High-precision as-built Management Achieved with RTK and AR Markers

Table of Contents

• Introduction: Challenges in as-built management and the need for DX

• What is RTK positioning: Technology that enables centimeter-level accuracy (inches)

• What are AR markers: The key to overlaying digital information on the real world

• High-precision as-built management enabled by RTK and AR markers

• Use cases: as-built management, pile driving and layout marking, buried utility visualization, etc.

• Automation of as-built inspection using point cloud data

• Complete with a single smartphone: improved work efficiency and reduced training burden

• Improved site safety and reduced human error

• Conclusion: Recommendation to introduce simple surveying with LRTK

• FAQ

Introduction: Challenges in as-built management and the need for DX

Surveying work and as-built management operations on construction sites have seen little major change for many years and face various challenges. Traditional surveying uses dedicated instruments such as total stations and levels, requiring multiple people to set out reference points and check as-built conditions. This method requires time and effort to set up equipment and install targets, and there is a limit to the number of points that can be measured in a day. In addition, converting survey results into drawings requires advanced knowledge and experience; with a growing shortage of skilled technicians, maintaining surveying quality and speed on sites has become difficult.

Furthermore, in construction management the task of confirming whether work has been executed “according to the drawings” also required many manual steps and visual checks. Relying on paper drawings and chalked batter boards, workers needed to visualize the finished form in their heads as they proceeded, creating a situation where inexperienced staff were prone to errors. Such inefficiencies and the risk of human error are recognized as important issues that the construction industry as a whole must solve.

Against this backdrop, the construction industry has been increasingly calling for ICT and DX (digital transformation). In particular, the surveying field is said to have had almost no major technological innovation for about 50 years, and there are concerns that productivity stagnation and loss of competitiveness due to labor shortages will continue if nothing changes. New technologies such as drone surveying, 3D scanners, and RTK positioning are attracting attention as trump cards to dramatically improve site productivity and accuracy.

Networked RTK positioning, in particular, is raising the positioning accuracy of GNSS from the meter class to the centimeter level (inches), and is becoming an indispensable foundational technology for as-built management and machine control of heavy equipment.

Alongside these centimeter-class positioning technologies, AR (augmented reality) technology has also rapidly matured and become practical in recent years. By simply holding a smartphone or tablet at a site, design data and survey data can be overlaid on the live camera view, enabling intuitive understanding of construction status.

A method that combines RTK positioning and AR (especially AR-marker-based alignment techniques)—what might be called “AR surveying”—has emerged, bringing about an era in which high-precision surveying and on-site verification of as-built conditions can be performed with a single smartphone.

This article explains the mechanism by which RTK positioning achieves centimeter-level accuracy (inches) and how data projection into the real world using AR markers is transforming sites. Using our smartphone RTK solution “LRTK” as an example, we introduce use cases and concrete benefits for as-built management and pile-driving work, clarifying the effects that site DX can bring. Finally, we answer frequently asked questions (FAQ) to resolve common doubts about RTK and AR usage.

What is RTK positioning: Technology that enables centimeter-level accuracy (inches)

RTK stands for “real-time kinematic,” a surveying method that achieves centimeter-level accuracy (inches) by correcting GNSS (satellite positioning such as GPS and GLONASS) errors in real time. Standalone positioning (such as a smartphone’s built-in GPS) can have position errors of several meters due to various radio error factors. With RTK positioning, a reference station (base station) with known accurate coordinates is installed separately, and a mobile receiver (rover) used on-site receives the same satellite signals simultaneously. Error information obtained at the base station is sent to the rover in real time and applied as corrections to measured values, reducing errors to on the order of a few centimeters.

In other words, by taking the difference between the results of a “stationary receiver” (the base) and a “moving receiver” (the rover), common error factors such as atmospheric delay and satellite clock errors are canceled out, allowing high-precision relative positions to be determined.

According to materials published by the Geospatial Information Authority of Japan, standalone positioning typically has errors of several meters, but using RTK can reduce errors to the centimeter range. This is made possible by analyzing the phase difference of signals from satellites to obtain a fixed integer solution (Fix solution) that pinpoints positions to the centimeter level. RTK-capable receivers are equipped with high-performance antennas and dual-frequency GNSS chips, and the rover performs these high-precision computations in real time.

