Tying Kilometer Positions to Absolute Coordinates with LRTK – High-Precision Positioning That Changes the Field

In infrastructure management, "kilometric position (chainage)" is an important keyword that links the field to data. When indicating locations along railway lines or roads, kilometer markings such as ◯◯ km + ◯◯ m (◯◯ ft) have long served as a common reference for both field staff and ledger data. However, as digital technology advances, the traditional management methods that rely solely on chainage are showing their limitations. This article explains the benefits and methods of linking chainage data to absolute coordinates using high-precision GNSS RTK positioning technology. By leveraging a compact RTK solution called LRTK, we will take a detailed look at how site surveying through asset management can be transformed, with concrete use cases.

Why is chainage management important? The interface between field operations and data assets

For long linear infrastructure such as railways and highways, "which location" is extremely important. In the field, when reporting accidents or inspection points, people share locations using expressions like "at km and m from the origin of line X" (e.g., ◯◯ km ◯◯ m). For example, railways install kilometer posts (distance markers) on catenary poles and signals, and roads have periodic distance markers; these serve as physical reference points on site. Field workers measure distances with tape measures based on kilometer posts to pinpoint the target equipment or defect locations. In this way, chainage has traditionally been used as the interface that connects spatial awareness in the field with ledger-based data management.

Chainage is also treated as basic information in management systems and ledger data. Route maps and wiring diagrams include chainage scales, and equipment ledgers list entries like "there is X at Y kilometers and Z meters." For engineers on the client side, understanding location by chainage is part of daily work. Construction plans and maintenance plans also use expressions like "bridge repairs near X km," and thus site and documents are linked by the common language of chainage. Good chainage management makes it possible to accumulate field events as data assets and utilize them cross-sectionally.

Is chainage alone insufficient? Conventional challenges and limits

Chainage is a convenient indicator, but relying on it alone has several noted problems. First is the issue of positional ambiguity. Chainage is merely a distance from an origin and does not indicate an absolute geographic position. For example, at switches or in double-track sections, the same chainage may refer to different tracks. In areas where tracks are interwoven, such as within stations, managing equipment by chainage alone can be insufficient to clearly indicate "which track at which location." Misunderstandings in the field can lead to the risk of working at the wrong location.

Second is vulnerability to alignment changes. When route realignments or extensions occur, discontinuities or shifts in the conventional chainage system can arise. At junctions between main and branch lines, chainage may be reset, and historical data may become unmatchable to new chainage. For example, if a level crossing that once existed is elevated and the track location changes, searching for the site based on old chainage will not match the actual position. From the perspective of long-term data utilization, records that rely only on chainage may be unsuitable for future spatial analysis or integration with other systems.

A third problem is that chainage-only information makes linking with maps and other spatial data cumbersome. For instance, displaying equipment or inspection points on a GIS typically requires coordinates such as latitude and longitude. Even if only chainage is available, linear referencing can plot locations on a map based on route distances, but that requires detailed route geometry data and distance-to-coordinate tables. Preparing and maintaining these is labor-intensive, and any discrepancy will map points incorrectly. In disaster situations when sharing location information with other organizations, expressions like "at km X of line Y" cannot be immediately located on a map, potentially delaying initial response. As described above, management relying solely on chainage has limits and is becoming increasingly ill-suited to modern, sophisticated information use.

Benefits of absolute coordinates: uniquely identifying locations anywhere

The key to solving these issues is absolute coordinateization. By assigning longitude and latitude (or plane rectangular coordinates) to each point, locations can be uniquely identified anywhere on Earth. Absolute coordinates are a globally shared "address of location" and are not bound to route-specific reference systems like chainage. Therefore, cross-analysis and sharing between different routes and other geographic data become easy.

Concrete benefits of introducing absolute coordinates include:

• Clarification of location identification: Managing locations by coordinates prevents confusion even in areas where multiple tracks run parallel or in complex track layouts. If equipment or damage points have coordinates attached, they can be pinpointed on a map and easily reproduced in the field with GPS devices.

• Ease of data integration: Standardizing on coordinates greatly simplifies data linkage with other systems such as GIS and CIM/BIM. For example, if inspection results are recorded with coordinates, they can later be overlaid on digital maps to analyze changes over time, or combined with other spatial data (terrain, weather, etc.) for visualization.

• Efficient information sharing and communication: With absolute coordinates, not only railway companies or road managers but also municipalities, disaster-response agencies, and contractors can handle location information on a common basis. In disaster response, one can indicate affected areas on a map by latitude/longitude and coordinate smoothly from the initial response. Locations can be shared without requiring internal knowledge of chainage, reducing explanation overhead.

