Achieved with LRTK’s AR guidance! Easy, high-precision rail misalignment measurement by a single person

Rail misalignment measurement (measuring track deviations) is an indispensable task for railway safety and efficient maintenance. The stability of running trains and passenger comfort are greatly affected by how well the straightness and flatness of the rails are maintained. However, traditionally, accurately measuring rail deformation required a lot of effort and skill. This article reviews the challenges of conventional deformation measurement methods and introduces an innovative measurement approach called LRTK, which combines a smartphone, RTK-GNSS, and AR technology. It explains that an era has arrived in which a single person can easily and highly accurately measure rail alignment and deformation, detailing the concrete advantages and future prospects. In the context of the recently emphasized “smart maintenance” (improving maintenance efficiency through digital technology), one-person high-precision measurement with LRTK holds great potential.

The importance of measuring rail deformation

Railway tracks (the track structure) are subjected to continuous loading as many trains run 24 hours a day, 365 days a year. As a result, over the years the roadbed and sleepers settle and wear, causing subtle deformations (track displacements) in the rails. So-called “track irregularities” can, when large, cause lateral sway or shocks to trains, reducing passenger comfort and, in the worst cases, posing risks of serious accidents such as derailment. If deformations are left unaddressed, they can cause uneven wear on wheels and rails, shortening equipment life and increasing maintenance costs. Therefore, railway companies are required to regularly measure and monitor track conditions and, if deformations exceed specified thresholds, to promptly carry out repairs (such as tamping the ballast or adjusting rail fastenings).

Rail deformations take various forms. Representative types include vertical irregularity, which indicates vertical undulations of the rail known as “profile irregularity,” and lateral alignment deviations, known as “alignment irregularity,” which indicate lateral shifts of the rail. Other categories include “level difference irregularity” (the height difference between left and right rails, i.e., cant defects), “gauge irregularity” (difference in rail spacing), and “planarity irregularity” (overall twisting of the track). Among these, vertical and alignment irregularities that directly affect running safety are particularly important indicators and serve as key criteria for quantifying how straight and smooth the rails are.

Each of these track irregularities has defined management thresholds; for example, on conventional lines vertical irregularity exceeding a certain value triggers maintenance action. High-speed lines such as the Shinkansen have even stricter standards, where deviations on the order of millimeters cannot be overlooked. For that reason, accurate measurement and early correction of deformations are crucial for safe railway operation.

Conventional deformation measurement methods and their challenges

Rail deformation measurement has long been performed through manual work or dedicated vehicles. Let’s look at representative conventional methods and their challenges.

• Manual measurement with string line: Field maintenance workers stretch a thin string (wire) of about 10 m (32.8 ft) tautly over or alongside the rail and measure the gap to the rail. For vertical irregularity, a string is stretched over the railhead and the vertical distance from the string to the rail at the center is measured. For alignment irregularity, a string is run along the rail side and the horizontal distance at the center is measured. Although simple, getting a long string perfectly straight requires skill, and measurements are difficult in strong winds—this method demands craftsmanship. Because measurements are taken manually every 10 m (32.8 ft), measuring long sections requires enormous effort and time, and measurement accuracy tends to depend on the operator’s experience. At least two people are required for the task, creating issues of labor cost and the burden of working in extreme heat or cold.

• Visual inspection: During routine patrols, maintenance crews may visually inspect track deformation by walking along the line. Experienced workers can detect slight irregularities, but judgments are largely subjective and do not yield quantitative records. Visual inspections are often performed frequently when rail temperatures rise dramatically in heat, but staring at rails under direct sunlight for long periods is grueling and carries risks of human error. Reliance on the intuition and experience of veterans makes the method subjective and limits precise management.

• Track inspection cars: On large networks such as JR, special track inspection vehicles equipped with laser measuring devices and gyro sensors periodically run to measure track irregularities at high speed. This allows efficient assessment of extensive track conditions, but introducing and maintaining such dedicated vehicles is very costly. Even famous inspection trains that run on the Shinkansen have limited schedules, so monitoring track deformation in real time at all times is difficult. Local private railways and small lines often lack inspection vehicles and frequently outsource measurements, making them unsuitable for routine fine-grained deformation measurement.

