Supporting Slope Construction with AR: Coordinate Guidance Enables On-the-Spot As-Built Verification

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

A slope (norimen) is an artificial incline formed by cutting or filling earth. Slope works are frequently performed in civil engineering projects such as roads and land development, and the process of confirming that the finished shape matches the design—known as as-built verification—is an indispensable step for ensuring safety and quality. For example, if the slope gradient is steeper than designed or the sprayed concrete layer is too thin, 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 locations and confirm they match the design values.

However, because slopes are high and inclined terrain, measuring dimensions and setting out positions is not easy. This is where construction management using coordinate guidance becomes important. Coordinate guidance is a method that directs workers and machines to the correct position and elevation based on predetermined design coordinates, achieving high-precision construction without relying on batter boards or visual estimation. In slope works, accurate position guidance is required in locations with few physical landmarks on the terrain, such as finished lines of cut slopes or anchor installation points. By using coordinate guidance, it becomes possible to proceed with work while checking on the spot whether the construction location matches the design position and shape, preventing rework and repairs caused by deviations in the as-built condition.

Challenges and Limitations of Conventional As-Built Management Methods

In conventional slope as-built management, many tasks have relied on craftsmen’s intuition and manual measurements, and various inefficiencies have been pointed out. Major issues include:

• Time-consuming and labor-intensive work: Each time a slope is completed, multiple points had to be measured with tape measures, levels, and total stations (TS), and then compared with drawings in the office. This often caused a time lag before problems were discovered on-site, leading to rework.

• Limited measurement coverage leading to oversights: Because the number of measurable points is limited, it is difficult to comprehensively check the entire slope. If only key cross-sections are measured, subtle irregularities in the middle areas may be overlooked, and inspectors may later point out “differences from the design,” prompting hurried corrections.

• Safety risks: Entering steep slopes to perform surveys inherently carries constant dangers such as falls or falling rocks. Methods using elevated work platforms or scaffolding are possible but are labor- and cost-intensive and thus cannot be performed frequently.

• Dependence on experienced technicians: There is a tendency to rely on veteran workers who can judge slope quality by experience, and sites staffed mainly by younger workers may have concerns about quality control. With labor shortages and aging technical staff, continuing with conventional methods raises concerns about future manageability.

As described above, traditional as-built verification methods face problems such as lack of efficiency and immediacy, limited coverage, safety risks, and human-resource concerns, and new methods to solve these issues have been sought at construction sites.

A New Approach to As-Built Verification Using AR and Coordinate Guidance

To address these issues, a recently highlighted approach is the use of AR (Augmented Reality) technology for as-built verification. AR is a technology that overlays three-dimensional digital information (models, guidelines, etc.) onto real-world images seen through a camera. Once considered an advanced experiment, AR is becoming an accessible tool for everyday construction management thanks to advances in smartphones and tablets. In particular, the latest iPhones and iPads are equipped with high-performance cameras and LiDAR sensors, and construction-focused AR apps leveraging these capabilities now enable intuitive on-site as-built verification. Instead of merely comparing measured point elevations numerically, the design model itself can be overlaid on the real scene, allowing anyone to visually grasp deviations at a glance without relying on intuition or experience.

AR’s true value is further realized when combined with high-accuracy positioning in coordinate-guided AR. For example, combining a smartphone with an RTK-GNSS centimeter-level positioning function enables the device to accurately determine its own position and attitude. As a result, 3D design models can be displayed aligned precisely to site coordinates. Even as the user moves with the device, the model on the screen remains correctly positioned as if a physical guide existed there. For instance, by displaying the design gradient line of a slope or a finish-surface model in AR and having an operator shape the surface while viewing the translucent model, the correct gradient can be achieved without installing physical batter boards. In this way, combining AR with coordinate guidance makes it increasingly possible to carry out construction and quality checks simultaneously on-site.

Expectations for AR use are growing across the industry. 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 utilization for as-built management become the new norm, coordinate-guided AR is likely to become a powerful tool for next-generation construction management.

Specific On-Site Use Cases and Benefits (Operational Efficiency, Visibility, Safety)

The as-built verification method combining AR and coordinate guidance has already been trialed at several sites, and its benefits are becoming clear. For example, in an expressway improvement project, a demonstration placed a 25 cm (9.8 in) thick foam material on a completed slope and used a tablet to color-code the differences from the design model. The heat map displayed in AR clearly showed the areas of excess fill, allowing operators and managers to intuitively identify where trimming was needed. With AR, even slight discrepancies between the as-built condition and the design can be visually revealed on-site, enabling immediate judgment and corrective action. Defects that previously might not have been noticed until inspection can be visualized on the spot, significantly reducing rework.

