Field verification at civil engineering sites! Practicality and Usability of AR Markers

Table of Contents

• What is an AR marker?

• Current status and issues of AR technology at civil engineering sites

• On-site verification of AR markers

• Practicality of AR markers

• Usability of AR markers

• Challenges and prospects for AR marker utilization

• Simple surveying with LRTK

• FAQ

In recent years, the use of AR (augmented reality) technology with smartphones and tablets has attracted attention on construction and civil engineering sites. By overlaying 3D design models and construction information onto real-world scenes, progress and the completed appearance can be intuitively “visualized.” This reduces the need to mentally match drawings with the site, helping to reduce construction errors, streamline quality checks, and promote communication between clients and site teams, among other benefits.

On the other hand, conventional smartphone AR often results in positioning errors of several meters, making it insufficient for surveying and as-built management tasks that require centimeter-level accuracy (half-inch accuracy). In the construction DX trend represented by the Ministry of Land, Infrastructure, Transport and Tourism-led *i-Construction* initiative, new methods such as AR markers and RTK methods using satellite positioning are expected to solve these high-accuracy challenges. In this article, we introduce the results of on-site tests in civil construction where we actually used AR markers to verify their practicality (usefulness) and usability (ease of handling). We also consider the challenges and future prospects of AR marker utilization, and briefly touch on LRTK, a next-generation simple surveying solution.

What is an AR marker?

An AR marker is a mark or image designed to be recognizable by a camera, used by AR applications as a “landmark” to accurately place digital information in the real world. Typical examples include black-and-white 2D codes or special geometric patterns. After registering AR marker images in the system in advance, you can point a smartphone or tablet camera at them on site to read them, and the corresponding 3D models or information will be overlaid at that location.

Simple rectangular black-and-white markers used to be mainstream, but advances in image recognition now allow photos, illustrations, QR codes, and various other items to be used as markers. However, for construction-site uses that require accuracy, patterns with clear contrast (for example, checkerboard-like markers or QR codes) are commonly used because they are easier to detect and yield higher recognition accuracy even when the distance or angle to the camera changes somewhat.

AR using AR markers is also called “marker-based AR,” where content is displayed only after the corresponding marker appears in the camera view. Conversely, methods that estimate position from surrounding feature points or planes without using markers are called “markerless AR (AR that does not require alignment markers).” The latter requires no prior preparation and is convenient, but position and orientation errors tend to occur when relying only on a smartphone’s GPS, gyroscope, and image feature points, causing the displayed model to drift from reality. To achieve the accuracy required on construction sites, using physical reference AR markers is often an effective approach.

Current status and issues of AR technology at civil engineering sites

Expectations for AR technology are growing in fields such as construction management and surveying. For example, if you overlay a 3D design model on the real scene through a tablet on site, you can visualize the completed appearance in place, preventing construction errors and enabling immediate as-built checks. Actual use cases have reported that AR allows even non-experts to perform layout (positioning) tasks accurately without relying on the experience of skilled workers, and to perform as-built checks immediately after construction. From the perspectives of addressing labor shortages and transferring skills, AR is seen as a promising tool to support young workers and small crews.

A key challenge in this context is positioning accuracy. Standard smartphone AR has limitations in pinpointing location with GPS and inertial sensors, sometimes resulting in errors on the order of meters. Such errors are acceptable for gaming but intolerable for surveying and as-built management in civil engineering. Therefore, in recent years, the use of AR markers to achieve higher-accuracy AR on site has garnered attention. By placing markers at pre-determined positions on drawings or at known control points, and using those markers to correct AR display, the gap between reality and virtual models can be minimized.

Under the *i-Construction* and construction DX initiatives promoted by the Ministry of Land, Infrastructure, Transport and Tourism, improving AR accuracy is an important theme. Experimental demonstrations of high-precision AR systems using AR markers have been conducted, and attempts to achieve AR displays with errors within a few centimeters by combining with RTK-GNSS (real-time kinematic positioning) have begun. Approaches that give smartphones high-precision positioning without markers, such as the LRTK described later, have also emerged. First, let’s look at the verification results of actually using AR markers on civil engineering sites.

On-site verification of AR markers

Although AR markers are regarded as a promising method for high-precision AR, we conducted field trials at an actual construction site to determine whether they are truly useful and worth the effort. The verification focused on using AR for rebar inspection of reinforced concrete structures. Multiple AR markers attached to the structure’s frame were used as landmarks; a 360° camera captured the entire site, and BIM design data was overlaid on that footage to enable digital measurement.

Specifically, we installed square markers about 20 cm (7.9 in) square at four or more locations on the site floor or near walls, and surveyed and recorded their position coordinates in advance. When a worker photographed the site with a 360° camera, the computer recognized the AR markers appearing in the footage and aligned the actual coordinate system with the BIM model’s coordinate system using the positions of those markers. As a result, virtual 3D models (such as the design positions of rebar) could be overlaid accurately at the correct positions within the captured footage.

