessary, perform point cloud scanning (3D measurement) with LRTK, and by comparing the design data and the as-built data in the cloud, numerical verification on the spot is also possible.

Additionally, LRTK also includes a feature called positioning photos, which lets you tag photos taken with a smartphone with high-precision coordinates and camera pose information and save them to the cloud. If you return to the same location later, previously photographed points will be shown as icons in AR, making it easier to track changes over time and identify areas that need repair. On-site management, which has traditionally relied on human labor and experience, will be greatly transformed.

In this way, LRTK is an all-in-one on-site DX tool that combines AR visualization and positioning capabilities. Its high-precision AR not only removes the stress of "drift", but also handles on-site surveying, inspection, and documentation seamlessly, so it is expected to dramatically improve work efficiency and quality. If you are looking for a solution that lets you easily survey and verify without being troubled by AR drift, why not consider experiencing LRTK’s new approach to surveying and construction management?

Frequently Asked Questions (FAQ)

Q: What equipment is required to perform high-precision AR surveying? A: Basically, you need a smartphone, a high-precision GNSS receiver (RTK-capable device), and an AR display app that supports them. A typical example is combining an RTK antenna that attaches to a smartphone (such as LRTK) with a dedicated app. The smartphone itself does not need to be the latest high-end model, but a device with sufficient performance that supports AR features (ARKit or ARCore) is desirable. Also, when using the equipment for long periods, it’s prudent to have a mobile battery pack ready to prevent the smartphone and receiver from running out of power.

Q: Can anyone handle it if they can use a smartphone? Is any special training or surveying knowledge required? A: Compared with conventional surveying instruments, it is more intuitive to operate, but it is ideal to receive basic training in advance. If you first learn how to operate the app, basic knowledge of RTK positioning, and operational precautions, the risk of confusion on site will be reduced. It is designed so that you can use it even without being a professional surveyor, and in practice site supervisors and construction management engineers have mastered it with a few hours of training and trial use and are putting it to practical use. At first, if you trial it while cross-checking measurements with an experienced surveyor and gradually expand the scope of application, it will be adopted smoothly.

Q: Do I need to prepare a base station (reference station) for RTK every time? Can it be used at sites without a communications environment? A: It depends on how you obtain RTK correction information. If there is a nearby public control point and cellular communications are available, you can receive VRS (Virtual Reference Station) services that use the Geospatial Information Authority of Japan’s electronic reference stations on a smartphone, allowing centimeter-level positioning without installing a dedicated base station. This assumes the smartphone can connect to the Internet via a mobile network. On the other hand, at remote sites such as deep in the mountains where there is no communications coverage, it is also possible to set up your own simple mobile station (base station) and transmit correction data by radio. Additionally, within Japan you can obtain correction information without the Internet by using CLAS (centimeter-level augmentation service) provided by the Quasi-Zenith Satellite System Michibiki. LRTK supports these multiple methods, so it can perform positioning tailored to the site’s conditions.

Q: How closely do AR overlays actually match reality? I'm worried about errors. A: Under good conditions, the horizontal position error is within about 1–2 cm (0.4–0.8 in). Height can also be kept within a few centimeters (a few inches), so in most cases you won't notice any misalignment visually. However, this assumes RTK maintains a solid "Fix" solution (a high-precision positioning solution) and the device's attitude sensors are properly calibrated. Accuracy degrades in environments with poor satellite reception, and if the smartphone's compass is off that will affect the display. We can't claim it will always be perfectly exact, but in typical outdoor conditions you'll get accuracy sufficient for practical use. The important thing is to check AR display accuracy against on-site known points and landmarks as you use it. Even with some error, if you reference real-world benchmarks and make corrections on site it is fully usable for field work and increases reliability.

Q: Is overlay display via AR possible in dark places or at night? A: Because positioning information itself is obtained via GNSS, positioning accuracy does not change at night. However, if the camera image does not have sufficient brightness the AR display becomes difficult to see, and the device’s AR functions (visual tracking) also find it hard to maintain accuracy. When using in dark places such as at night or inside tunnels, illuminate the target with a floodlight or similar to ensure sufficient light, or use a device equipped with LiDAR, which in some cases can remain stable even in somewhat dark conditions. That said, considering safety, it is preferable to conduct surveying and AR checks during daylight hours when possible. In dark sites, rather than relying on camera-image AR, switching to coordinate guidance (numeric guidance) based on high-precision positioning is also an effective approach.

Q: How can AR be used indoors or in underground spaces where GNSS is not available? A: In places where satellite positioning is not possible, such as inside buildings or tunnels, you cannot obtain absolute positions with RTK. In that case, as an alternative you can use AR in local coordinates using known points. For example, you can pre-install reference points on a building floor with a total station, manually enter those coordinates into an app, and use them instead of the smartphone’s current position to perform a simple AR display. Recently, AR systems using indoor positioning technologies such as UWB signals or Visual SLAM have also been researched. At present, achieving centimeter-level (cm level accuracy (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 used as substitutes. The point is that in environments where GNSS cannot be used, combining other positioning methods can broaden the ways AR can be utilized.

Q: I'm worried about the initial cost — is the equipment really expensive? A: Compared with traditional surveying instruments and 3D laser scanners, a smartphone + RTK receiver configuration can be introduced at a relatively affordable cost. Specific prices depend on the model and service form, but in many cases it comes at only a fraction of the cost of a dedicated surveying GNSS set. In addition, reducing outsourced surveying work and cutting rework thanks to improved surveying accuracy can be expected to lower overall costs. Depending on the scale of the site, when labor and time savings are taken into account, reports indicate that payback on the investment is relatively quick. There are also products like LRTK available as subscriptions (monthly), allowing you to reduce initial expenses and try a pilot deployment. It is recommended to start with small-scale sites, evaluate the effects, and expand gradually.