GNSS Rovers Accelerate ICT Construction! Easy 3D Measurement Is Changing As-Built Management

Introduction: The need to advance as-built management in ICT construction

As construction sites become more digitalized, as-built management methods in ICT construction (commonly known as *i-Construction*) are changing dramatically. Traditionally, measurements were taken at prescribed cross-sections, documents were prepared by handwriting or manual input, and inspections were conducted accordingly. Today, however, there is a shift toward delivering results as electronic 3D data by comparing point cloud data obtained from 3D measurement technologies such as drone photogrammetry and laser scanner surveys with design data. The establishment of such electronic delivery rules requires unified data formats that anyone can understand anywhere and anytime, which helps reduce document errors and improve work efficiency. As a result, there is strong demand for the advancement and efficiency of as-built management, which is a quality control process.

Against this backdrop, the use of high-precision 3D surveying technologies on site has become indispensable. Among these, surveying with a GNSS rover is attracting attention. Rapid 3D measurement using satellite positioning is being introduced across sites as a pillar of on-site DX (digital transformation). This article explains how GNSS rovers work and their roles, and describes how the latest easy-to-use devices are changing job sites. It also introduces concrete ways to apply them to 3D as-built management and key points for complying with the Ministry of Land, Infrastructure, Transport and Tourism (MLIT) guidelines.

How GNSS rovers work and their role in ICT surveying

A GNSS rover is a mobile receiver that receives signals from multiple positioning satellites such as GPS, GLONASS, and QZSS (Michibiki) and can measure its position in real time. Standalone GPS positioning can result in errors of several meters, but GNSS rover surveying can achieve centimeter-class accuracy using the RTK (Real Time Kinematic) method. RTK works by communicating observation data between a reference station (base station) set up on site and the rover (mobile station) to cancel out positioning errors between them and achieve high accuracy. For example, while a typical smartphone GPS may deviate by about 5-10 m (16.4-32.8 ft), RTK-GNSS can improve accuracy to about 1-2 cm (0.4-0.8 in).

There are two main types of GNSS rover surveying: the traditional RTK method that requires setting up a base station, and the network RTK (VRS) method that uses the Geospatial Information Authority of Japan’s permanent GNSS network. In the conventional RTK that requires a base station, you set up your own reference station on site; with VRS, correction information is obtained from a virtual reference point via communication, so you only need a single receiver and a mobile communication environment. Each has trade-offs in initial cost and communication requirements, but with the spread of VRS services, environments that allow simple high‑precision positioning without a base station are becoming common. Choose the method according to site conditions and project scale to obtain high-precision position information.

The role GNSS rovers play in ICT construction is that a single operator can efficiently obtain 3D coordinates on site. By operating the rover from a dedicated controller terminal or an app on a tablet or smartphone and placing the receiver at the pole tip on the point to be measured, you can obtain the X, Y, Z coordinates of that point in real time. The acquired coordinate data can be used directly in the design coordinate system, allowing immediate comparison of survey results with design data or use for as-built judgments. In particular, the accuracy obtained with RTK-GNSS is said to be approximately ±1-2 cm (±0.4-0.8 in) horizontally and ±2-3 cm (±0.8-1.2 in) vertically, which is sufficient for typical civil engineering staking and as-built management. As a result, many tasks that were formerly performed with total stations (TS) are now being replaced by GNSS rovers to improve efficiency.

However, when using GNSS rovers, attention must be paid to the process of “localizing” the positioning coordinates to the site’s coordinate system. It is necessary to correct discrepancies between latitude/longitude and ellipsoidal height obtained from GNSS and the plane rectangular coordinates (public coordinate system) used in design drawings. Specifically, you compare the coordinates of known points (control points) on site with values observed by the GNSS rover, calculate the shift and rotation values, and perform a coordinate transformation. This RTK localization (site calibration) allows the point clouds and measured points obtained by the rover to match the public coordinate system. As a result, survey data can be handled on the public coordinates stipulated by national or local governments (for example, the JGD2011-based plane rectangular coordinates in Japan), maintaining high consistency with existing drawings and other survey results. In ICT surveying it is important to perform this coordinate alignment reliably and ensure positioning that has no discrepancy with existing control points or design data.

