In recent years, DX (digital transformation) using smartphones has been progressing in the civil engineering and construction sectors. In particular, on surveying and construction management sites, attention to AR (augmented reality) technology that overlays digital information on real-world scenes is increasing. However, conventional smartphone GPS positioning was coarse at about several meters (several ft), and insufficient for the precise alignment required to match drawings. Enter smartphone RTK, which enables centimeter-level (half-inch accuracy) high-precision positioning. Smartphone RTK is a technology that combines high-precision GNSS positioning (RTK method) with a smartphone so that positions can be determined to within a few centimeters (a few in) of error without dedicated surveying equipment. With this technology, it has become possible to use accurate coordinate data acquired only with a smartphone in CAD drawings, or to display design models from CAD drawings on site in AR.

In this article, we explain the mechanism of centimeter-class surveying using smartphone RTK and how to utilize the coordinate data obtained in CAD drawings. We also introduce specific examples such as the technology for overlaying CAD drawings on a smartphone for AR display and its practical utility, visualization of underground buried objects, applications in structural construction, and uses in as-built management. Finally, we feature LRTK as a solution that makes it easy to introduce these cutting‑edge technologies on site and provide guidance on how to get started with simple surveying.

How centimeter-level positioning is achieved with smartphone RTK

Smartphone RTK is a technology that integrates real-time kinematic (RTK) GNSS positioning into mobile devices such as smartphones. Normally, smartphone GPS positioning is said to have an error of about 5-10 m (16.4-32.8 ft), making it unsuitable for accurate on-site positioning. RTK is a method that corrects error sources in real time through simultaneous measurement with a reference station, achieving accuracy of several centimeters (a few in). Traditionally, expensive GNSS receivers, base stations, and skilled surveyors were required, but in recent years, by equipping or connecting compact RTK-GNSS receivers to smartphones, anyone can easily achieve centimeter-level positioning (half-inch accuracy).

With smartphone RTK, the phone’s built-in or external high-precision GNSS antenna receives signals from multiple satellites and calculates position by combining them with correction information delivered from reference stations or satellites. In Japan, by using centimeter-level augmentation services (CLAS) broadcast by the Quasi-Zenith Satellite System "Michibiki" — i.e., cm-level accuracy (half-inch accuracy) — high-precision positioning is possible without the internet even in mountainous areas where mobile signals do not reach. With smartphone RTK, horizontal position can be determined to ±1–2 cm (±0.4–0.8 in), and vertical position within ± a few centimeters (± a few inches), yielding accuracy comparable to professional surveying instruments. With this level of precision, the difference between design coordinates on drawings and on-site survey values becomes negligible, dramatically improving surveying and construction accuracy in the field.

Utilizing high-precision coordinate data obtained by smartphone RTK for CAD drawings

Coordinate data obtained with smartphone RTK can be used directly in CAD drawings in the same way as conventional survey data. The latitude, longitude, and height of points measured with high accuracy can be automatically converted and output by the smartphone app into the Japan Geodetic System or the plane rectangular coordinate system (public coordinates), making it easier to match the coordinate system of design drawings. For example, if you export the coordinates of key points measured on site as CSV or DXF files and import them directly into design CAD, you can instantly verify the deviation between the as-built and the design drawings digitally. It also eliminates the hassle and errors of jotting down point coordinates in a paper field notebook and entering them later by hand, and a major advantage is that you can reflect measurements in the CAD drawing on the spot.

As a specific use case, you can survey the existing site topography before construction with smartphone RTK to obtain point clouds and terrain data, and overlay them on CAD drawings to review design plans. During construction, you can measure key as-built points with smartphone RTK and compare the results with the design model in the CAD drawings to verify quality or assist in correcting construction deviations. After construction, you can measure important points of the completed structure and record them in the CAD drawings, using the data to create as-built drawings and reports. By importing the highly reliable coordinate data obtained with smartphone RTK into CAD in this way, data linkage from design through construction and maintenance becomes seamless, making it possible to eliminate the gap between the field and the drawings.