• Asset value for the future: Coordinate data serve as an unchanging standard over the long term. Even if chainage systems change due to route improvements, past coordinate records allow cross-referencing between old and new positions. Furthermore, if new systems or technologies emerge in the future (e.g., autonomous vehicles or advanced simulations), having basic spatial coordinates enables flexible adaptation.

Thus, linking chainage to absolute coordinates clarifies location management and expands the scope of information utilization. Next, let’s look at the technical mechanisms for obtaining high-precision absolute coordinates.

Mechanism of coordinate acquisition and position correction using RTK-GNSS

RTK-GNSS (real-time kinematic positioning), which has become widely available in recent years, is key to easily obtaining absolute coordinates in the field. Standalone GPS positioning typically has errors on the order of several meters, but the RTK method uses two GNSS receivers—a base station and a rover—and cancels error factors by using the difference between the satellite signals received by both, achieving centimeter-level accuracy (the base station is set at a point with a known accurate coordinate, and the rover is placed at the field point to be measured. The base station sends correction information calculated from its observations to the rover via radio or the Internet, and the rover applies the correction to its own position results to obtain high-precision coordinates). This relative positioning mechanism reduces errors that used to be several m (several ft) down to several cm (several in), enabling coordinate acquisition with the accuracy required for infrastructure management.

To use RTK positioning, the known coordinates of the base station must be aligned with the national geodetic datum. In Japan, it is common to obtain known-point information from the GEONET network of continuously operating reference stations or from commercial correction services, and to receive correction data via network RTK (Ntrip). Additionally, recently the Quasi-Zenith Satellite System "Michibiki" provides the CLAS (Centimeter-Level Augmentation Service), which allows nearly RTK-equivalent accuracy with a single receiver even when outside communication coverage.

For example, LRTK receivers support both network corrections via Ntrip and direct corrections from Michibiki, enabling optimal high-precision positioning in mountainous or underground sections as appropriate. By combining such compact RTK-GNSS devices with smartphone apps, high-precision positioning that used to require specialized surveying equipment and personnel becomes extremely easy. Positioning results can be displayed in real time on a smartphone map or CAD drawing, allowing immediate confirmation of coordinate values on site. In other words, it is now an era in which anyone can carry a high-precision positioning device to measure any point in the field instantly and obtain absolute coordinates.

How LRTK changes the management style to "measure in the field and share immediately"

What does this new field management style using RTK-GNSS look like? One example is the workflow of using LRTK to "measure in the field and share immediately." Traditionally, to reflect field-surveyed data into ledgers and drawings, one had to bring the data back to the office for processing and input. But with an LRTK system, measurements can be uploaded to the cloud and shared with stakeholders in real time.

For example, coordinate lists, photos, and point cloud data obtained by an LRTK app are automatically saved on the smartphone and can be sent to a dedicated cloud web service with a single tap. Since data measured in the field are shared immediately, by the time workers return to the office, supervisors, colleagues, or clients can already review the results. The cloud map will plot measured points and captured images, and stakeholders can view them via a browser without specialized software. This is a true example of seamless connection between field and office, allowing decisions and subsequent work planning to be based on measurements taken immediately.

This immediate sharing reduces the double handling of field records. Tasks such as noting chainage in a paper field book and digitizing later become unnecessary. Since measured points' absolute coordinates are registered directly in the ledger database, human error is reduced. Photos and point clouds also carry location information, so for example it becomes easy to identify the exact location on a map while viewing 360-degree inspection images in the office. If LRTK makes "measure in the field and share/record immediately" the norm, the barriers between field and management departments will be lowered, accelerating and improving overall workflow efficiency.

Use cases in route geometry management, inspection, and GIS integration

Chainage data linked to absolute coordinates is powerful in many aspects of infrastructure management. Here are some concrete use cases.

• Application to route geometry management: Managing the exact route geometry of railways and roads using accurate coordinates makes it easy to compare design values with field measurements. For example, if the track centerline data of a railway is obtained by RTK, small discrepancies between theoretical values on design drawings and current conditions can be visualized in 3D. Small settlements or displacements can be captured as coordinate differences, leading to improved accuracy in maintenance planning.

• Equipment inspection and maintenance management: Defects discovered during routine inspections, when recorded with coordinates, make downstream processes smoother. For example, rail track recording cars have historically managed track deformation amounts by chainage, but by using RTK, each measurement can be assigned absolute coordinates. Consequently, points from inspection data can be accurately plotted on maps, and managing defect locations by GNSS coordinates enables instant identification of problem spots during repair work. On roads, inspectors measuring and photographing cracks or potholes with LRTK during patrols can immediately share that information to the cloud, enabling headquarters to plan repairs in real time.

