Supporting Slope Construction with AR: Coordinate Guidance Enables On-site As-built Verification

The Importance of As-built Verification and Coordinate Guidance in Slope Construction

A slope is an artificial inclined surface formed by cutting or filling. Slope works are frequently carried out in civil engineering projects such as roads and land development, and an essential process for ensuring safety and quality is the "as-built verification," which confirms that the finished shape matches the design. For example, if a slope’s gradient is steeper than designed or the sprayed concrete thickness is insufficient, the risk of future collapse or erosion increases, and the work may fail inspection. Therefore, it is necessary during construction to measure heights, gradients, and thicknesses at various points and verify that they conform to the design values.

However, because slopes are high and steep, taking measurements and laying out positions (survey marking) is not easy. This is where coordinate guidance-based construction control becomes important. Coordinate guidance is a method that directs workers and machines to the correct positions and heights based on pre-determined design coordinates, enabling high-precision work without relying on batter boards or eyeballing. In slope works, precise position guidance is required in places with few physical reference points—such as the finish line of a cut slope or anchor installation locations. By using coordinate guidance, it becomes possible to progress work while confirming on-site whether the construction location and shape match the design, preventing rework and repairs caused by discrepancies in the as-built condition.

Challenges and Limits of Traditional As-built Management Methods

Conventional slope as-built management has often relied on craftsmen’s intuition and manual measurements, and various inefficiencies have been pointed out. Major issues include the following:

• Labor- and time-intensive work: Each time a slope is completed, multiple points had to be measured with tape measures, levels, or total stations (TS) and then checked against drawings in the office. As a result, there is a time lag before problems are discovered on-site, often causing rework.

• Limited measurement coverage leading to oversights: Because the number of measurable points is limited, it is difficult to thoroughly check the entire slope. If only major cross-sections are measured, subtle bumps in intermediate areas can be missed and later flagged as "different from the design" in inspections, prompting hurried corrections.

• Safety risks: Entering steep slopes to take measurements carries constant risks such as falls or rockfalls. While aerial work platforms or scaffolding can be used, they require time and cost and therefore cannot be deployed frequently.

• Dependence on experienced technicians: There is a tendency to rely on veteran personnel who can judge slope quality by experience, creating uncertainty in quality control on sites staffed mainly by younger workers. With labor shortages and an aging workforce, there is concern that traditional methods will eventually become untenable.

As described above, traditional as-built verification methods face inefficiencies, lack of immediacy, limited coverage, safety risks, and human-resource concerns, and the construction field has been seeking new methods to resolve these issues.

A New Approach: As-built Verification Using AR and Coordinate Guidance

To address these challenges, a new approach gaining attention is the use of AR (Augmented Reality) technology for as-built verification. AR overlays three-dimensional digital information (models, guidelines, etc.) onto real-world images viewed through a camera. Once an advanced endeavor, AR is becoming increasingly accessible for everyday construction management as smartphone and tablet performance improves. In particular, the latest iPhones and iPads include high-performance cameras and LiDAR sensors, and construction-focused AR apps leveraging these capabilities now allow intuitive on-site as-built verification. Rather than just comparing measured point elevations numerically, the design model itself can be overlaid on the real scene, enabling anyone to visually grasp discrepancies at a glance without relying on intuition or experience.

AR realizes its full potential when combined with high-accuracy positioning in coordinate-guided AR. For example, combining a smartphone with centimeter-level positioning such as RTK-GNSS allows the device to accurately determine its position and orientation. As a result, a 3D model from design data can be displayed precisely aligned with the site coordinates. Even as the user moves with the device, the model remains fixed in the correct location on the screen, behaving as if an actual physical guideline existed there. For instance, by displaying the slope’s design gradient lines or finished surface model in AR and having operators shape or excavate while viewing this semi-transparent model, the correct gradient can be achieved without installing physical batter boards. By combining AR and coordinate guidance in this way, it is becoming possible to carry out construction and inspection concurrently on-site.

The industry’s expectations for AR use are rising. Under the Ministry of Land, Infrastructure, Transport and Tourism-led *i-Construction* initiative and the broader push for construction DX, AR technology has been positioned as one solution to simultaneously improve site efficiency and quality. As three-dimensional data and ICT use for as-built management become the new norm, coordinate-guided AR can be considered a promising tool for next-generation construction management.

