Dramatically improving kilometer post re-survey efficiency! A new method realized by high-precision positioning and AR guidance

Kiloposts ([distance markers](https://ja.wikipedia.org/wiki/距離標)) are important signs indicating the distance from the origin for roads and railways. On expressways, small numbered signs are installed at 100 m (328.1 ft) intervals, and on railways kilometer posts are placed along the line at regular distances. Although they are not often noticed, they are indispensable for identifying accident locations and managing maintenance work sections. However, when these kilometer posts themselves become deteriorated or damaged, or are temporarily removed due to roadworks, re-surveying work is required to re-install them in the exact correct positions. Conventional methods required considerable effort and time, but new techniques that utilize high-precision GNSS positioning (RTK) and AR technology have recently emerged and are dramatically improving the efficiency of kilometer post re-surveying and reinstallation. This article explains the importance of kilometer post re-surveying, the challenges of conventional methods, and the solutions and benefits offered by new technologies.

The necessity of kilometer post re-surveying and the challenges of conventional methods

Kiloposts on roads and railways may appear modest, but they play an important role as reference points for infrastructure management. For example, on expressways, the operator companies and the police exchange accident or broken-down vehicle locations using the numbers on kilometer posts. In roadworks, the work range is indicated by a section description such as “from kilometer post ○○ to kilometer post △△.” It is of course assumed that kilometer posts are installed at their designated exact locations. However, with years of use the sign plates may be damaged or lost, or when they are temporarily removed for road widening works, it becomes indispensable to restore them exactly to their original positions.

The situation is similar in railways. Distance markers are reference points indicating track mileage for train operations and are used for distance management between junctions and stations. When existing kilometer posts are relocated during line improvement works (e.g., double-tracking or elevated structures), failure to restore them accurately can cause problems in timetable settings and maintenance planning. Performing surveys over long sections between train services is not easy, and even more than on roads, more efficient re-surveying methods have been needed.

Conventionally, re-surveying and reinstallation of kilometer posts relied heavily on manual work by experienced survey technicians. A typical method starts with checking the coordinate values or distances from the origin recorded in design drawings or road ledgers, then measuring distances on site using tape measures or distance-measuring wheels (kyori chain), and indicating the position with temporary stakes or markings. In some cases, reference point stakes are installed on the road shoulder or beside the tracks in advance, and offsets are measured from there. However, when long distances are measured manually, small measurement errors inevitably accumulate and can cause deviations from the target position. Furthermore, in curved road sections or areas with elevation differences, it is difficult to obtain strictly horizontal distances with a tape, and correction calculations that take gradient and curvature into account may be required.

An advanced surveying method uses total stations (TS) for coordinate surveying. This method measures the installation coordinates of a kilometer post from known reference points using a TS, but it also requires considerable effort. Setting up a TS requires placing the instrument in a location with good lines of sight, checking it against reference points, and having a staff member stand near the target point carrying a prism to be guided. It usually requires two or more people, and for long distances the TS must be re-sited and recalculations performed multiple times. On busy trunk roads, in particular, safety constraints limit where surveyors can be positioned, sometimes preventing daytime work and requiring temporary road closures at night.

Thus, traditional kilometer post re-surveying had several challenges:

• Accuracy: Human and mechanical errors such as tape sagging, misreading, disappearance of markings, and instrument setup errors are always possible. Even tiny errors accumulate over long distances and can lead to errors of several kilometers in absolute position from the origin.

• Efficiency: Even reinstalling a single distance marker requires multiple people and long hours, making the restoration of many kilometer posts extremely inefficient.

• Safety: Survey work on roads or tracks poses risks to workers. Measuring with a tape on an expressway or guiding a staff member carries danger even with caution, so minimizing time and personnel is essential.

• Reproducibility: Accuracy can vary depending on the procedure and personnel, so the same location measured by different people may not yield consistent results. The way records are kept also varies by person, making later verification difficult.

Centimeter-level high-precision positioning with RTK-GNSS

Lately, RTK-GNSS positioning has attracted attention as a technology that can address these challenges. RTK (Real Time Kinematic) uses signals from multiple GNSS satellites and applies differential corrections between a base station and a rover to dramatically improve real-time positioning accuracy. Conventional GPS measurements often produced errors of several meters, but with RTK it is possible to obtain current positions with errors of only a few centimeters or less.