Changes brought to sites by easy-to-use GNSS rovers

In recent years GNSS rover devices have become smaller and simpler, evolving into easy-to-use surveying tools that anyone can handle. As a result, they have brought major changes to site workflows and staffing arrangements.

First, work efficiency improves dramatically. One technician can carry a rover around the site and quickly collect survey points, eliminating the need for multiple people to set up equipment or maintain line-of-sight as in the past. For example, total station surveys often required an operator and a staff member to maintain mutual line-of-sight, but GNSS can proceed even when sightlines are blocked by obstacles. Consequently, the time required for measurement is greatly reduced, enabling wide-area surveys to be completed in a short time.

Next, there are benefits from reduced personnel requirements (labor saving). GNSS rover surveying can be performed by a single person, reducing the burden of arranging survey crews and schedules. This is a major advantage in the construction industry, which faces severe labor shortages, allowing small teams to manage construction more efficiently. Also noteworthy is the improvement in safety. Measurements in hazardous areas such as zones with operating heavy equipment or unstable slopes can be completed quickly, reducing risk. Survey staff do not need to remain on site for long periods, lowering the chance of secondary accidents or collisions. Moreover, intuitive operation apps make the tools easy to use for newcomers and non‑survey personnel, enabling satisfactory surveys without relying solely on veteran skills. From the perspective of on-site DX, being able to perform high-precision surveys with a smartphone-like interface will smooth the digital shift across the site, even for workers unfamiliar with digital technology.

Additionally, because GNSS rovers complete digitization during measurement, real-time information sharing is possible. With cloud-enabled systems, coordinates and elevation data measured on site can be shared instantly with the office, allowing immediate decisions on pass/fail judgments or instructions for additional measurements. Compared to the era of writing notes in paper field books and bringing them back, electronic-first data eliminates transcription errors and supports faster decision-making. In this way, GNSS rovers that enable measurements that are “fast, small-team, safe, and reliable” significantly increase productivity and reliability in construction management and thus drive the promotion of ICT construction.

Practical methods for 3D as-built management combining point clouds, photos, and AR

To advance as-built management, it is effective to combine not only GNSS position information but also point cloud data, photographs, and AR technology to capture site conditions from multiple perspectives. Below we describe the roles of each technology and how to combine them.

• Point cloud data (3D measurement): High-density 3D data obtained by terrestrial laser scanners, drone photogrammetry, or LiDAR-equipped smartphones. Point clouds allow detailed recording of post-construction terrain and structures, enabling measurement of dimensions on arbitrary cross-sections and calculation of quantities such as excavation or fill volumes. Whereas previously only key cross-sections could be grasped, point clouds allow planar and volumetric evaluation of as-builts, greatly improving quality control accuracy. Acquired point clouds can be overlaid with design 3D models or visualized with heat maps to show compliance at a glance.

• Photographs (geotagged photos): On-site photos are also important records. Recently, managing photos taken with smartphones or tablets that include position information and linking them to survey data has become widespread. If you take photos at each survey point and save them with coordinates and orientation, you can visually check the condition of points later in the office and keep site records like an electronic field book. Using photogrammetry, point clouds can be generated from multiple images to create 3D models. It is now realistic to generate terrain point cloud models from numerous drone images or to create detailed point clouds by photographing structures from various angles with a smartphone. Utilizing such photo data enables intuitive as-built management including color information and detection of subtle changes that the naked eye might miss.

• AR (augmented reality): AR overlays digital 3D models or information on camera images from smartphones or tablets. In civil construction, AR is attracting attention for displaying design data over the actual construction. For example, if a design 3D model is displayed in AR on top of a completed structure, you can directly compare the as-built shape and dimensions with the model. Small differences in elevation or tilt can be visually detected on-site, making inspections and rework smoother. There are reports that “overlaying design models on real structures during construction allowed discovery of surveying deviations and construction errors in advance, reducing rework.” Moreover, AR enables owners/inspectors and contractors/workers to share the same completed-image while checking as-builts, improving communication and streamlining inspection attendance. AR is thus a powerful tool for intuitive on-site pass/fail judgments and stakeholder information sharing.

