Smartphone RTK On-Site Simulation: How AR Is Revolutionizing Clearance Design

In railway and road infrastructure design, the concept of "clearance"—the safe space that must be ensured between vehicles and structures—is indispensable. When a train passes through a tunnel or a station platform, or when a truck travels under an elevated structure, insufficient space can lead to contact accidents or damage. Therefore, a margin of clearance is provided between the vehicle's maximum dimensions (vehicle gauge) and the structure's minimum permitted space (structure gauge) to guarantee safety. This clearance is an important standard to be strictly observed from the design stage in both railway and road projects.

However, the conventional approach has generally been to study this clearance on paper drawings or two-dimensional plans. It is difficult to grasp three-dimensional spaces from flat drawings, and even overlaying different drawings can miss small interferences. Even when design and construction are thought to be faithful to the drawings, problems often arise such as "when the actual item was installed on site, it interferes with another structure and does not fit." Especially on urban sites with limited space, issues that exceed the clearance may be discovered after construction, posing a risk of major rework.

A new approach attracting attention to solve these issues is on-site simulation using a smartphone combined with RTK (real-time kinematic) technology. Smartphones that support high-precision GNSS positioning can obtain centimeter-level position information (cm level accuracy (half-inch accuracy)) on site. By overlaying a clearance model onto the real scene on that smartphone using AR (augmented reality) technology, clearances planned during design can be directly simulated on site. Clearance conditions that were difficult to understand on paper can be intuitively grasped through AR, greatly improving on-site decision accuracy.

Below we explain in detail the basics and importance of clearance, the limitations of conventional methods, and the benefits brought by on-site AR simulation using smartphone × RTK. Finally, we touch on the effects of introducing the latest solution, LRTK, on on-site DX and explore the future outlook for clearance verification.

What Is Clearance? Its Importance in Railway and Road Design

Clearance refers to the safety space that must be ensured so that a moving vehicle does not collide with surrounding structures. In railways, this includes distances to be kept from the inside of tunnels or the edges of station platforms; in roads, it includes heights to be maintained under viaducts or signs. While the vehicle gauge defines the maximum dimensions of the vehicle itself, the structure gauge specifies the minimum space the structure must maintain. The margin set between the vehicle gauge and the structure gauge is the clearance, determined with consideration for dynamic vehicle movements such as sway and tilt.

In the railway sector, laws strictly define separation distances between the structure gauge and the vehicle gauge. For example, in straight sections near platforms, a gap of at least several centimeters (several in) must be secured between the train and the platform. In road law enforcement ordinances, it is also stipulated that obstacles must not be placed in spaces of 4.5 m (14.8 ft) over roadways or 2.5 m (8.2 ft) over sidewalks. These standards prevent moving vehicles from contacting bridge girders, tunnels, overhead lines, and the like, protecting passengers and drivers.

Proper design and maintenance of clearance underpin safe operation. If this space is insufficient, a vehicle could contact a structure when passing, causing a serious accident. Conversely, excessive clearance leads to oversized facility cross-sections and increased construction costs, so it is important to set it within a necessary and sufficient range. For engineers planning railway and road infrastructure, understanding and adhering to clearance is a basic requirement to balance safety and efficiency.

Challenges and Limitations of Conventional Methods

Clearance verification has long relied on work carried out on drawings. Designers draw cross-sections of vehicle and structure gauges and overlay them on paper or CAD screens to check for interference. However, it is not easy to accurately grasp three-dimensional clearances using only planar drawings. When cross-referencing multiple drawings (for railways, civil structure drawings and vehicle drawings; for roads, structural drawings and vehicle trajectory diagrams), there is a risk of small oversights.

In railways, special gauge templates attached to vehicles have also been used to inspect interference with obstacles along tracks, but this method is labor-intensive and limited in where inspections can be performed.

On-site, it is not uncommon that "there seemed to be no problem on the drawings, but after construction a structure intruded into the vehicle’s passage space." For example, the underside height of a newly built viaduct may have been expected to meet the required truck clearance, but site errors or design mistakes could result in insufficient height and reveal a risk of contact by large vehicles after opening. In railways, even if platform or catenary pole positions match the drawings, insufficient consideration of lateral sway during train passage can reveal problematic locations during test runs. In such cases, urgent corrective construction after completion causes rework, leading to schedule delays and cost increases.