• Other portable measuring devices: Small measuring devices such as hand-pushed inspection trolleys or laser distance meters attached to the rail have been developed. These are designed to be operable by one person but often measure a limited set of items or require preparation and setup, limiting their wide use in daily inspections.

As described above, conventional methods had the problems of “taking time,” “requiring manpower,” and “being limited in accuracy and frequency.” Methods range from qualitative visual checks to expensive inspection trains, but none are convenient for frequent use. As a result, there has been increasing demand in the field for “a way to measure rail deformation more easily and more accurately.”

How smartphone × RTK × AR change rail deformation measurement

A new approach has emerged recently to solve these problems. That is LRTK, which combines a smartphone, high-precision GNSS (RTK positioning), and AR (augmented reality) technology for deformation measurement. LRTK consists of a compact RTK-GNSS receiver attachable to a smartphone and a dedicated app, offering centimeter-class position measurement, 3D scanning, and AR display in a single device. Because the LRTK receiver can obtain high-precision correction information over the internet (such as satellite CLAS signals or electronic reference station networks), it also offers the convenience of stable RTK positioning without requiring a dedicated base station.

Using LRTK, measurements that traditionally required a veteran team can be performed by a single person. Specifically, a worker walking along the track with a smartphone and LRTK device in hand can digitize the track’s deformation. RTK-GNSS continually provides the smartphone’s position with centimeter-level accuracy, enabling precise recording of height and lateral displacement at each measurement point. For example, if the measurement pitch is preset to “measure every 10 m (32.8 ft),” the app will automatically guide the user to the next measurement point with AR navigation. By following arrows and markers on the screen, one person can easily perform evenly spaced measurements, which previously required tape measures and markers.

The LRTK system also captures the site with the smartphone camera and LiDAR sensor and performs on-site data analysis. Once measurements are complete, deformation amounts for that section (for example, vertical and alignment irregularity every 10 m (32.8 ft)) are immediately calculated and displayed on the smartphone. This enables real-time understanding at the site of “how much deformation exists at which points,” allowing immediate temporary repairs or decisions on speed restrictions if necessary.

A notable feature is LRTK’s support for AR display. Not only numeric results but also virtual reference lines and color-coded deformation maps based on data can be overlaid on the actual track. For example, when holding up the smartphone, the difference between an ideal straight reference line and the actual rail position can be shown as colored bands, allowing you to discover deformed areas at a glance. Deformation assessment that previously required reviewing numeric tables or paper records can now be intuitively confirmed by overlaying data on live camera views, enabling even non-experts to detect abnormalities easily.

Visualizing deformation with point cloud scanning and high-precision AR

LRTK does not rely solely on single-point positioning: combined with a smartphone’s LiDAR point cloud scanning, it can digitize rails and surrounding equipment as complete 3D data. Recent high-end smartphones include short-range LiDAR sensors that can capture the shapes of rails, sleepers, and the roadbed while walking, producing detailed point clouds. Normally, standalone smartphone LiDAR scans can distort over long walks due to self-positioning errors, but with LRTK’s RTK positioning, self-position is constantly corrected so that point cloud data does not suffer positional drift or distortion. In other words, even when a long section is continuously scanned by a single person, all point cloud points are accurately recorded with absolute coordinates.

Acquired point cloud data can be used immediately on site as well as later in the office. On the LRTK app, point clouds can be overlaid with design data and displayed as color-coded heat maps to visually confirm deviations from design lines. For example, if you display rail deformation as green within ±○ mm (±○ in) and red for exceedances, it becomes immediately clear which locations exceed allowable limits. Numeric values can also be tagged at each point for quantitative evaluation needed for later repair planning.

AR display transforms work that previously required comparing drawings or screens with the actual site. By overlaying data directly onto the field of view, it becomes intuitive to grasp “how far it is off” and “which direction to correct.” Even for night work or low-visibility situations, AR guidance enables precise alignment, reducing operational mistakes. Moreover, once point cloud data is acquired, you can later measure rail spacing, cant (left-right height difference), and even positional relationships with overhead lines and signal equipment—allowing free measurement and analysis after the fact. If you realize at the site that you “forgot to measure something,” you can extract cross-sections or measure distances from the point cloud, reducing the need to remeasure.