The recent spread of network RTK has significantly lowered the barrier to using RTK positioning. Traditionally, users had to install a dedicated base station near the site, but today correction information can be obtained over the Internet from networks of continuously operating reference stations or commercial correction services. By obtaining base station data (corrections) from the network using the Ntrip protocol, centimeter-class positioning is possible without placing your own base station at the site. For example, in Japan, the Geospatial Information Authority’s nationwide network of about 1,300 GNSS reference stations and VRS correction services provided by local governments and private companies can be used to obtain data for virtual reference stations near the observation point, enabling stable high-precision positioning.

Japan also offers a unique high-precision positioning service, CLAS (Centimeter-Level Augmentation Service), provided by the Quasi-Zenith Satellite System “Michibiki.” With a CLAS-compatible receiver, even in mountainous areas or remote islands without mobile communications coverage, augmentation signals directly broadcast from satellites can be received to maintain centimeter-level positioning. With both network RTK and CLAS available, an environment that allows easy centimeter-class positioning across almost the entire country is taking shape.

What are AR markers: The key to overlaying digital information on the real world

An AR marker is a specially patterned tag or symbol that can be recognized through a camera and functions as a reference point for accurately placing virtual objects in the real world using AR technology. When a smartphone or tablet camera captures an AR marker, the app instantly calculates its position and orientation and overlays pre-associated digital information (for example, a 3D model or a drawing point) at the corresponding location.

On construction sites, AR markers can serve as “pins” that connect the virtual and the real. For example, if you place a marker at an arbitrary location and observe its center position with RTK, then register that coordinate value in the AR app, the design reference and the actual location will automatically align whenever the marker is captured by the camera.

If you project the design model or as-built inspection results at the positions associated with the markers, you can accurately verify them within the real scene.

AR markers themselves can be deployed on site using printed sheets or boards resistant to dirt. Their shapes vary—two-dimensional codes like QR codes, patterns with markers at four corners, etc.—but all are designed to be easily recognized from camera images.

The main advantage of using AR markers is that they provide high positional accuracy and stability for on-site AR displays.

Normally, placing 3D models using only a smartphone or tablet’s AR capabilities can cause “drift” where model positions gradually shift with extended use or movement. But when virtual models are aligned using AR markers, the models remain correctly fixed as long as the reference marker is in view. Especially when RTK positioning provides absolute coordinates for the markers and the device itself, model drift becomes negligible and centimeter-level consistency (inches) is maintained constantly.

High-precision as-built management enabled by RTK and AR markers

Combining centimeter-level positioning (inches) with the precise alignment provided by AR markers enables high-precision on-site verification and management of as-built conditions. By overlaying digital data such as design drawings or 3D models onto the real world based on RTK position information, you can intuitively judge whether construction has been executed according to the design.

For example, if you overlay the design model semi-transparently on the live view of an actual structure on a smartphone or tablet screen, you can immediately spot slight positional or elevation differences. Without setting up surveying instruments to read fine numerical values, you can visually detect discrepancies through the screen, allowing even less-experienced technicians to reliably perform as-built checks.

Moreover, by using the sensors and camera in a smartphone or tablet, you can capture three-dimensional as-built data on site and evaluate it immediately.

For instance, you can scan freshly placed embankments or paving with a smartphone, overlay the resulting point cloud data with the design model, and display a heatmap that color-codes elevation differences. Even a seemingly flat roadbed will show low spots relative to the design height on the spot by color, enabling prompt corrective action such as additional filling without waiting for later inspections. Traditionally, surveying crews would confirm after completion and rework if problems were found, but with an RTK+AR workflow teams can verify accuracy immediately after work and prevent rework.

Use cases: as-built management, pile driving and layout marking, buried utility visualization, etc.

Solutions that combine RTK and AR markers are effective across many civil engineering and construction scenarios. Here are representative use cases.