With this method, the captured footage and the design model are recorded in perfect alignment. For example, in rebar placement, virtual design position lines are overlaid on photographs or videos of the actual rebar, allowing immediate visual identification of where actual conditions differ from the design. Digital measurement is also possible, enabling confirmation of concrete cover thickness and spacing without physically measuring the object.

The verification work was conducted collaboratively by construction and surveying personnel, and the entire workflow from marker installation to shooting and data processing was tested on site. Now, let’s look at the concrete results regarding the practicality and usability obtained from this field verification.

Practicality of AR markers

The verification first made clear the effects of workflow efficiency improvement and accuracy enhancement by using AR markers. Measurements and checks of rebar dimensions that supervisors or inspectors traditionally performed manually with scales or tape measures were greatly streamlined by AR markers and 360° photos. For example, there is data showing about a 40% (44%) reduction in labor hours for rebar photography and report creation compared with conventional methods. This is because dimensions can be measured digitally on captured images, reducing the effort of on-site measurement and photo organization.

Using AR markers also confirmed the accuracy of measurements. In images where actual rebar was overlaid with BIM models, the misalignment could be visually confirmed to be within a few centimeters (within a few inches). Of course, it is a prerequisite to measure the AR marker positions accurately, but even with some installation error the impact on results was small. In the demonstration, even if the input marker coordinates had errors of about 10 cm (3.9 in), the digital measurement error remained below 2%. However, when the shift exceeded 20 cm (7.9 in) the accuracy degraded significantly, so proper and accurate installation of markers is important.

Because measurement data obtained via AR markers is digital, remote verification and analysis are also easy. If the 360° footage captured on site is sent to the office, specialists in remote locations can check rebar conditions on a PC while viewing the BIM model overlaid on that footage. This reduces the need for experts to visit the site every time, cutting travel time and enabling remote technical support—another practical advantage.

Overall, AR markers were shown to be a practical tool that can contribute to workflow efficiency while meeting the required accuracy on civil engineering sites.

Usability of AR markers

Next, regarding on-site usability: the workflow using AR markers was generally intuitive and easy for site staff to handle. Marker installation itself only required sticking printed markers onto designated positions; no special tools or complex adjustments were needed. In this verification, we prepared markers adhered to thick paper or plastic plates and fixed them to concrete walls or floors with tape; the work was completed in a short time with a few people.

Deciding where to install markers is straightforward if based on site reference marks or design drawings, so staff with surveying knowledge can handle it without difficulty. At the actual site, survey personnel measured marker coordinates with a total station and input the values into the system, but this task can be performed as an extension of conventional surveying work and was not considered a major burden. At some sites, it may be practical to pre-install markers at required locations during breaks in construction and measure and register coordinates during available time.

Shooting and scanning with a 360° camera or tablet was also easily handled by site staff. Walking once around the designated area while holding the camera was sufficient; no special shooting technique was required. During shooting it was necessary to ensure that markers appeared clearly, but because we placed four or more markers around the site, the relaxed condition “as long as at least one marker appears in any shot” proved operationally acceptable. In the actual verification, we did not encounter cases where markers within the shooting area failed to appear and caused problems.

Moreover, site staff reported that they did not have trouble finding places to put the markers. Plate-like markers about 20 cm (7.9 in) square can be attached to small spaces on walls or floors without interfering with materials or equipment. Care should be taken to prevent them from being blown away by wind or getting dirty, but these issues can be addressed by securing them with protective tape or laminating them to improve durability.

Overall, the introduction and operation of AR markers seemed feasible for site staff without special skills. With basic smartphone operation skills and elementary surveying knowledge, site personnel can use them on site after a simple set of instructions. This is a major advantage in that new technology can be introduced without increasing on-site burden.

Challenges and prospects for AR marker utilization

The field verification showed many practical benefits of AR markers, but several challenges also emerged. First, the time and effort to install and survey markers in advance are inevitable. For a few locations as in this verification, the burden is small, but covering a wide area would require a corresponding number of markers. If an area changes, you must also consider the effort of repositioning markers.

Also, AR markers are only useful if they are in positions visible to the camera. If a marker is hidden from view during shooting or AR display, or if it is too far away for the camera to recognize, accuracy cannot be guaranteed. Therefore, markers should be arranged to surround the measurement target as much as possible, and their height and orientation should be chosen to be easy for the camera to see. In this verification we avoided placing markers in a straight line and instead positioned them at the four corners.

When using AR markers outdoors, attention to environmental factors is also necessary. Measures such as preventing markers from being blown away by strong winds, avoiding rain or dirt that could obscure markers, and using matte printing to prevent glare under strong sunlight can improve stability with small adjustments. In dark places, adding lighting to compensate camera exposure may be required. In other words, while AR marker methods provide high accuracy, they are somewhat sensitive to surrounding environmental conditions.

Going forward, technological development to make AR marker operations more efficient and simpler is expected. For example, saving the coordinates of installed markers to the cloud so that other site staff can use them immediately is an operational improvement to consider. In addition, high-precision markerless AR that realizes accurate alignment without markers will likely become more widespread. A representative example is the RTK-GNSS-based method introduced next. By utilizing satellite positioning, AR display and positioning with centimeter-level positioning accuracy (half-inch accuracy) are becoming possible without placing physical markers.