Immediate sharing via cloud integration and integration into maintenance workflows

Deformation measurement data acquired with LRTK can be linked directly to the cloud for immediate sharing. Traditionally, field measurements involved writing notes in a notebook or transcribing into Excel, making real-time sharing difficult. LRTK allows numerical data, point clouds, and geotagged photos captured on the smartphone to be uploaded directly to the cloud from the site, enabling near-real-time information sharing with office personnel and other team members.

For example, if a section’s rail deformation is measured and uploaded to the cloud, the results can appear on a management PC within minutes, and highly urgent deformation points can trigger instant notifications to relevant personnel. This reduces communication loss between the field and the office and significantly shortens the lead time from anomaly detection to countermeasure planning. Accumulated data in the cloud also facilitates analysis of long-term changes and centralized management of maintenance histories. Comparing past and current measurements helps identify deformation progression trends and verify repair effectiveness, supporting PDCA cycles. The accumulated data can serve as a digital twin of track conditions, forming a foundation for integrated understanding of track status over time.

Cloud integration also enables LRTK-acquired data to be integrated with other systems. For instance, importing measurement results into a railway company’s maintenance management system can automatically link to repair planning and material ordering workflows. Measurement values taken by inspectors in the field become immediate digital records, enabling alert issuance or automatic creation of work orders if necessary, advancing DX (digital transformation) of the entire maintenance workflow. Smooth field–cloud integration reduces human error and strengthens information transmission, raising organizational safety management standards.

Prospects and use cases brought by LRTK adoption

LRTK’s smartphone + RTK + AR approach to rail deformation measurement has the potential to significantly impact railway infrastructure maintenance and could become the standard style going forward. Some railway operators have already begun pilot introductions and reported halving work time compared to conventional methods. In one trial, a railway company used LRTK to measure track deformation and surrounding equipment positions; not only was work time greatly shortened, but point cloud data and geotagged photos managed in the cloud also streamlined repair planning. Detailed deformations that were previously difficult to digitize are now recorded without being missed, strengthening preventive maintenance efforts.

Future use cases include not only integration into routine inspections but also scenarios where rapid measurement is required, such as emergency inspections immediately after disasters or post–night-work verification. For example, after an earthquake or extreme heat that raises concerns about track conditions, LRTK can quickly check wide-area deformation and support decisions on whether to resume operations. LRTK’s point cloud scanning and AR comparison also enable immediate on-site verification that newly installed tracks or turnouts were constructed according to design. Millimeter-level offsets that might be missed by visual inspections are now supported by data, enhancing quality control.

Efficiency gains in measurement work also directly reduce field workers’ burdens. Shorter track closure times and reduced nighttime work contribute to both safety improvements and cost reductions.

Beyond rails, LRTK can be applied to position measurement of roadside equipment and displacement monitoring. Regularly measuring positions of catenary poles or signals with LRTK can detect issues caused by ground subsidence or structural displacement early. Consolidating measurement tasks that were previously handled separately by discipline into one common platform—LRTK—can dramatically improve overall equipment management efficiency and accuracy.

Summarizing the benefits that LRTK measurement brings:

• Labor and personnel reduction: Measurements can be performed by a single person, greatly reducing personnel arrangements and workload

• High accuracy: RTK-GNSS enables tracking of track irregularities at millimeter-scale precision that was previously difficult to achieve

• Real-time visualization: Measurement results can be AR-displayed on site, allowing intuitive identification of anomalies on the spot

• Data sharing and accumulation: Measurement data can be shared and accumulated in the cloud immediately, supporting maintenance planning and long-term analysis

• Versatility: Applicable to a wide range of field tasks beyond rails, such as structure measurement and as-built inspection

Finally, the barrier to adopting LRTK is not high. The dedicated equipment is a small add-on device for a smartphone, the unit itself weighs only a few hundred grams and operation is simple—just follow the app’s on-screen instructions. Even crews new to RTK positioning can handle it after a short training period, making LRTK a reliable ally for maintenance sites facing technician shortages and generational turnover, enabling anyone to measure accurately. The Ministry of Land, Infrastructure, Transport and Tourism is also promoting three-dimensional measurement and digital technology use for infrastructure inspections, and LRTK aligns with such policy directions. LRTK is poised to change the conventional wisdom of rail deformation measurement. Let’s bring cutting-edge technology to the field and build the next-generation maintenance system.

If your site has felt limitations with conventional methods, why not try this new measurement approach at least once?