• As-built management: You can verify and measure completed structures and terrain on site. By displaying a design 3D model over real objects based on high-precision coordinates acquired by an RTK receiver attached to a smartphone, you can intuitively check whether a finished structure matches the design to the last detail. For example, after concrete casting you can check finished surface elevations with a smartphone AR view to instantly see whether any spots are too high or too low relative to the design elevation. Automatically calculating volumes and areas from acquired point cloud data also greatly reduces the labor required to compute earthwork volumes or prepare as-built drawings.

• Pile driving and layout marking: AR-guided displays are helpful when setting out reference points or indicating the precise installation positions of structures. Based on pre-configured design positions, the smartphone screen can show “this is where a pile should be driven” or “this is the chalk line for the wall” in real time, allowing workers to perform pile driving or line marking exactly following on-screen prompts. There is no need to measure with tape measures or chalking tools as before, shortening work time and improving accuracy. Because even inexperienced staff can follow AR guidance to perform accurate pile driving and layout marking, this also aids skills transfer.

• Buried utility visualization: AR can display underground infrastructure (pipes, cables, etc.) visually. If you link drawing data of buried utilities to corresponding markers on the surface, simply pointing a smartphone at the marker lets you intuitively understand what is buried and where. For example, projecting the routes of water or gas pipes onto the ground reduces the risk of accidental damage during excavation. In tunnel construction, linking surface markers with shield machine position data allows AR visualization of the subterranean shield machine from the surface, enabling understanding of underground machine movements that are normally invisible and aiding construction management and stakeholder communication.

Automation of as-built inspection using point cloud data

The fusion of AR and RTK also greatly streamlines and automates the as-built management process itself. Traditionally, computing earthwork volumes, excavation quantities, and comparing as-built conditions with design drawings were tasks performed later in the office by hand.

Using an RTK-enabled smartphone and an AR app, these processes can be largely automated on site. By overlaying captured point cloud data with the design 3D model, differences between them can be visually checked immediately.

For example, you can instantly generate a heatmap that color-codes elevation differences between the design surface and measured point cloud, highlighting locations that exceed allowable tolerances. This reduces the risk of discovering issues only after work completion and requiring rework.

Because quantities needed can be calculated immediately from on-site point clouds, time spent preparing as-built drawings and quantity reports is also shortened.

For instance, calculating the volume of embankment from a smartphone-scanned point cloud or measuring the finished pavement area can be done at the push of a button.

Tasks that previously took days—creating as-built drawings and computing volumes—can be completed on site, contributing to shorter schedules and cost savings.

Moreover, manual recording, transcription errors, and calculation mistakes are reduced, enabling consistent digital-quality control. As part of site DX, decisions that used to rely on craftsmen’s intuition and experience are replaced by data-driven objective judgments, improving reproducibility and reliability of construction.

Complete with a single smartphone: improved work efficiency and reduced training burden

With a smartphone, a compact RTK receiver, and an AR app, surveying and as-built verification on site can now be performed quickly by anyone. This brings significant benefits in both operational efficiency and human resource development.

First, there are great advantages in portability and responsiveness. A smartphone fits in your pocket and can be used immediately for measurement and verification, greatly improving on-site mobility. There is no need to transport and set up heavy surveying equipment, and a single person can tour the site and check conditions in real time, enabling labor and effort savings. Rapid response to sudden measurement needs reduces waiting time in workflow and smooths the overall schedule.

Second, it is effective for skill transfer and training. Intuitive AR displays allow proper as-built checks and layout tasks to be performed even by non-experts. There is no need to teach complex surveying instrument operation from scratch; workers can follow the guidance shown on the smartphone screen to carry out necessary tasks, enabling young or inexperienced staff to become productive sooner.

Reducing reliance on veterans’ “feel and experience” and creating an environment where anyone can work to a consistent quality is critically important for future construction sites. In that sense, smartphone- and AR-based solutions will become the new normal.

Measurement data is also automatically shared via the cloud, eliminating the need to return to the office to create drawings or reports. Real-time connectivity between site and office dramatically speeds up information sharing and decision-making.

