Conforming to MLIT Standards for Peace of Mind! Centimeter-level (cm level accuracy (half-inch accuracy)) Smartphone Surveying LRTK Enables Single-Person Public Surveying

In civil engineering surveying, quality assurance hinges on inspecting as-built conditions to verify that structures have been built according to design. However, conventional as-built inspections and surveying tasks have required considerable labor and personnel, and posed the problem of being able to evaluate quality only at a limited number of measurement points. With a shortage of experienced survey technicians and the need to handle hazardous work areas, labor savings and accuracy improvement have been long-standing themes on site.

As a trump card to solve these issues, the Ministry of Land, Infrastructure, Transport and Tourism (MLIT) has been promoting smart construction using ICT such as *i-Construction* since 2016. Recently, technologies such as drone surveying and 3D laser scanners have been increasingly adopted, and by 2025 the “Guidelines for As-Built Management Using 3D Measurement Technologies (draft)” are scheduled to be revised, accelerating the incorporation of digital technologies into public surveying. Among these, a new technology attracting attention is LRTK, which combines a smartphone with GNSS to enable single-person centimeter-level (cm level accuracy (half-inch accuracy)) surveying. This article explains in detail LRTK’s technical advantages, the reassurance of MLIT compliance, concrete use cases, and future prospects.

Current State and Challenges of Civil Surveying: Limits of Conventional Methods and Expectations for ICT

Traditional civil surveying has been manpower-centered, using levels and total stations. For as-built management, crucial heights and widths are measured using measuring tapes and staffs to verify conformity with design specifications. However, this method limits the number of measurable points, making it difficult to grasp the overall shape of embankments or slopes. For example, when checking embankment gradients, one could only measure a few heights and judge roughly whether they match the design, risking overlooking small irregularities or partial excesses/deficits.

Survey work itself also requires significant manpower and time. Total station surveying typically requires two or more workers (operator and prism staff), and surveying large areas involves repeatedly relocating the instrument. In urgent situations—such as ground checks after heavy rain or night inspections after work—preparing heavy equipment and securing multiple personnel become bottlenecks. In addition, surveying on steep slopes or in deep excavations can be hazardous, and measurements are sometimes abandoned to ensure safety. Given these limitations, a method capable of measuring entire sites efficiently, safely, and with high precision using fewer personnel was needed.

Against this backdrop, MLIT’s ICT-driven construction strongly promotes the use of 3D technologies in surveying and as-built management. 3D laser scanners and UAV (drone) photogrammetry enable planar measurement of sites and new inspection methods—such as displaying differences from design data using color maps—are being put into practical use. Even with limited personnel, leveraging digital technologies to ensure quality and dramatically improve construction management efficiency and accuracy is expected. In this trend, the smartphone surveying system LRTK has emerged as an innovative solution that overturns conventional civil surveying assumptions.

What is LRTK: Centimeter-level (cm level accuracy (half-inch accuracy)) Positioning and Point Cloud Measurement with a Smartphone

LRTK (pronounced 'el-arr-tee-kay') is an innovative high-precision positioning system that turns a smartphone into surveying equipment. Developed by Reflexia Inc., a startup spun out of Tokyo Institute of Technology, it is a pocket-sized device that, when a compact RTK-GNSS receiver is attached to an iPhone or iPad, enables centimeter-level (cm level accuracy (half-inch accuracy)) positioning, 3D point cloud measurement, and even AR-based as-built inspection with a single unit. While a smartphone’s built-in GPS can have errors on the order of meters, LRTK achieves errors of only a few centimeters or less through real-time kinematic (RTK) corrections. Precision positioning that previously required expensive dedicated GNSS equipment and base stations can now be performed easily by anyone with a smartphone and a small device.