By combining multiple information sources such as point clouds, photos, and AR, you can practice 3D as-built management more reliably and understandably. For example, measure control points with a GNSS rover while scanning as-built surfaces with a smartphone LiDAR to automatically compute fill volumes from the resulting point clouds—upload the results to the cloud to share with stakeholders—and finally perform on-site verification with a tablet’s AR function. Processes that once required separate equipment and software are now being integrated around handheld devices. By conducting accurate surveys in accordance with the 3D as-built management guidelines and supporting them with advanced technologies, construction management that balances quality and efficiency becomes possible.

MLIT “3D As-Built Management Guidelines” compliance and data utilization

Since FY2016 (Heisei 28), the Ministry of Land, Infrastructure, Transport and Tourism has developed and revised the “Guidelines for As-Built Management Using 3D Measurement Technology (draft)” in step with the expansion of ICT-utilized works. The March 2022 revision expanded applicable work types so that 3D as-built management can be applied not only to earthworks and paving but also to foundations, retaining walls, and bridge substructures, among others. These guidelines (draft) define procedures, accuracy management, and report formats when applying 3D measurement technology to civil engineering construction management. The key point is that even if measurement technology changes, the inspection items and acceptance criteria for as-builts remain the same. In other words, whether you measure with 3D scanners or GNSS, final acceptance is still judged by conventional standards (e.g., whether deviations from design values fall within tolerance). Therefore, contractors must perform sufficient accuracy management to ensure quality even when using new technologies.

A concrete example of guideline compliance is that RTK-GNSS as-built measurements require a prior GNSS accuracy verification test. This involves observing site control points multiple times with the GNSS rover and verifying the difference between the mean observed values and the true values. By objectively demonstrating rover surveying accuracy through such procedures and recording results in the prescribed format, reliability can be assured during later inspections. For 3D scanner surveys, it is also necessary to plan and manage so that the point cloud acquisition range and density meet the standards. For example, the guidelines show procedures such as extracting points within 50 mm (1.97 in) from the scanner-acquired point cloud to create cross-sections and visually reading measured points to calculate dimensions. When submitting data, the three-dimensional data used for as-built management (design surfaces, measured point clouds, and verification results) must be electronically delivered in prescribed formats. Typically this involves submitting LandXML-format design data, as-built point cloud data, inspection images such as heat maps, and inspection reports together. Such data are stored and utilized by the client side and can be valuable for future maintenance and as-built level analysis.

Furthermore, effective utilization of as-built management data is an important perspective. 3D as-built information obtained is not merely for passing inspection at the end of a project but becomes an asset that can be used for future asset management and maintenance. For example, point cloud data and survey photographs at completion can serve as baseline materials for future defect detection or repair planning for roads and hydraulic structures. Analyzing measurement history data obtained during construction can accumulate know-how for improving construction processes and as-built accuracy. If 3D data are shared on cloud-connected systems, owners, designers, and contractors can continuously grasp site conditions like a digital twin, contributing to DX in infrastructure management even after construction. Compliance with MLIT guidelines is of course important, but the real value of ICT construction is realized by effectively utilizing high-quality 3D as-built data obtained while meeting those requirements.

Points for on-site implementation: institutional, accuracy, and operational viewpoints

When introducing new surveying technologies and devices on site, several points should be noted. Below are key considerations from the viewpoints of institutional requirements, accuracy, and operations.

• Institutional requirements: Carefully confirm in advance which guidelines and standards (MLIT’s as-built management guidelines (draft) and local government operational standards) apply to your projects. When applying 3D as-built management, electronic delivery requirements may be specified in contract documents. Identify required data formats, inspection procedures, and necessary forms (such as GNSS accuracy verification test reports), and plan operations to meet deliverable requirements. Also note that surveying work itself may be regulated under laws such as the Survey Act. Public survey control point measurements may restrict GNSS use in first- or second-class control surveys, so consult licensed surveyors as necessary and ensure legal compliance. Update internal regulations and inspection flows to a digital-data-first process and share data-check methods with inspectors in pre-meetings to avoid issues.

• Accuracy: Thorough accuracy management is essential when using GNSS rovers or 3D scanners. For GNSS, check satellite reception conditions in a location with a clear view before starting surveys and check base station or VRS settings as needed. Site localization (site calibration) using known points is mandatory; perform precise known‑point surveys at three or more control points before the main survey. During surveying, periodically return to control points to monitor errors and verify whether measurements differ between morning and afternoon. For laser scanners and photogrammetry, measure verification points before and after acquisition to evaluate point cloud accuracy, or place scale bars or targets to check scale errors. Keep equipment calibrated and firmware updated; follow manufacturer-recommended calibration procedures. In short, even with new technologies, avoid “just measuring” — incorporate stricter accuracy verification and confirmation procedures than before so you can confidently submit as-built data.