Improved site safety and reduced human error

New measurement and management methods using RTK and AR also contribute to improved site safety and reduced mistakes. For example, measurements that used to require hazardous access to high places or steep slopes can increasingly be performed safely from a distance. If AR markers are placed appropriately, you can capture a marker with a camera from a distance and measure without directly approaching, reducing risky measurements in areas with poor access or active heavy machinery. The ability to measure with fewer people and in less time also reduces worker burden and the risk of heatstroke and other hazards.

Digital technology also brings large benefits in reducing human error. AR visualization prevents misreading drawings and recording mistakes, and measured results are automatically saved and shared, eliminating errors like transcription mistakes from handwritten notes. Because data keeps a record of who measured what and when, double checks are easy.

Real-time verification also allows immediate discovery and correction of construction mistakes, preventing major rework. As a result, delays are avoided and quality is secured, contributing to safe and reliable site operations.

Conclusion: Recommendation to introduce simple surveying with LRTK

The construction industry is now on the verge of major change through digital technology. We have seen how combining centimeter-level positioning (inches) with AR-marker-based intuitive visualization can dramatically streamline how as-built management and surveying are performed on site.

If you have not yet introduced such technologies, consider starting with simple surveying using a smartphone and LRTK. Even small first steps will let you experience the effects of site DX, and over time this will lead to productivity improvements across your organization.

We encourage you to try smartphone RTK and AR technologies—the future standard—at your sites.

FAQ

What is RTK positioning?

RTK positioning is a technology that corrects positioning errors from GNSS satellites in real time to obtain highly accurate positions. Using a dedicated reference station to send error information and applying corrections at the rover reduces typical errors of several meters down to the order of a few centimeters. The spread of network RTK, which distributes reference station data over the Internet, has made it easy to use RTK without installing your own base station at the site.

What is CLAS?

CLAS (Centimeter-Level Augmentation Service) is an augmentation signal service provided by Japan’s Quasi-Zenith Satellite System “Michibiki” for high-precision positioning. With a compatible receiver, augmentation signals broadcast directly from satellites allow centimeter-level positioning even in mountainous areas or remote islands without mobile communication coverage. CLAS is distributed to cover almost all of Japan and is attracting attention as a means of performing RTK positioning at sites without network connectivity. Our LRTK devices include models that support CLAS reception and perform well at sites outside communication range.

Is the position of virtual models displayed in AR stable?

Yes. Combining high-precision RTK positioning with AR markers results in very stable positioning of models overlaid on the real world. With normal smartphone AR, “drift” can cause slight shifts in model placement while moving. However, if you place models using the device’s or markers’ absolute coordinates from RTK, you generally do not need to worry about display shifts. Even when walking around a site, virtual objects remain correctly positioned, and their orientation and scale do not change unexpectedly. Temporary sensor anomalies can cause minor errors, but as long as the positioning maintains a Fix solution (integer solution), positional accuracy remains at the centimeter level (inches).

What should be done where satellite signals cannot be received?

Because RTK positioning uses GNSS satellite signals, it cannot be used in principle in environments where satellites cannot be captured at all, such as deep indoors or deep inside tunnels. In such cases, you need to take measures such as setting local coordinates based on known points obtained outdoors and using conventional total stations for layout. If satellite visibility is only partially obstructed, maintaining accuracy may be possible by taking longer observation times to average results or moving observation points to secure a clearer view. Also, if satellites can be received even without communications, CLAS or a local base station can be used to continue high-precision positioning. Higher-end models in the LRTK series include a local base station mode using short-range radio and CLAS reception, enabling surveying at sites without Internet access.

Can AR surveying be done without AR markers?

It is possible to roughly place virtual models using only a smartphone’s GPS and inertial sensors without AR markers. However, in that case, orientation sensor errors and AR drift limit placement accuracy.

For as-built management, where high precision is required and even slight misalignments are unacceptable, strict alignment using AR markers is indispensable. AR markers provide absolute reference points on site, and combined with RTK centimeter-level positioning (inches) enable consistently accurate AR displays. Even in indoor environments where GNSS cannot be used, placing markers whose positions were measured in advance allows AR-only alignment. To achieve high-precision AR surveying, using AR markers is the most reliable method.