High-precision RTK positioning used to require specialized expensive receivers and radio equipment, but recent technological advances have greatly simplified the setup. For example, by connecting over the Internet to the Geospatial Information Authority of Japan’s GNSS reference station network or regional correction services, RTK positioning can be achieved simply by attaching a small antenna to a smartphone or tablet. On open roads with a wide view of the sky, positioning can initialize in a few seconds to a few tens of seconds, allowing handheld devices to obtain current positions in a global geodetic coordinate system.

The greatest advantage RTK positioning brings to kilometer post reinstallation is that the coordinates of the point to be installed can be identified accurately in absolute terms. If the coordinate values of the kilometer posts required at the design stage (for example, latitude/longitude or plane rectangular coordinates) are known in advance, the point can be plotted on an RTK-capable device’s map, and by comparing it with the device’s current on-site position, the approximate direction and distance are immediately known. In other words, instead of measuring distances on site from the origin step by step, RTK satellite positioning can directly navigate to the goal point. Unlike conventional chain-measuring methods, this approach does not accumulate error in theory, and positioning accuracy for each point is dramatically improved.

Intuitive position guidance using AR technology

Even if RTK-GNSS provides centimeter-level current positions, clear guidance that humans can easily use on site is necessary. This is where AR (augmented reality) guidance using smart devices comes in. With a dedicated app, virtual markers and guidance arrows indicating the target location can be overlaid on the camera view of a smartphone or tablet. For example, by selecting the coordinates of the kilometer post to be reinstalled that are stored in the cloud and tapping “Start Navigation” in the app, the screen displays a directional arrow and the straight-line distance to the target point in real time. The worker simply walks in the direction of the arrow and can intuitively tell they are approaching the target. When the distance display reaches around zero, that spot is the kilometer post installation position. Since the screen shows an AR marker as if a pin or post were standing on the ground, the worker makes fine adjustments so that the virtual stake lines up with their feet on the ground to complete the positioning.

AR navigation’s advantage is that people without surveying or coordinate expertise can intuitively achieve accurate positioning. Reading numeric coordinates and moving “2.5 m east and 1.3 m north,” for example, is difficult unless you are experienced, but with AR arrows you just follow the direction. AR displays provide the same information to anyone viewing them, so there’s no worry about someone missing or misinterpreting a marker tape or chalk line. Because physical marking is unnecessary, problems such as marks being erased by heavy machinery or weather are avoided. Moreover, on paved surfaces or dangerous spots where you cannot drive stakes into the ground, AR can indicate points virtually from a distance. For example, even on expressway shoulders that are difficult to access, AR can show “there is a stake here” from a safe location on the screen, reducing situations in which workers are put at risk. In this way AR functions as a tool that radically simplifies and makes on-site position guidance safer.

One-person re-survey workflow with a smartphone + LRTK

Combining high-precision RTK positioning with AR guidance makes it possible for a single person to complete kilometer post re-surveying that traditionally required multiple people. Below is the specific procedure.

• Pre-work: Collect the design coordinate values for each kilometer post to be reinstalled and register them in the dedicated app. Prepare the coordinates obtained from road ledgers or past drawings as text data and load them into the smartphone via the cloud. Multiple points can be imported at once.

• Equipment setup: The worker attaches a small GNSS receiver (e.g., an LRTK device) to the smartphone. This receiver connects to the phone via Bluetooth or similar and performs high-precision RTK positioning. The smartphone should be connected to the Internet and logged into the regional GNSS correction service.

• Start positioning: Launch the app on site and check the GNSS positioning status. Once RTK achieves a FIX solution and the current position is confirmed to be accurate to a few centimeters, navigation is ready to begin.

• Point guidance: Select the target kilometer post coordinates on the app and start navigation. An arrow for guidance appears on the camera screen, showing the direction and approximate distance to the target. The worker follows the instructions while watching the phone screen. If necessary, moving by vehicle is acceptable (for example, driving slowly under safe conditions).

• Position confirmation: Follow the arrow until the distance display becomes very small, and confirm that the virtual marker (pin) on the phone screen overlaps the area near your feet. For fine adjustments, move the phone up, down, left, and right so the AR marker aligns precisely with the actual ground. At this point, the difference between the current coordinate shown on the phone and the target coordinate should be within a few cm (a few in) to a few mm (a few 0.0 in).

• Install the sign: Once the position is confirmed, install the kilometer post sign at that point. Typically, a signpost is erected and the appropriate plate attached. By driving the stake where the AR marker indicates, the marker can be placed accurately at its designated position. If multiple people are involved, the navigator can point at the screen and say “this is the location,” allowing others to immediately understand and begin stake-driving without unnecessary discussion.