The LRTK device itself features a slim, lightweight design weighing approximately 125 g and measuring approximately 13 mm (0.51 in) thick, and it contains a built-in battery (approximately 6 hours of operation). It can be attached to a dedicated smartphone case with one touch, eliminating complicated wiring or mounting. If necessary, it can be mounted on the included monopod, allowing the smartphone and receiver to be separated and used like a stationary GNSS surveying instrument. Despite its small size, performance is full-scale: standalone positioning (instantaneous measurement) yields errors of about 1–2 cm, and by averaging data for several tens of seconds, planar accuracy can be improved to under 1 cm (under 0.4 in). This level of accuracy is beyond what conventional optical surveying or standalone GNSS could achieve and fully meets the standards required for public surveying.

By launching the dedicated app on the smartphone, the latitude, longitude, and height of measured points are displayed in real time. Measurement results can be automatically converted to any Japanese plane rectangular coordinate system zone, and geoid height (elevation) corrections are applied instantly, so global positioning coordinate system data that can be used directly as public surveying deliverables are obtained on-site. The app also has a photo function for each measured point; photos are tagged with high-precision coordinates and stored in the cloud. Furthermore, acquired data can be uploaded to the cloud from the smartphone with one touch, enabling immediate sharing and verification from office PCs and comparison/analysis with design data. The ease of completing all of this on-site by a single person is perhaps LRTK’s greatest attraction.

MLIT Compliance for Peace of Mind: Conformance with Public Surveying and As-Built Management Guidelines

One reason smartphone surveying with LRTK is attracting attention is its high-precision measurements that conform to MLIT standards. In the 2020s, MLIT has promoted the 3D-ization of as-built management guidelines and inspection standards, indicating procedures for grasping as-built conditions in plan using RTK-GNSS, laser scanners, and photogrammetry. In FY2025 (Reiwa 7), supervisory and inspection guidelines (draft) for as-built management using 3D measurement technologies will be revised across multiple work types such as earthworks, paving, and slopes, considerably expanding the scope for digital measurement use. As a result, advanced quality inspections—such as automatically analyzing acquired 3D point cloud data by overlaying it with design models and visualizing finishing errors with heat maps—have become possible.

Under these new standards, tools capable of performing high-precision positioning on-site and planar, wide-area measurement are required. LRTK precisely meets these requirements. RTK-GNSS positioning accuracy is within a few centimeters, and measured point coordinates can be directly aligned with the design coordinate system. This allows detection of deviations from the client’s specified tolerances down to millimeter levels and on-site confirmation. Because it supports wide-area point cloud acquisition using smartphone-mounted LiDAR, planar as-built understanding that was previously difficult is now achievable by a single person. If acquired data are reconciled with design data in the cloud, nonconforming areas can be identified immediately, significantly reducing inspection effort.

Moreover, LRTK supports correction signals provided by Japan’s Continuously Operating Reference Station network and the Quasi-Zenith Satellite System “Michibiki” (CLAS). Even at mountain sites or remote islands outside cellular coverage, receiving CLAS signals from Michibiki enables centimeter-level accuracy without internet access, allowing stable public surveying anywhere. Previously, surveying deep in the mountains required setting up a base station or pre-measuring known points for coordinate transformation, but with LRTK, correction information from satellites alone allows immediate acquisition of global positioning coordinates. Because it enables surveying in the coordinate system standard used by the Geospatial Information Authority of Japan, LRTK can be introduced into public surveying with confidence. Its high affinity with MLIT’s i-Construction initiatives makes it a powerful solution for on-site DX (digital transformation).

Technical Features and Advantages of LRTK

Centimeter-level (cm level accuracy (half-inch accuracy)) smartphone surveying enabled by LRTK offers many advantages over conventional technologies. Below are the main technical points and the resulting benefits.

• Centimeter-level GNSS positioning accuracy: LRTK receives signals from multiple satellites—GPS, GLONASS, Galileo, and Michibiki (QZSS)—and corrects positioning errors in real time using the RTK method. Even without a dedicated base station, network RTK (VRS method) or Michibiki’s CLAS signal achieves positioning accuracy of a few centimeters. High accuracy applies not only horizontally but also vertically, making height management, which was previously difficult, easy. This accuracy is sufficient for public surveying and as-built management and can be used directly for stakeout and control point establishment.