AR markers and RTK positioning each have pros and cons. For example, GNSS (satellite positioning) cannot be used indoors or inside tunnels, so image-recognition-based methods like AR markers are effective. On the other hand, GNSS-based approaches are suitable for wide-area positioning and AR display while moving. In the future, using these technologies appropriately by situation or combining them will create more flexible and powerful site DX tools.

Simple surveying with LRTK

Alongside methods using AR markers, a high-precision AR and surveying solution that has attracted attention in recent years is LRTK. LRTK connects a compact high-precision GNSS receiver to a smartphone or tablet and corrects positioning errors in real time, allowing easy acquisition of centimeter-level positioning accuracy (half-inch accuracy). In a sense, it turns a handheld smartphone into a versatile surveying instrument. This enables surveying and staking-out tasks that previously required specialized equipment and multiple personnel to be carried out by one person with a smartphone in a short time.

With LRTK surveying, there is no need to place markers on site, and in environments with a visible sky you can obtain high-precision positioning information over a wide area. For example, when staking out pile-driving positions according to drawing coordinates, you can reach the target location simply by walking while following a guide displayed on the smartphone screen. Dramatic efficiency improvements have been reported, such as staking-out work that previously took multiple people half a day being completed quickly by one person using LRTK.

Another advantage is that you do not need to prepare expensive dedicated equipment. Since you can start with a smartphone or tablet and a palm-sized GNSS receiver, small construction companies and local governments can more easily introduce the system. LRTK system apps are designed with intuitive interfaces that site workers can use, so you can operate them after basic training without specialized knowledge. In practice, there are cases where site supervisors who are not surveying specialists have used LRTK to perform surveying and position checks themselves, expanding their responsibilities.

Thus, simple surveying with LRTK is gaining attention on site as a new option alongside AR marker methods. While AR markers are strong for localized measurements and indoor use, LRTK excels at flexible, high-precision positioning and AR display across large outdoor sites. By choosing between or combining these approaches according to site conditions and objectives, future construction DX will become even more efficient and smarter.

FAQ

Q: What preparation and equipment are needed to use AR markers? A: Basically, you need images to serve as markers, printed versions of those images (or display devices showing them), and an AR-capable smartphone/tablet to get started. To improve accuracy, it is desirable to survey and record the coordinates of the positions where markers will be placed in advance. No special expensive equipment is particularly required; you can operate with standard camera-equipped devices.

Q: How accurate can measurements be when using AR markers? A: If AR markers are properly placed, you can align real space and digital data with an accuracy on the order of a few centimeters (within a few inches). Experiments confirmed high accuracy—for example, when marker position measurement errors were 10 cm (3.9 in) or less, measurement errors were kept below 2%. However, if markers are poorly placed or incorrect coordinates are input, accuracy will degrade, so it is important to install multiple markers in sufficient numbers and survey them accurately.

Q: Can anyone on site use AR marker technology? A: Yes. If you can operate a smartphone or tablet at a basic level, it is not difficult. The AR display is handled automatically by the app, so users only need to place markers and hold up the camera. More and more tools are available that non-surveyors can use, and with a short instruction session the site staff in this study were able to perform shooting and measurement themselves. Using apps with an intuitive feel, even those unfamiliar with IT can operate them after brief training.

Q: Can markerless AR do the same things? A: Markerless AR (methods that extract features from the surrounding landscape for alignment) can provide simple AR displays, but it still faces challenges for high-accuracy applications. A smartphone’s standalone GPS and sensors have large errors, and it is particularly difficult to achieve accurate alignment across wide outdoor areas. Therefore, for situations requiring a few centimeters of accuracy (a few inches), combining AR markers or RTK-GNSS corrections is more reliable.

Q: What are the advantages of AR using RTK-GNSS (for example, LRTK)? A: The biggest advantage is obtaining centimeter-level accuracy (half-inch accuracy) across wide areas without installing markers. By using satellite positioning, you can obtain high-precision coordinates wherever you are, making it suitable for AR display while moving and for wide-area surveying. Eliminating the need to prepare physical markers reduces pre-work, and the mobility to perform surveying or stake-out immediately when needed is attractive.

Q: Can small sites or companies introduce these AR technologies? A: Yes. In fact, sites with small crews may benefit the most from AR technologies. Recent AR solutions often do not require expensive dedicated equipment and can be used with existing smartphones or tablets. Systems like LRTK can be introduced with limited initial investment, making them accessible to small-to-medium construction firms and local governments. By using AR, work efficiency improves and small crews can maintain high productivity despite labor shortages.

Q: Will AR make conventional surveying and construction management unnecessary? A: AR is a tool to assist site work, not a replacement for conventional surveying and management knowledge and verification processes. Technical judgment and expert scrutiny remain important to interpret AR-provided information and ensure quality. However, because AR dramatically shortens the time for manual measurement and drawing checks, engineers can focus more on higher-level decisions and overall optimization. In other words, AR should be seen as complementing and strengthening traditional methods rather than replacing them.