Improvements in operational efficiency through AR as-built management are also notable. Even on large slopes, AR-compatible 3D scanning can capture measurement data for the entire surface at once. One site reported that a simple survey using a smartphone (iPhone 3D scan feature and a dedicated app) completed in about 5 minutes of actual work—tasks that previously required several workers and half a day. Because volume (earthwork) and cross-sectional shapes can be calculated immediately from the acquired point cloud data, there is no need to return to the office for drawing preparation or manual calculations. Real-time on-site understanding of progress (earthwork quantities) and pass/fail judgments for as-built conditions allow work to proceed while reducing machine idle time.

Improved visibility is another key benefit. AR-presented heat maps and 3D models of as-built conditions provide visual information that can be intuitively understood even by non-experts. By immediately conveying “which areas are higher or lower than the design” and “which portions are deficient,” differences in color and shape help the whole site share a common understanding. This reduces communication loss and errors from differing opinions, making it easier for the team to collaboratively manage quality.

Moreover, safety improvements are significant. Because terrain can be measured non-contact, workers need not enter dangerous steep slopes or operate measuring instruments on cliff edges as frequently. It is also possible to scan slopes remotely as needed and verify as-built conditions from a safe distance. Situations that previously did not allow human approach, such as slope failure checks or high-elevation anchor inspections, can now be conducted safely via AR. Overall, on-site adoption of AR plus coordinate guidance yields multifaceted effects: efficiency gains from shorter work times, quality improvements through visualization, and safety assurance via non-contact measurement.

Connectivity with MLIT Guidelines and Promotion of ICT Construction (Institutional Background)

One reason AR-based as-built management is attracting attention is changes in national policies and guidelines. In public projects in particular, as-built inspection methods and evaluation criteria are specified by as-built management standards and manuals, and clients (national or local governments) require contractors to perform reliable as-built verification and recordkeeping. While level surveys and point-by-point tape measurements were once central, under the Ministry of Land, Infrastructure, Transport and Tourism–promoted *i-Construction* initiative, ICT-based construction methods have spread, and in recent years three-dimensional as-built management using drone photogrammetry and 3D laser scanners has been introduced in earnest. In fact, the draft as-built management manual includes a chapter on “as-built management using 3D measurement technologies,” and trials using unmanned aerial vehicles or terrestrial LiDAR to evaluate as-built conditions on slope works are progressing nationwide.

The ministry has introduced a new approach called “surface management,” which evaluates errors between the design surface and measured point clouds in an areal manner, enabling comprehensive quality checks beyond traditional point-by-point inspections. For example, slope inspections that once measured representative cross-section heights and widths now obtain point cloud data for the entire slope and check deviations from the design across the whole surface. This makes it easier to detect previously overlooked localized defects and improves inspection reliability. AR heat map displays are highly compatible with the surface management concept because measured point cloud data can be visualized on-site for areal as-built evaluation, which is expected to assist client inspections and remote presence of supervisors.

Furthermore, *i-Construction 2.0*, which became full-scale in FY2024 as part of the construction productivity revolution, sets three pillars: “automation of construction,” “automation of data linkage,” and “automation of construction management.” On-site as-built verification using AR is positioned precisely as a technology that supports the automation and sophistication of construction management. A ministry survey on the effects of ICT construction reported that sites adopting 3D surveying and machine guidance reduced total man-hours by about 30% on average. This is largely due to digitization and automation of drawing creation and quantity calculations. By streamlining as-built management through AR and point-cloud technologies, further labor and power savings can be realized, directly addressing labor shortages and shortening construction schedules. Thus, coordinate-guided AR for as-built verification aligns with national promotion of ICT construction and DX, and both institutional and technological foundations for adoption are being established.