Concrete Field Use Cases and Benefits (Efficiency, Visibility, Safety)

The combined use of AR and coordinate guidance for as-built verification has already been trialed on several sites, revealing measurable benefits. For example, in a highway improvement demonstration, a 25 cm-thick foam material was deliberately placed on a completed slope, and a tablet was used to color-code the differences from the design model. The AR-displayed heat map clearly showed the excess fill areas, enabling operators and managers to intuitively identify where to cut back. Using AR in this way can visually expose even slight discrepancies between the as-built condition and the design on-site, allowing immediate decisions and corrective actions. Issues that previously went unnoticed until inspection can be largely eliminated by making them visible on the spot, significantly reducing rework.

The introduction of AR-based as-built management also brings notable efficiency gains. Even on large slopes, AR-compatible 3D scanning can capture the entire surface’s measurement data at once. On one site, a simple survey using a smartphone (iPhone’s 3D scan function and a dedicated app) completed a measurement that would normally require several people and half a day in about five minutes of actual work. From the obtained point cloud data, volumes and cross-sectional shapes can be calculated immediately, eliminating the need for drawing preparation or manual calculations back at the office. Real-time on-site understanding of progress (earthwork volume) and pass/fail judgment for as-built conditions reduce machine idle time and allow work to proceed more smoothly.

Improved visibility is another key benefit. AR-displayed heat maps and 3D models are visual information that anyone—not just veterans—can intuitively understand. Color and shape differences instantly show "where the surface is higher or lower than the design" and "which areas are lacking," enabling the entire site team to share a common understanding. This reduces communication loss and errors caused by differing opinions, facilitating collective quality control.

Moreover, safety is greatly enhanced. Because surrounding terrain can be measured non-contact, workers need not enter dangerous steep slopes or operate instruments at cliff edges as frequently. If necessary, slopes can be scanned remotely and as-built conditions checked from a safe distance. Locations of slope failures that were previously inaccessible and high-elevation anchor installations can also be safely inspected through AR. Overall, the field introduction of AR plus coordinate guidance yields multifaceted benefits: time savings and efficiency, quality improvement through visualization, and safety assurance through non-contact measurement.

Connection with MLIT Guidelines and ICT Construction Promotion (Regulatory Background)

The rising attention to AR-based as-built management is also driven by changes in national policies and regulations. In public works, the method and evaluation standards for as-built inspection are set out in as-built management standards and as-built management guidelines, and clients (the national government and local authorities) require contractors to perform reliable as-built verification and record-keeping. While traditional methods centered on leveling surveys and point-by-point tape measurements, MLIT-promoted *i-Construction* initiatives have popularized ICT-based construction methods, and 3D as-built management using drone photogrammetry and 3D laser scanners has been increasingly implemented. In fact, the draft as-built management guidelines include a chapter on "as-built management using 3D measurement technologies," and trials are underway nationwide to evaluate as-built conditions using unmanned aerial vehicles and terrestrial LiDAR for slope works.

MLIT has introduced a new method called "surface management," which evaluates the error between the design surface and measured point clouds over an area, enabling more comprehensive quality checks than traditional point-by-point inspections. For slope inspections, rather than measuring only representative cross-sectional heights and widths, sites now obtain point cloud data for the entire slope and check for deviations from the design across the whole surface. This makes it easier to detect local defects that were previously overlooked and improves the reliability of inspections. AR heat-map displays align well with the concept of surface management because they allow visualization of measured point cloud data on-site for area-based as-built evaluation, which is expected to be useful for client inspections and remote participation by supervisors.

Additionally, *i-Construction 2.0*, which entered full-scale implementation in fiscal 2024 as part of efforts to revolutionize construction productivity, emphasizes three pillars: "automation of construction," "automation of data linkage," and "automation of construction management." On-site as-built verification with AR is precisely positioned as a technology that supports "automation and sophistication of construction management." An MLIT survey found that sites adopting 3D surveying and machine guidance reduced total work hours by about 30% on average. This reduction is largely due to digitization and automation of drawing creation and quantity calculations. If as-built management is streamlined through AR and point-cloud technologies, further labor-saving and manpower reductions can be realized, helping address labor shortages and shorten construction schedules. Thus, coordinate-guided AR for as-built verification aligns with national ICT construction and DX initiatives, and both regulatory and technical foundations for adoption are being established.