Final on-site checks using conventional methods have also relied on manual effort. After construction, measurements are taken with surveying equipment or by visual inspection to confirm compliance with standards, but these checks heavily depend on the judgment and experience of skilled personnel and are not always foolproof. In designs that push limits, measurement errors of a few centimeters (a few in) can determine safety, so visual inspection by humans leaves room for concern. As a result, clearance checks have been conducted with ongoing challenges in on-site responsiveness and verification accuracy.

High-Precision On-Site Positioning with Smartphone × RTK

Recently, technology enabling centimeter-precision positioning using smartphones has become practical. The key is the satellite positioning method known as RTK (Real Time Kinematic). RTK calculates the relative position to a GNSS receiver at a reference point in real time, achieving high accuracy that standalone GPS cannot provide. While RTK positioning once required large surveying instruments, it is now possible to obtain one’s position within an error range of a few centimeters (a few in) by attaching a small receiver to a smartphone or by combining high-precision GNSS chips built into smartphones with correction information. Moreover, using VRS (virtual reference station) methods that utilize nationwide reference station data via the Internet or enhancement signals such as CLAS from the quasi-zenith satellite system Michibiki, high-precision positioning is possible without installing dedicated base stations.

The biggest advantage of smartphone × RTK positioning is a dramatic improvement in on-site positional alignment accuracy. Ordinary smartphone GPS has meter-level errors, which is insufficient to map design positions to the actual site. But with an RTK-capable smartphone, target points based on drawing data can be accurately identified on site. For example, planned column locations or heights on a map can be indicated on site with an error of only a few centimeters (a few in).

This is crucial when verifying clearance. When placing vehicle or structure models into real space, if the user’s smartphone position is highly accurate, discrepancies between the digital model and reality are minimized and reliable simulations can be achieved.

Being able to perform high-precision positioning on a smartphone is also changing fieldwork styles. Portable smartphones can substitute for some surveying instruments, and tasks that formerly required surveyors or multiple people can often be completed by one person. Additionally, RTK coordinates can be obtained in unified geodetic reference frames, making it easy to share on-site data via the cloud and immediately cross-check with office design drawings. Smartphone × RTK serves as a technological foundation that seamlessly links field and design, bringing major transformation to the clearance design process.

AR Display of Clearance Models and Interference Simulation

With high-precision position information from a smartphone, AR (augmented reality) technology can be used to overlay clearance models onto real space. A dedicated app can call up 3D models that indicate vehicle and structure gauges, and viewed through the smartphone screen they appear as if virtual models exist within the actual scene. For example, pointing a smartphone at the trackside might reveal a semi-transparent silhouette of a train’s vehicle gauge cross-section or the outline of a structure gauge.

Consider a specific on-site image. Suppose a new pedestrian overpass is planned at a road construction site. If a smartphone displays an AR model showing a truck’s vehicle height gauge, you can immediately tell whether the overpass secures sufficient height clearance. Similarly, on a railway site you can project a vehicle cross-section model onto the platform and easily verify the positional relationship between the arriving vehicle and the platform. These checks, which used to be verifiable only by sectional drawings, can now be realistically simulated on site through AR.

AR display makes the design-required clearance space immediately obvious on site. If any existing or temporary structure overlaps that silhouette even slightly, it is instantly visible that “this part is interfering.” Unlike following numbers on paper drawings, overlaying the model on the real scene lets you intuitively grasp the margin of clearance. For example, whether a ventilation duct installed inside a tunnel fits within the clearance can be judged at a glance via AR. If CAD data for planned structures such as new viaducts or signal poles are displayed in AR simultaneously, the positional relationship with the vehicle’s dynamic envelope can also be simulated. Additionally, smartphones equipped with LiDAR can scan surrounding structures as point cloud data and quantitatively evaluate clearance dimensions through digital measurement.

AR-based clearance simulation can be used broadly from the design stage through construction. Designers can discover subtle interference risks that drawings did not reveal by conducting on-site AR checks. During construction, completed structures can be verified on the spot to ensure they provide the designed clearance. Processes that previously relied on post-completion test runs or visual inspections can now be performed in real time and with high precision through AR-based verification. This not only improves safety but also speeds up on-site decision-making, providing time to implement corrective measures early if problems are found.