• Record: Finally, digitally record the measured coordinates of the installed kilometer post. The app automatically saves the coordinate values and positioning accuracy information (estimated error) at that time, so there is no need for manual note-taking. Back at the office, check the data in the cloud to verify whether it matches the ledger coordinates or is within acceptable limits to complete the process.

With the above steps, kilometer post re-surveying, which traditionally required long hours and multiple people, is dramatically simplified. For example, even in a case of reinstalling 50 distance markers over a 5 km (3.1 mi) section, work that previously took several teams and several days using conventional methods could potentially be completed by one person in a few hours to one day with the RTK+AR approach.

Quality improvement through consistency with point cloud data and ledger coordinates

Another strength of the new method is that it can improve consistency with past data. The position of a kilometer post is a virtual point not physically visible, but digital tools make it possible to manage it precisely. For example, if a road manager has previously conducted a 3D point cloud survey of the entire road, the positions of kilometer posts will be recorded in that point cloud data. Using RTK+AR, you can erect a new sign in exactly the same coordinates as in the point cloud. What was traditionally restored by “roughly around here” can now be reproduced from past precise measurements.

Because design drawings and ledger coordinate systems can be directly linked to on-site surveys, spatial data consistency is preserved. RTK positioning places all on-site points in a unified coordinate system, making it easy to match and compare separately acquired data sets (for example, pre-construction design data and post-construction as-built data). The positions of kilometer posts reinstalled this time can be referenced directly when re-surveyed again in the future. Such high reproducibility contributes to strengthening quality assurance in infrastructure asset maintenance.

The benefits of digital records should not be overlooked. Whereas survey records were previously kept by hand, after introducing an RTK+AR system they are automatically saved in the cloud. Who measured what, where, and with what accuracy at what time can be tracked by data, making it easy for a third party to verify that installation was performed correctly. This provides reassurance to client-side technical staff and improves the reliability of as-built documentation.

Efficiency effects and implementation benefits of high-precision positioning and AR guidance

The new method using RTK-GNSS and AR offers very large benefits for on-site kilometer post re-surveying and reinstallation. Below are the main effects summarized.

• Dramatic improvement in work efficiency: Steps such as surveying→marking→confirmation can be greatly shortened, and the measurement of multiple consecutive points becomes smooth. Compared with conventional methods, the time required for positioning can be drastically reduced, contributing to shorter construction periods and reduced night work (shorter work times directly reduce labor and traffic regulation costs). In one comparison, using GNSS+AR navigation reduced the time for positioning tasks to about 1/6 of that required by traditional optical surveying methods.

• Improved accuracy and error prevention: Absolute accuracy from satellite positioning combined with visual guidance from AR minimizes positioning errors and marking mistakes. Parts that relied on human estimation or intuition are replaced by digital guidance, ensuring stable accuracy regardless of who performs the work.

• Labor savings and improved safety: Enabling single-person operations leads to major labor savings. In sites with serious personnel shortages, the benefit of being able to operate with few people is invaluable. Also, reducing workers’ time on site and their need to enter hazardous areas improves safety management. Shorter traffic regulation periods and reduced staff for guiding heavy equipment can also be achieved.

• Data utilization and quality management: Digital measurement, guidance, and recording promote data utilization. Once coordinate data are acquired, they can be used in future works and inspections and accumulated continuously as assets. This contributes to DX (digital transformation) promotion in as-built management and maintenance operations.

In fact, expressway companies and railway operators are conducting demonstrations to reduce labor in surveying and stake-driving using GNSS and AR, and full-scale implementation is becoming feasible.

This kind of high-precision positioning + AR guidance for kilometer post re-survey support has the potential to become a new standard in infrastructure maintenance. With solutions like “LRTK” that combine smartphones and GNSS receivers already available, the era in which anyone can easily perform the re-surveying and stake-guidance described above on site is arriving. By adopting this paradigm-shifting method that overturns conventional wisdom, railway and road managers and survey professionals can achieve more efficient and higher-quality infrastructure management. These initiatives are in line with the Ministry of Land, Infrastructure, Transport and Tourism’s i-Construction initiatives and DX strategies in the infrastructure field, and they contribute significantly to productivity improvements and labor savings through on-site digitalization. Even the seemingly mundane task of kilometer post re-surveying is now experiencing a wave of technological innovation, and it may be worth actively considering adopting these technologies. Digital innovation is now sweeping into kilometer post re-surveying sites.