Fusion of AR and RTK:
Next-Generation Construction Surveying Enabled by Smartphones

Surveying technology in the construction industry has evolved significantly in recent years. Traditionally, surveying was mainly conducted using instruments such as transits, total stations, and GPS. However, with the advancement of ICT and the promotion of DX, RTK surveying and drone surveying are becoming more widespread on construction sites. In particular, the use of RTK technology in construction is gaining attention, with real-time kinematic (RTK) technology, which offers millimeter-level accuracy, being utilized for construction management and as-built measurements. Moreover, in recent years, a new method combining RTK and AR technology (augmented reality) has emerged, and we are now entering an era where high-precision construction surveying can be done with just a smartphone. Reports are emerging of groundbreaking examples where smartphones, equipped with high-precision GNSS receivers, transform into "universal surveying devices," capable of measuring point clouds and benchmark coordinates with centimeter-level accuracy.

In this article, we will introduce the next-generation construction surveying made possible by the fusion of AR and RTK, covering the fundamental technologies, use cases, and specific examples. We will also explain the benefits of smartphone-based surveying and how to utilize "LRTK," the solution we provide, offering insights for those in the construction industry on how to leverage the latest technology on-site.

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Basic Technologies of AR and RTK
What is AR (Augmented Reality)?
AR (Augmented Reality) is a technology that overlays digital information onto the real world’s visual display through devices such as smartphones, tablets, or smart glasses.

By synthesizing 3D models, text information, and other digital content onto the real-world scenery in real-time, AR technology virtually extends the world in front of you. In the construction industry, the adoption of AR technology is progressing. For example, it allows overlaying a model of a building to be completed onto the actual site’s video feed or projecting the pipe routing from drawings onto the actual structure for verification. In fact, Shimizu Corporation developed a system called "Shimz AR Eye," which overlays BIM data with real-world footage to assist with the construction management of equipment piping and structural elements. This system helps verify hidden piping before finishing and displays the next process plans for information sharing. In this way, AR technology is useful for visual information sharing on construction sites and for sharing completion images, contributing to improved operational efficiency and error prevention.

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The Mechanism and Accuracy of RTK Surveying
RTK (Real Time Kinematic) surveying is a technology that significantly enhances the positioning accuracy of GNSS (Global Navigation Satellite System), including GPS. By conducting simultaneous GNSS observations with a reference station placed at a known point and a rover (moving station), and sending real-time error correction information from the reference station to the rover, RTK allows for high-precision measurements with errors reduced to just a few centimeters. According to the Geospatial Information Authority of Japan, traditional standalone positioning or simple corrections (DGPS) have errors of several meters, but with RTK-GNSS, the error is reduced to just a few centimeters.

To achieve centimeter-level accuracy, the rover side uses high-performance GNSS antennas and receivers, and combines correction data from the base station (obtained via radio or the internet) to perform position calculations. In recent years, network-based RTK systems (such as VRS) and Japan's quasi-zenith satellite Michibiki's CLAS (Centimeter-Level Augmentation Service) have enabled high-precision positioning without the need to set up a base station. RTK surveying is increasingly being used by surveyors and engineers for infrastructure measurements and construction precision management, and it is becoming an essential technology on civil engineering and construction sites.

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New Possibilities Enabled by the Fusion of AR Technology and RTK Surveying

By combining AR and RTK, "AR display based on absolute coordinates," which was previously difficult, becomes possible. In typical AR apps, initial calibration using markers or plane recognition is required to align the real-world location with the CG model. However, this often leads to positioning drift, where the model shifts relative to the real world as the user moves.

By using the high-precision self-position coordinates from RTK-GNSS, the virtual AR model can be directly aligned with the Earth's coordinate system, ensuring stable display without any shift, even as the user moves around the site.

In other words, AR linked to surveying coordinates equals "AR that doesn't require coordinate alignment." This eliminates the need for initial calibration, allowing 3D models of structures to be instantly and accurately overlaid on the desired locations on-site. For example, when installing a sign on a roadside obscured by vegetation with poor visibility, the AR model can pinpoint the exact location for placement, making it easy to see where it should go. Additionally, with RTK, the user's orientation (heading) is captured with high precision, so even when walking around and viewing the AR model from different angles, its position and orientation remain accurate.

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By combining AR and RTK, design data can be displayed at accurate locations on the smartphone screen. For example, the image below shows a 3D model of the Statue of Liberty overlaid on an outdoor space in front of Tokyo Tower using an iPhone equipped with the RTK-compatible device "LRTK." Normally, with GPS, the positioning accuracy is too low for such precise alignment, but with RTK-GNSS's centimeter-level positioning (indicated by "Fix" on the top of the screen), the virtual model perfectly matches the real-world coordinates. The display coordinates in the AR app are based on the public coordinate system (WGS84), and by directly placing virtual objects at the design positions obtained from surveying diagrams or BIM models, intuitive on-site verification and instructions are dramatically streamlined.