• Extremely compact and lightweight for portability: Conventional GNSS surveying instruments and 3D scanners are large and weigh several kilograms, making transport and setup cumbersome. In contrast, the LRTK device weighs approximately 125 g and is ultra-light; together with a smartphone it fits in one hand. With a thickness of about 1 cm (0.4 in), it can be carried in a pocket and quickly brought out to start surveying when needed. It has a built-in battery for long operation and supports USB charging, allowing continued use on site with spare batteries. No cable connections or complicated initial setup are necessary, so equipment preparation does not consume time. This high portability enables surveying anytime, anywhere, including narrow or remote locations that were difficult to measure with conventional equipment.

• Easy operation through smartphone integration: Because LRTK works integrated with a smartphone, operation is intuitive. The dedicated app displays current coordinate values and accuracy indicators in real time, making it easy even for beginners to grasp positioning status. Recording measured points is a one-tap operation, and names or codes can be assigned on the spot for management. Measurement data is automatically synchronized with the cloud, eliminating the need for manual note-taking or later PC transcription. The cumbersome control panels and specialist knowledge often associated with conventional surveying instruments are unnecessary; technicians familiar with smartphones can master it in a short training period. This lowers the barrier to surveying, enabling all on-site staff to participate in high-precision measurements.

• 3D point cloud scanning and AR visualization: Another strength of LRTK is 3D measurement and AR using smartphone sensors. Using the LiDAR scanner on iPhone or iPad Pro, site shapes can be captured as point cloud data and tagged with high-precision positions. For example, walking while scanning the ground surface produces dense planar terrain data in a short time. Acquired point clouds can be overlaid with 3D design models in the cloud and compared, allowing intuitive inspection of embankments and pavement finishing using color-coded displays. With AR features, design data can be overlaid on actual structures on the smartphone screen, enabling on-the-spot visual checks of differences between construction results and design. Slight deviations that paper drawings or tape measures could not detect become apparent at a glance with AR. Thus, LRTK goes beyond mere surveying to allow real-time verification and decision-making on site.

• Cloud utilization and data sharing: Positioning, point cloud, and photo data acquired with LRTK are automatically uploaded and stored in the cloud. There is no need to transfer data via USB after returning to the office; remote supervisors or clients can share information in real time from the field. In the cloud, accumulated data can be managed as lists, reconciled with design drawings or BIM/CIM models, and analyzed with tools such as volume calculations. Analysis results are synchronized back to the smartphone; for example, a heat map of as-built conditions generated from point cloud data can be displayed on the phone and overlaid on the real scene via AR. This allows the entire process from surveying to inspection reporting to be completed digitally, dramatically improving work efficiency and data reliability.

• Single-person surveying and improved safety: LRTK’s defining advantage is that surveying can be completed by a single person. GNSS surveying and as-built inspections that traditionally required two-person teams can be performed solo with LRTK, from positioning to data processing. This not only addresses manpower shortages and enables significant labor savings but also enhances safety through non-contact measurements in hazardous areas. On steep slopes or deep excavations, operators can measure from a safe location using a pole, or perform remote point cloud scans without entering danger zones. The need for personnel in areas where heavy equipment is operating is reduced, contributing to overall site risk mitigation. Realizing single-person surveying not only decreases manpower but also produces qualitative benefits—measuring places that people could not reach and digitizing areas where people cannot enter—greatly expanding the possibilities of civil surveying.

On-Site LRTK Use Cases: Applications Across Various Public Surveying Tasks

High-precision smartphone surveying with LRTK proves powerful across a wide range of civil construction fields. Below are representative use cases where tasks that were formerly time-consuming or hazardous can now be performed easily by one person.