• Data linkage with GIS and other systems: Chainage × coordinate data can be combined with other information layers on GIS maps or CIM models. For example, if bridge and tunnel locations are managed with coordinates, then in the event of an earthquake they can be instantly cross-referenced with nearby active fault data or elevation data to assess damage risk. If each asset’s attribute information is unified by coordinate keys, inspection histories and component replacement records can be selected and referenced directly from a map. Attaching coordinates as tags to chainage greatly increases compatibility with various digital tools.

In this way, coordinate-enabled data is useful across a wide range of tasks—from improving route geometry management accuracy to daily maintenance management and even risk management using hazard maps. It truly becomes the information base for infrastructure DX.

The value of centrally managing chainage × coordinate data in the cloud

The value of chainage-linked absolute coordinate data is maximized when centrally managed in the cloud. Traditionally, drawings and ledgers were managed on paper or local servers, and data tended to be fragmented by department or route. However, by building an integrated database on a cloud platform and linking each point’s chainage and coordinates with ancillary information (equipment name, photos, inspection history, etc.), sharing and utilization within and outside the organization are dramatically improved.

In a centrally managed database, access to required information becomes quick and precise. For example, to check the maintenance history of equipment in a particular section, clicking the location on a map can list past records. Searching by chainage or equipment name will highlight the results on the map, allowing data to be understood in a spatial context. In the cloud, access rights can be configured so that railway operators, municipalities, and disaster-response personnel refer to the same up-to-date data. This resolves discrepancies like "field information has been updated but the headquarters ledger remains outdated."

Furthermore, having master chainage × coordinate data in the cloud makes it easier to develop new applications and analyses. Data users can retrieve coordinate data from the integrated DB for their own GIS analysis or incorporate location parameters into AI degradation prediction models—enabling flexible secondary use. In this way, data becomes a living "asset" and forms an important foundation that supports DX in infrastructure management.

Asset management in the DX era: the foundation of digital twins and spatial information

In next-generation asset management predicated on digital technologies, the concept of a digital twin—replicating real infrastructure entirely in data—is gaining attention. Accurate spatial information is indispensable for faithfully mapping routes and structures into virtual space. Data sets that link chainage with absolute coordinates form the foundation for building such digital twins. For instance, incorporating measured coordinate information into BIM/CIM 3D models enables sophisticated management like visualizing differences between current conditions and design. With point cloud data and photos obtained by LRTK, these can be overlaid on models and compared over time. It is not an overstatement to say that the precision of spatial information determines the success of DX, and coordinate-backed data will remain a useful asset into the future.

High-precision coordinates are also compatible with future technologies. For example, AR-based work support is beginning to allow overlaying equipment design positions or buried utilities on-site through a tablet, which is only possible when coordinates are accurate. If IoT sensor monitoring data carry clear location tags, anomaly detection points can be immediately identified. In disaster damage simulation, having asset spatial distribution data prepared makes it easier to quantitatively evaluate the extent of damage. In short, coordinate-enabled infrastructure data is becoming the core of digital-era infrastructure operations.

Start with one line or one section: recommendations for introducing LRTK

As shown so far, linking chainage data to absolute coordinates has many advantages, but when introducing it for the first time you may worry, “Can we actually use this in-house?” or “Where should we start?” For that reason, we recommend starting with a pilot on a single line or a single section. With a portable RTK-GNSS positioning system like LRTK, field staff can begin measurements as part of routine work without assembling a large surveying team. For example, choose a particular section that urgently needs aging countermeasures and obtain coordinates of major structures and past defect locations during patrol inspections. Import the acquired data into a cloud-based management system and try comparing it with existing ledgers or displaying it on a map.

Even a small trial with few people over a short period should quickly demonstrate benefits. The experience of “measuring coordinates in the field and sharing them with stakeholders on the same day” sharply highlights the difference from traditional methods. Once established, coordinate data can be reused repeatedly in subsequent operations. Data accumulated steadily over years becomes a valuable asset, and the know-how gained facilitates expansion to other sections.

Building a data foundation using high-precision positioning technology is part of a forward-looking DX strategy for infrastructure management. Start with a small step: incorporate simple LRTK surveying into your fieldwork. Once you bridge chainage and absolute coordinates, you will experience transformative effects across daily maintenance and future planning.