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Use Cases for Next-Generation Construction Surveying
High-precision AR surveying (AR + RTK) is expected to be utilized in various scenarios on construction sites. Let's take a look at some of the representative use cases.

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Overlaying Design Data on Site
One of the biggest advantages of AR × RTK technology is the ability to overlay design data from drawings or BIM models onto real-world site footage. For example, by displaying the model of the design ground surface in AR on a site for earthworks, it becomes immediately clear where and how much excavation or embankment is required. During the planning of structure construction, projecting the position models of columns and beams onto the site allows for early verification of temporary enclosures and interactions with the surrounding environment. What used to be done on drawings can now be visually and intuitively checked on-site, facilitating smoother communication and shared understanding between designers and contractors. In actual construction projects, as seen with Shimizu Corporation’s "AR Eye," efforts are already underway to cross-check plumbing and equipment with BIM models during construction, helping to prevent errors and rework. With high-precision positioning from RTK, this AR-based projection of design data on-site can be done with errors within a few centimeters, significantly improving the accuracy and efficiency of construction management.

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Efficiency in As-Built Management
As-built management involves inspecting and recording whether a structure, after construction, matches the design specifications in terms of shape and dimensions. By utilizing AR + RTK, this process can be made more efficient. For example, the ground or structure after construction can be scanned with the smartphone's LiDAR scanner, and the resulting point cloud data can be overlaid and compared with the design 3D model in AR. The point clouds obtained via RTK are positioned in the global coordinate system, so point clouds measured separately, as well as design data, are automatically aligned.

This allows for the traditionally time-consuming process of comparing measurement data with design drawings to be carried out in real-time and intuitively, enabling immediate identification of discrepancies or deficiencies in the as-built condition. For example, in highway repair work, the post-repair road surface shape can be measured on-site and compared with the design profile in AR to immediately check if the flatness and drainage slope are within the required standards. Similarly, for railway track replacement work, the position of the replaced track can be cross-checked with the planned alignment in AR, allowing for adjustment of even the smallest deviations, such as a few centimeters. These capabilities lead to faster as-built inspections and prevent rework, contributing to both quality assurance and improved efficiency.

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Automation of Surveying Tasks and Improved Accuracy
AR + RTK is revolutionizing how surveying itself is conducted. Traditionally, on-site surveying required specialized surveying technicians using total stations and levels, with multiple people needed to set benchmarks and observe measurement points. However, with a smartphone equipped with high-precision location information, it has become increasingly common for individuals to handle surveying tasks on their own. For example, Obayashi Corporation has released the "Smartphone Survey® AR Edition" app for iPhone/iPad, which allows anyone on-site to measure earthworks, such as embankment or excavation volumes, and calculate the required amounts instantly, streamlining the process.

In this type of smartphone AR surveying, the on-site information captured by sensors and cameras is automatically analyzed, and required dimensions, areas, and volumes can be calculated, eliminating the need for manual tape measurements or hand calculations. When combined with RTK, which ensures almost no error in positioning coordinates, it also makes the automation of layout (positioning) tasks possible. For instance, when approaching a pre-set installation position, the smartphone screen can display "This is the reference point," allowing workers to install stakes or make markings with high precision simply by following the on-screen instructions. This concept is similar to GNSS-guided heavy machinery operations but can be used more easily on small sites since no specialized machinery is required. Overall, the use of AR + RTK enables the reduction of labor and personnel for surveying tasks and is expected to help compensate for the shortage of skilled surveyors.

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Real-World Use Cases of LRTK
Now, let's take a look at some examples of how AR × RTK is being utilized on-site, as well as the use cases for the "LRTK" solution provided by our company.

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Examples from Civil Engineering and Construction Sites
LRTK is a system developed by Lefixia Co., Ltd., consisting of an ultra-compact RTK-GNSS receiver and a smartphone app. Since its release in 2022, it has become a quiet trend among on-site professionals. By attaching an antenna-integrated receiver to smartphones such as iPhones and obtaining correction data from network-based RTK or the Michibiki CLAS system, smartphones can be used as centimeter-precision surveying instruments. For example, on a civil engineering site, a smartphone with LRTK was used to perform benchmark surveying, followed by point cloud scanning of the as-built section and an AR overlay check with the design model. What used to take several days using total stations, laser scanners, and computers was completed in a single day with just the smartphone.

In another construction site, CAD data showing the planned excavation area was loaded into the LRTK-compatible app, and excavation work proceeded while the AR display was shown on-site. Workers were able to guide heavy machinery along the virtual excavation guidelines displayed on the smartphone screen, achieving accurate excavation shapes even without the need for benchmarks. In this way, on-site digital transformation (DX) utilizing LRTK is beginning to progress in various aspects of civil engineering and construction projects.