Mechanism and Practicality of Smartphone AR That Overlays CAD Drawings

When high-precision positioning becomes possible with smartphone RTK, using that data makes AR that overlays CAD drawings onto the real world a powerful tool. If design CAD drawings and 3D models can be composited into the actual scenery through a smartphone or tablet camera, you can directly compare the design and the as-built conditions on site. However, with typical smartphone AR, GPS and sensor errors cause position offsets of several meters (several ft), making precise registration difficult. With smartphone RTK, because the device’s position can always be determined accurately at the centimeter level (half-inch accuracy), the position of the design data shown in AR can be matched to the real world with almost no discrepancy. Therefore, it is no longer necessary to place markers on the ground to align AR models as was previously required.

As for how AR display works, first the design data created in CAD (2D drawings and BIM/CIM 3D models, etc.) are mapped to real-world coordinates and loaded into a smartphone AR app. Based on the high-precision current coordinates provided by the smartphone RTK and the orientation sensor information, the CAD data is projected to the corresponding position and orientation on site. For example, design lines drawn on the ground or a predictive 3D model of a completed structure are overlaid on the real-world view on the smartphone screen. Thanks to RTK position correction, the key point is that even if the user walks around, the model does not float or drift but remains fixed in the correct location. Recent smartphones have high screen brightness and improved visibility even outdoors in daylight, making outdoor AR use practical. The experience of pointing a tablet on site and seeing the design lines from the drawings directly on the ground is truly the future of construction management.

Specific examples of on-site applications

By combining smartphone RTK and AR, various field operations that were previously difficult can be streamlined and enhanced. Here, we introduce representative cases such as visualization of underground buried objects, application to structural construction, and application to as-built management.

• AR visualization of underground buried objects: Accurately identifying in advance the locations of pipes and cables buried underground is important for safe excavation work. By using smartphone RTK and AR, the design positions of buried utilities and the coordinates obtained from preliminary surveys can be used to visualize and display underground piping routes on the ground. Workers can excavate while confirming on the smartphone screen the lines along which underground pipes run, greatly reducing the risk of accidentally damaging lifelines. It also aids in locating buried assets: by overlaying the positions on drawings with the actual site using AR, workers can efficiently find the target buried pipes.

• Use of AR in structural construction: Even in foundation work and the assembly of structures, RTK-capable smartphone AR proves powerful. For example, in piling work, surveying teams traditionally set batter boards and pile markers based on coordinates from drawings to stake out positions. With RTK-capable smartphone AR, a virtual "AR pile" can be placed at the pile positions on the design drawings and displayed on site, so workers are guided to the designated positions by on-screen arrows and the virtual piles. Even inexperienced operators can intuitively and accurately identify pile locations, allowing one person to stake out multiple piles in a short time. Even on steep slopes or in areas with obstacles, points can be indicated through the screen from a safe location, improving safety. Furthermore, during structural assembly, displaying the design's 3D model on site in AR and overlaying it on the current construction state allows immediate verification of whether component installation positions are misaligned. Inspections such as steel frame installation positions and bolt-hole locations can also be performed without errors by comparing them with the design model in AR.

• Immediate AR Checks for As-built Management: AR and smartphone RTK also prove powerful in the process of verifying whether completed work conforms to the design after construction. Traditionally, as-built management involved surveying after construction and comparing the results to drawing data to make determinations, but with AR it is possible to verify on the spot. For example, when constructing embankments for roadworks, the completed design model or reference surfaces can be pre-displayed in AR, allowing the operator to adjust the amount of fill while viewing the design height lines superimposed on the actual terrain. When the fill reaches the design line, the line becomes hidden by the terrain on the screen, making it immediately obvious that the required height has been achieved. Minor irregularities can also be checked through AR, and additional fill or excavation can be instructed immediately as needed. Combined with the LiDAR scanners and photogrammetry functions built into iPhones and iPads, point cloud data of the finished terrain can be captured on the spot, and advanced checks—such as automatically calculating differences from the design 3D model in the cloud—can be completed in a short time. This dramatically improves the accuracy and speed of as-built management, minimizing rework and additional effort.

Moreover, other applications can be envisioned: for example, visualizing property boundary lines with AR so that boundaries that are only discernible on drawings can be intuitively understood on site, or overlaying past record photos with current footage during structural inspections to check changes over time. The combination of smartphone RTK and AR will support fieldwork in all kinds of situations by making the invisible visible.