• Operations: Practical measures are needed to operate ICT surveying smoothly on site. For human resource development, share basic operation methods and data interpretation with not only the person in charge but related construction management staff so the whole site gains understanding of digital tools. Even with simple GNSS rovers, initial confusion can occur, so allow sufficient trials and training time before construction starts. Next, prepare infrastructure such as communication and power. If using network RTK, ensure stable mobile communications (4G/5G) on site; in mountainous areas with poor reception, consider base-station methods. Establish habits for managing battery levels of tablets, smartphones, and GNSS receivers and carry backup power. As a data management system, use cloud integration for automatic backups and build systems to reliably store data on internal servers. Since large point cloud datasets can be huge, if wireless transfer is impractical consider transporting data on SSDs. Finally, regarding safety management, even if single-person surveying is possible, always maintain surrounding safety checks and communication systems, and use multiple people to support operations in hazardous areas. When introducing new technologies, reconfirm basic safety and quality rules and aim for operations that everyone can use with confidence.

Case study: Introducing a smartphone-linked simple GNSS rover using the LRTK Phone

The recently introduced LRTK Phone is gaining attention as a new-generation simple GNSS rover that links with smartphones and tablets. This thin, lightweight RTK-GNSS receiver attaches to the back of an iPhone and instantly turns the phone into a centimeter-accuracy surveying instrument. Weighing about 150 g and with a thickness of about 1 cm (0.4 in), it is compact enough to fit in a pocket, yet its positioning accuracy rivals that of high-end GNSS equipment: horizontal ±1-2 cm (±0.4-0.8 in) and vertical ±3 cm (±1.2 in). Turn on the power and launch the dedicated app, and initialization completes in tens of seconds, after which high-precision positioning begins. No specialized setup is required, and the ease and immediacy of being able to start surveying as soon as you arrive on site make it a true game changer.

This smartphone surveying system is steadily being adopted in the field. For example, a local government used LRTK Phone in disaster recovery sites. In management of collapsed areas after heavy rain or earthquakes, rapid surveying and safety assurance are critical. Even in situations where conventional methods required setting up a total station and two or more people, LRTK Phone allowed a single person to quickly record survey points in damaged areas. Photos and notes of measured points could be recorded and synced to the cloud in real time via the smartphone, dramatically improving situational awareness and report preparation in emergencies. The local official reported that “we could cover wide areas of damage in a short time, reduce the risk of secondary disasters, and achieve accurate as-built management.” The mobility of simply taking a smartphone and rover from a pocket makes surveying possible even when transporting and setting up conventional equipment would be difficult, which is a major advantage not only in disaster response but also in routine civil works.

What makes LRTK Phone excellent is that it enables one-stop workflows from surveying to data utilization. The dedicated app integrates many functions: single-point coordinate measurement, acquisition of point cloud data by combining with the iPhone’s LiDAR sensor for 3D scanning, instant calculation and display of distances, areas, and volumes between two points, and generation of heat maps showing elevation differences from design data. Measured points can have photos and notes attached, preserving site records like an electronic field book. In AR mode, it can overlay design drawings or BIM/CIM models on site and provide navigation for staking (coordinate layout). Measured survey data are automatically saved to the cloud and can be shared immediately within the company or with partners. With LRTK Phone, surveying planning through as-built data analysis and sharing are all achievable, allowing even small sites or teams with limited manpower to implement full-featured 3D as-built management.

In summary, case studies of the smartphone-linked GNSS rover LRTK Phone show that its ease of use and sufficient accuracy strongly support on-site DX. Intuitive interfaces and all-in-one functionality make surveying simpler and lower the barrier to as-built management. This is truly the realization of “as-built management DX anyone can do.” If your company’s sites face time or labor issues with traditional as-built management methods, consider a solution that combines the latest GNSS rover technology with smartphone apps. Leveraging compact, high-precision devices like the LRTK Phone can maximize the benefits of ICT construction and realize efficient, reliable as-built management. On-site DX starts with surveying—this new norm is rapidly becoming the standard in the construction industry.