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Cases in Highway and Railway Maintenance

In the field of infrastructure maintenance, the combination of high-precision positioning and AR proves to be a powerful tool. In highway inspections, AR can display the location information of repair points and buried objects, which are registered in the GIS beforehand, helping workers to accurately identify and avoid overlooking the areas that need repairs. With LRTK, even in locations with no mobile signal, such as tunnels or mountainous areas, models that continue centimeter-level positioning by receiving correction signals (CLAS) from Michibiki are available, ensuring reliable use even in out-of-coverage areas.

For example, in railway track maintenance, AR markers can be used during nighttime work to display the location of parts to be replaced or cable routes, helping workers to perform their tasks without confusion in dark areas. In the future, a scenario where a JR maintenance worker holds up a tablet and sees markers for the next bolt to tighten is becoming increasingly realistic. Additionally, this technology can be applied to measuring displacements in bridges and tunnels. By regularly observing displacements at multiple locations using an LRTK-equipped smartphone and aggregating the results into a 3D model on the cloud for AR visualization, the health of infrastructure structures can be assessed in a three-dimensional and quantitative manner. In the maintenance of large-scale infrastructures such as highways and railways, high-precision location information from LRTK and AR visualization will contribute to both improved efficiency and safety.

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Specific Workflow for Surveying Tasks
The flow of next-generation surveying using LRTK is outlined in simple steps below.

1. Preparation (Data Setup): Import the data to be displayed and measured on-site, such as design drawings or BIM models, into the LRTK app. Additionally, set up the reference coordinate system and prepare RTK correction data (via network connection or CLAS reception). If necessary, perform checks at known points.

2. Smartphone + LRTK Setup: Attach the LRTK receiver to the smartphone and launch the dedicated app. Connect the receiver and smartphone via Bluetooth to begin satellite positioning. If satellite conditions are favorable, the "Fix solution" will be obtained in about a few seconds, and the current position will be displayed with centimeter-level accuracy on the screen.

3. Switch to AR Surveying Mode on Site: Once the positioning is stable, switch to the AR mode in the app. When you point the camera, the pre-prepared design data will be overlaid on the real-world footage. For example, if there is a point to be surveyed, a corresponding virtual marker will appear on the ground at that location.

4. Conducting Surveying and Inspection: While checking the AR display, perform measurements or markings at the required points. By tapping the button on the screen at any point, you can save the 3D coordinates of that location. Using the photo capture feature, you can record on-site photos with high-precision location coordinates in the cloud. With LRTK’s tilt correction feature, measurements can be automatically adjusted to correct for the position of the pole, even if it is tilted, making surveying through obstacles easier.

5. Data Sharing and Utilization: The measured point cloud data and photo data are uploaded to the cloud and can be instantly checked on the office PC. Since the data is centralized in the global coordinate system, it is possible to overlay and analyze measurement results from multiple sites. The measurement data can also be directly utilized for creating daily reports or as-built drawings, reducing the effort required for post-processing.

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With the flow described above, using LRTK allows the entire process from surveying to design verification and documentation to be completed on a smartphone. The data obtained on-site is visualized and shared instantly, enabling real-time decision-making and feedback, which leads to faster execution of the construction PDCA cycle.

In particular, the ease of use with a smartphone and the ability to cross-check design and surveying information in real-time are significant advantages that were not possible with traditional methods. However, it is important to note that this approach does not completely replace traditional methods. In locations with extremely poor signal environments or in cases where millimeter-level accuracy is required, such as benchmark surveying, the continued use of traditional optical surveying equipment should still be considered. Nevertheless, in construction management and routine surveying tasks, AR + RTK technology will strongly complement traditional methods and help improve on-site productivity.

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Introduction to LRTK

By implementing LRTK, a next-generation surveying solution that is gaining attention, you can enjoy the benefits mentioned above on-site. If you are interested after reading this article, we encourage you to check out more details about LRTK. We also offer free material requests for LRTK, so if you'd like documents summarizing case studies and technical specifications, feel free to contact us. Additionally, the official website provides product specifications, pricing, and application videos.

When comparing with other RTK devices or total stations, you will be able to see the differentiating points in terms of portability and AR integration.

In today's construction industry, where efficiency and digital transformation (DX) are essential, the fusion of AR and RTK technology is becoming the new standard for surveying and construction management. By utilizing smartphone surveying tools such as LRTK, we encourage you to adopt cutting-edge surveying methods for your site. The future of construction surveying has already begun. Take this opportunity to step into the next-generation solution and achieve smart site operations that balance both precision and efficiency.