Verification of Accuracy, Reproducibility, and Reliability, and Broader Internal Data Use

When introducing new technologies on site, concerns about accuracy and reproducibility are natural. However, AR plus coordinate-guided as-built management has already undergone demonstrations that show adequate reliability. Three-dimensional point cloud data obtained by combining smartphones with GNSS and LiDAR can, when properly operated, be compared to design values with accuracy within a few centimeters. In fact, in 2021, cases were reported in which iPhone LiDAR sensors combined with RTK-GNSS achieved high-accuracy 3D point cloud measurements in the public coordinate system, enabling smartphone-only workflows for earthwork volume calculations and structural displacement monitoring. Such technological advances have brought as-built measurement—once requiring expensive surveying equipment and specialized personnel—within reach of reproducible, widely accessible workflows. Of course, basic accuracy management such as instrument calibration and cross-checking with control points is still necessary, as in traditional surveying. On the contrary, AR systems perform measurements and evaluations based on consistent digital numerical criteria, reducing variability between operators and enabling highly reproducible inspections.

On-site as-built data can be stored and shared in the cloud, enabling effective internal use of data. Accumulating point clouds, photogrammetry-based 3D models, and AR-recorded footage from construction can later be analyzed to review construction processes or serve as reference data for other projects. For example, referencing point clouds from past slope works under similar terrain conditions can help estimate earthwork volumes and construction procedures in advance, improving estimate accuracy and aiding method selection. Recorded data also provides high-trust evidence for third-party verification or in the event of disputes, contributing to building trust with clients and supervisors.

Digital as-built data is also useful for internal training and knowledge sharing. By converting veteran know-how into point clouds and AR visualizations, companies can achieve data-driven preservation and capitalization of experience. With DX tools becoming inexpensive and easy to use, one-device-per-person smartphone surveying is becoming realistic, and an era in which each employee routinely collects and utilizes as-built data is approaching. The accumulated large volume of on-site data may in the future enable optimization of construction planning and quality prediction through AI analysis. The ripple effects of AR adoption extend beyond efficiency gains at a single site to improve a company’s overall technical capability and credibility.

Simple Surveying and AR Coordinate Guidance Enabled by LRTK

One solution that makes the AR-based as-built verification and coordinate guidance described above easily achievable on-site is LRTK. LRTK is a simple surveying system using a smartphone: a small RTK-GNSS receiver (LRTK Phone device) is attached to an iPhone or iPad, and a dedicated app performs measurements and AR display. By attaching a device weighing just a few hundred grams, a smartphone can achieve centimeter-level positioning and AR projection of BIM/CIM 3D design models onto the site. Because high-precision positioning prevents display drift when moving the device, marker placement and complex calibration are unnecessary. Simply pointing a smartphone makes the design slope model or structure align with the real scene, effectively turning the site into a virtual drawing.

Using LRTK, tasks that previously required multiple separate processes can be completed with one smartphone. For example, the following functions enable multifaceted on-site as-built management:

• As-built verification for slopes, revetments, and similar works (on-site checking of finished shapes)

• Measurement and verification of underground piles and anchor positions (coordinate guidance to design positions)

• Real-time display of dimensional values and elevation levels

• Layout support by AR projection of lines and points from drawings

Combining these functions makes it possible to instantly support as-built management—e.g., projecting the slope design model on-site to check finish quality and displaying required heights on the screen as needed. The system can also automatically generate heat maps showing deviations from the plan, color-coding areas of insufficient fill or excessive excavation, so as-built conditions before and after construction can be visualized with a single smartphone. Photo-based measurement (photogrammetry) is also supported, allowing generation of point clouds from photos for areas beyond LiDAR range. LRTK is truly an all-in-one tool that supports as-built management from every angle.

LRTK also includes robust functions for immediate utilization of on-site data. From measured point clouds, volume calculations (earthwork quantity) can be performed on the spot, and as-built quantity reports and drawings can be auto-generated. Measurement results can be cross-compared directly with CAD drawings or BIM models, and photos and measurement notes can be linked and synchronized to the cloud. Data stored in the cloud can be accumulated without capacity limits and easily shared with designers and clients from office PCs. The acquired point cloud data itself meets the accuracy standards set out in MLIT’s as-built management guidelines and can be used as deliverables for electronic submission.

In this way, LRTK supports the entire workflow—from high-precision coordinate-guided layout and navigation to AR-based on-site as-built verification, point cloud generation, report creation, and data sharing. The operation is simple: attach the device to a smartphone and walk to complete the survey, providing intuitive usability that even non-specialist site staff can quickly master. Because staff can use it effectively from day one, LRTK can deliver DX benefits immediately upon introduction. In slope construction as-built management, adopting simple surveying tools like LRTK heralds an era in which anyone can safely and efficiently perform on-site inspections and quality checks. LRTK—pioneering AR and coordinate guidance for next-generation slope construction management—has the potential to fundamentally change site practices.