• Earthworks: In road and development excavations and embankment works, verifying the as-built ground surface is a key quality control point. With LRTK, the finished ground surface can be thoroughly 3D-scanned, and arbitrary cross-sections can be extracted later to check whether widths and heights meet design specifications. Embankment slope gradients and flatness can be exhaustively verified by analyzing the obtained surface data. Because measurements become surfaces instead of points, subtle irregularities are not overlooked, improving as-built management accuracy. Point cloud data can also be used directly to calculate embankment/excavation volumes, enabling efficient earth quantity management simultaneously with as-built measurement.

• Paving: Road paving projects typically require much time to inspect surface flatness and pavement thickness. Scanning the freshly finished pavement with LRTK produces high-density point clouds containing millions of points, from which elevation differences relative to design can be automatically calculated, allowing surface irregularities and areas with insufficient thickness to be identified at a glance. Based on heat map results, defective areas can be remedied immediately and rescanned to verify post-repair conditions, completing the PDCA cycle within the day. Digitalizing inspections that previously relied on craftsmen’s intuition or straightedges dramatically improves the reliability of pavement quality.

• Exterior works: In exterior works around buildings—such as parking lots, sidewalks, and park development—LRTK’s flexible measurement capability is advantageous. Heights and positions of widely distributed inlets and curbs can be measured quickly by one person, checking on-site whether required design slopes are achieved. Even on complex terrain, repeated scanning of finished surfaces allows checking flatness and drainage gradients to prevent construction defects like subsequent puddling. Recording finished ground elevation and structure placements as 3D data provides valuable deliverables for handover documentation and maintenance records.

• Buried utilities: For buried sewer pipes and cable conduits, recording pipe positions and terrain before and after burial is important. With LRTK, the slope and depth of conduits in open trenches can be easily measured non-contact. Even in deep trenches, inserting a GNSS receiver mounted on a monopod allows measurements of the bottom without personnel descending. After burial, only the ground surface is visible, but high-precision 3D data of underground pipe positions can be retained for future excavations or repairs. As-built management of buried facilities, which was previously difficult, can thus be carried out safely and reliably with LRTK.

LRTK’s non-contact and remote surveying not only improves quality management as described above but also greatly contributes to worker safety. Reducing surveying work on steep slopes or busy roads lowers accident risk while ensuring reliable data acquisition. Even in confined sites or night works, LRTK allows a single operator to move agilely and measure without disturbing the surroundings. In this way, various public construction surveying tasks are evolving into smart and safe processes with LRTK.

Future Outlook: The Future of Civil Surveying Paved by Smartphone Surveying

Centimeter-level (cm level accuracy (half-inch accuracy)) smartphone surveying enabled by LRTK is poised to set a new standard in civil surveying. As surveying equipment continues to be miniaturized and advanced, it will become commonplace to manage and verify many on-site tasks based on digital data. High-precision site data accumulated through single-person surveying will be used not only as post-construction quality records but also for future maintenance and infrastructure management planning. For example, AI could analyze accumulated cloud-based point cloud data to automatically detect signs of construction defects or deterioration, bringing next-generation construction management into view.

Within MLIT’s drive toward labor-saving and automation on construction sites (i-Construction 2.0), smartphone-integrated surveying solutions like LRTK will be indispensable. Making infrastructure conditions visible down to the millimeter with devices anyone can operate directly boosts productivity across the industry. Furthermore, in the future, integration with AR glasses and robots may enable fully hands-free, automated systems from surveying to inspection. Toward such a future, it is important first to establish smartphone surveying as the new norm on-site and to realize safe and reliable construction management through the synergy of people and ICT.

Centimeter-level (cm level accuracy (half-inch accuracy)) smartphone surveying LRTK breaks the constraints of traditional civil surveying and is an innovative tool that dramatically improves quality control and work efficiency. With MLIT-compliant accuracy and ease of use, engineers, contractors, and municipal staff can all benefit. The era of single-person surveying driven by LRTK is now well underway, and the future in which smartphone surveying accelerates on-site DX to realize safe, secure, and highly productive public infrastructure development is within sight.