Verification of Accuracy, Reproducibility, and Reliability, and the Growing Use of Internal Data

When introducing new technologies to the field, concerns about accuracy and reproducibility inevitably arise. However, AR plus coordinate-guided as-built management has already undergone validation and shown that sufficient reliability can be secured. Three-dimensional point cloud data obtained by combining smartphones with GNSS and LiDAR, when properly operated, can be compared to design values with accuracy within a few centimeters (within a few inches). In fact, in 2021 there was a reported case where an iPhone’s LiDAR sensor combined with RTK-GNSS achieved high-accuracy 3D point cloud measurements on a public coordinate system and realized earthwork volume calculations and structural displacement monitoring using only a smartphone. Such technological advances have brought as-built measurement—previously requiring expensive surveying instruments and specialist technicians—to a level where anyone can reproducibly perform it. Of course, basic accuracy management such as instrument calibration and cross-checking with control points is necessary during measurement, but this is the same as with conventional surveying. Indeed, AR systems perform measurement and evaluation based on digital numeric criteria at all times, reducing variability between operators and enabling reproducible inspections.

On-site as-built data can be saved and shared in the cloud, leading to effective internal data utilization. Accumulating point cloud data, photogrammetric 3D models, and AR-recorded construction footage allows later analysis to review construction processes or use the data as reference for planning other sites. For example, referencing point clouds from past slope works may help estimate earthwork volumes and construction procedures for similar terrain conditions in advance, improving estimate accuracy and method selection. Recorded data also serve as reliable evidence for third-party verification or in case of disputes, contributing to building trust with clients and supervisors.

Moreover, digital as-built data are useful for in-house training and knowledge sharing. By converting veteran experience into point clouds and AR-visualized data, experiential knowledge can be shared with younger staff, enabling the datafication and capitalization of expertise. As site DX tools become affordable and easy to use, “one smartphone per person” surveying becomes realistic, and each employee can routinely collect and utilize as-built data. The accumulated vast amount of field 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 overall enhancement of a company’s technical capability and credibility.

Simple Surveying and AR Coordinate Guidance Realized by LRTK

One solution that enables the AR as-built verification and coordinate guidance described above to be easily implemented 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 measurement and AR display. By attaching a device weighing just a few hundred grams to the phone, the smartphone supports centimeter-level positioning (half-inch accuracy) and can AR-project BIM/CIM 3D design models onto the site. Because high-precision positioning prevents display drift when the device is moved, marker placement and complex calibration are unnecessary. Simply pointing the smartphone makes the design slope model or structure appear perfectly overlaid on the real scene, turning the site itself into a virtual drawing.

Using LRTK, tasks that previously required separate processes can be completed with a single smartphone. Combined functions enable multifaceted on-site as-built management, such as:

• As-built verification of slopes and revetments (on-the-spot checking of finished shapes)

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

• Real-time display of dimensional values and elevation levels

• Setting-out support by AR projection of lines and points from drawings

By combining these functions, LRTK allows immediate as-built management support—for example, projecting the slope design model on-site to check finish and displaying heights of construction points on the screen as needed. It can also automatically generate heat maps showing deviations from planned data, color-coding areas of fill deficiency or excessive cutting, enabling visualization of pre- and post-construction as-built conditions with just a smartphone. Additionally, LRTK supports photogrammetric measurement (photogrammetry), allowing generation of point clouds from captured images in areas beyond the reach of the LiDAR sensor. It is truly an all-in-one tool that supports on-site as-built management from every angle.

LRTK also offers functions to immediately utilize the data obtained on-site. From measured point cloud data, volume calculation (earthwork quantity calculation) can be performed on the spot, and production of progress quantity tallies and as-built diagrams can be automated. Measurement results can be directly compared with CAD drawings or BIM models, and photos and notes of measurement points can be linked and synchronized to the cloud. Data saved in the cloud can be stored in unlimited capacity and easily shared with design staff or clients from office PCs. The acquired point cloud data itself meets the accuracy requirements of the MLIT as-built management manual and can be used directly as deliverables for electronic submission.

Thus, LRTK supports the entire process—from high-precision coordinate-guided setting-out and navigation, to AR on-the-spot as-built verification, point cloud generation, report creation, and data sharing—in a single, integrated workflow. Its use is simple: attach the device to a smartphone and walk to complete surveying with intuitive operation. Because site staff without specialized training can quickly master it, the DX effects can be realized from the day of introduction, which is a major appeal. In slope construction as-built management, adopting simple surveying tools like LRTK heralds an era in which anyone can safely and efficiently perform on-the-spot verification and quality checks. AR and coordinate-guided next-generation slope construction management—led from the forefront by LRTK—is dramatically changing conventional site practices.