Effects of AR Visualization on Safety and Work Efficiency

Improved safety: AR visualization using smartphone RTK significantly enhances on-site safety. Because it enables situation awareness and guidance via AR displays from safe locations without entering physically hazardous areas, it can reduce the risk of accidents in heavy equipment operation zones and on slopes prone to collapse. Making buried utilities visible can prevent accidental damage and also help avert the risk of structural defects caused by construction errors. Furthermore, sharing information through AR reduces communication loss among site personnel, helping to prevent rework and accidents caused by miscommunication.

Improving work efficiency: The use of AR dramatically enhances the efficiency of on-site operations. Tasks such as placing and removing survey points and performing as-built inspections can be completed on-site in real time, eliminating the back-and-forth of measuring, bringing data back to the office, and comparing it with drawings as was traditionally required. A single person can measure and verify many points in a short time, contributing to reduced staffing and lower costs. Furthermore, AR visualization allows anyone to work accurately without relying on the intuition or experience of skilled workers, reducing quality variation and minimizing rework in downstream processes. Additionally, smoother information sharing between the field and the office shortens the time required for various inspections and reporting tasks. Overall, this shortens construction cycles and improves productivity, strongly supporting the DX (digital transformation) of on-site operations.

Labor and Personnel Savings Achieved with a Single Smartphone and the Benefits of Eliminating the Need for Skilled Workers

By leveraging smartphone RTK and AR, on-site tasks that once relied on specialized equipment and large teams can be performed with a single smartphone. The main benefits this brings include the following:

• Labor savings: Because surveying and positioning can be done using only a compact smartphone RTK system, the physical burden of carrying heavy surveying equipment and of having to set out many survey stakes is reduced. The app also performs advanced calculations automatically, eliminating the need for manual calculations and reducing the overall workload.

• Labor-saving: Tasks that used to be carried out by a team of a surveyor and assistants can be completed by a single person with smartphone RTK. Because stake positioning and as-built checks can be handled sufficiently by one operator, construction management can be performed with a small crew even on sites facing labor shortages. In addition, since it does not tie up other workers, personnel can be reassigned more efficiently, leading to improved productivity.

• No expertise required: Visual navigation using AR and automatically corrected numerical displays make it possible to perform highly accurate work without advanced surveying skills. Even new staff can record survey points or determine stake locations by following instructions on a smartphone screen, shortening the time required for skill transfer and personnel training. Reliance on veterans’ intuition is reduced, enabling consistent results regardless of who carries out the work.

Simplified Surveying and High-Precision AR Display with LRTK

As shown above, smartphone RTK and AR technologies bring very large benefits to surveying and construction sites. That said, some people may think, "Isn't it difficult to actually introduce such cutting‑edge surveying into the field?" Enter the solution developed by Refixia: LRTK. By using LRTK, you can achieve the centimeter-level surveying (cm level accuracy, half-inch accuracy) and high‑precision AR display I have just described all in one with a single smartphone.

LRTK uses a dedicated ultra-compact RTK-GNSS receiver attached to a smartphone. Weighing approximately 125 g and small enough to fit in a pocket, it has a built-in battery, making it easy to handle. Its price is also significantly lower than traditional surveying instruments, making it realistic for each person to carry one as a field tool at all times. Through a smartphone and a dedicated app, it acquires highly accurate position coordinates in real time and can consistently perform tasks such as recording survey points, taking photos, point cloud scanning, and overlay display of design data via AR. Positioning data are automatically uploaded to the cloud for sharing, allowing immediate verification of field survey results from the office. Even at sites without internet access, it can receive augmentation signals from Japan's satellite positioning system (Michibiki) to maintain centimeter-level accuracy (half-inch accuracy), making it effective in mountainous and disaster areas.

Thus, LRTK is a versatile surveying tool that condenses into a single smartphone the accuracy and functionality comparable to conventional dedicated surveying instruments. It has a low barrier to on-site adoption and can be used immediately without special qualifications or training. It is a solution that also meets the needs for the use of 3D data in public works—such as i-Construction promoted by the Ministry of Land, Infrastructure, Transport and Tourism—and is expected to become the standard going forward. If you want to reduce labor and improve accuracy in on-site surveying and construction management, consider introducing LRTK, which combines smartphone RTK and AR. Harness the latest technology to achieve a safer and more efficient on-site DX.