Benefits of On-Site Verification Using Smartphone AR

Combining a smartphone, RTK, and AR for clearance simulation brings various benefits to on-site verification. Preventing unnecessary rework and schedule delays can lead to overall project cost reductions. The main advantages are summarized below.

• Work that can be completed by one person: With just a smartphone, survey and verification tasks that formerly required multiple people can be carried out solo. This reduces personnel coordination and improves field productivity.

• Labor savings and improved efficiency: Instead of carrying paper drawings and a tape measure, needed information is immediately available via AR display, greatly shortening task times. There is no longer a need to repeatedly travel between the site and the office to compare drawings.

• Cloud integration for information sharing: Model data and positioning data used in AR can be managed via the cloud, allowing the latest design information to be reflected on site instantly. Photos of AR screens and verification results captured on-site can be uploaded to the cloud and shared with the office team easily.

• Simplified recordkeeping and reporting: Recording AR-based checks with photos or videos provides visual evidence of where and how much clearance existed. Compared to traditional text- and number-based reports, more intuitive materials can be created, facilitating explanations to clients and stakeholders.

• Reproducibility and objectivity of verification: Because verification is based on digital models and coordinates, the same results can be obtained regardless of who performs the check. This shifts decision-making from reliance on veteran experience to data-backed objective evaluation. If another person performs the same AR check later at the same location, they can confirm the same result, increasing trust in the verification.

• Improved safety: AR checks can be performed without waiting for actual trains or vehicles to pass. They reduce the need for personnel to enter hazardous areas for direct measurement, contributing to worker safety. Detecting and correcting problems early during construction also reduces accident risk during operational use after completion.

In these ways, smartphone AR clearance verification greatly surpasses conventional methods in efficiency, accuracy, and safety. Preventing unnecessary rework and schedule delays can result in project cost savings. It is a highly useful approach for promoting on-site DX (digital transformation).

Benefits of LRTK Implementation for AR Guidance and Clearance Verification

To put the smartphone RTK and AR combination described above into practical use on site, one option is to introduce a solution called LRTK. LRTK is an innovative system that turns smartphones into centimeter-precision positioning terminals, designed so that anyone can easily handle high-precision GNSS with dedicated devices and apps. Its intuitive smartphone app operation makes it easy for field staff—not just specialized survey technicians—to use. As a result, a smartphone becomes a versatile surveying tool capable of coordinate-linked photography, point cloud data acquisition, AR navigation to target points (coordinate guidance), and the AR display of clearance models discussed in this article, all with a single device.

Introducing LRTK on site can be expected to produce the following effects in the clearance verification process. First, because high-precision positioning and AR display are seamlessly linked, work time from surveying to verification is significantly reduced. Tasks that previously required separate equipment and procedures—positioning, measurement, and recordkeeping—can be completed with a single smartphone, achieving labor savings and speed improvements. This contributes greatly to overall site productivity. LRTK also supports the Michibiki system’s high-precision augmentation signals (CLAS), maintaining stable accuracy even in mountainous or communication-limited sites, enabling reliable clearance checks in any environment.

Furthermore, the DX ripple effects brought by LRTK should not be overlooked. By integrating with cloud platforms for data management, on-site information can be shared instantly within and outside the organization, allowing all stakeholders to discuss based on the same models and coordinates. Designers, contractors, and managers can collectively grasp situations in real time and discuss safety measures or design adjustments on site. Clearance verification, a once-niche task, can be digitized and standardized through LRTK implementation and stored within the organization as a reproducible workflow. This digitalization aligns with infrastructure DX strategies such as the Ministry of Land, Infrastructure, Transport and Tourism’s i-Construction initiative. The utilization of high-precision positioning and AR technologies is expected to be a key lever for improving productivity and strengthening safety management across the industry, and their adoption is likely to accelerate.

Ensuring clearance is always a critical issue in infrastructure safety management, and the new combination of smartphone RTK × AR × LRTK is fundamentally transforming how this work is done. From an era of relying on paper drawings to one of projecting digital models into real space for verification—this change reduces the burden on field engineers and significantly enhances project efficiency and safety. AR utilization in clearance design is expected to spread further, and solutions like LRTK will become indispensable tools for the future of railway and road infrastructure.

Smartphone RTK and AR clearance simulation may well become the new standard at infrastructure sites.

If you feel limited by conventional verification methods, consider introducing this technology on site to experience dramatic improvements in safety and productivity.