Table of Contents

• Introduction

• What Is an Inflection Point?

• Conventional Surveying Methods and On-Site Challenges

• Benefits AR Technology Brings to the Field

• How High-Precision Coordinate Navigation Works

• How to Visualize Inflection Points with AR

• Use Cases for AR

• Simple Surveying with LRTK

• FAQ

Introduction

In recent years, the use of digital technologies at construction and surveying sites has accelerated, and on-site DX (digital transformation) has become a major trend. Faced with industry challenges such as labor shortages and an aging workforce, expectations are growing for smart measurement solutions that anyone can use. One technology attracting attention is high-precision AR (augmented reality) that combines smartphones with GNSS (satellite positioning). Efforts to display inflection points with AR and visualize site conditions are drawing interest for their potential to improve construction management and surveying efficiency.

This article explains what inflection points are and why their verification is important, then describes the benefits and specific methods for visualizing inflection points on site using AR. It also introduces RTK-GNSS technology, which is key to enabling high-precision coordinate navigation, and presents examples of on-site AR use. Finally, as an easy-to-adopt solution, we touch on simple surveying using LRTK and offer hints for on-site deployment. Now, let us guide you into the world of AR high-precision navigation that will support the construction sites of the future.

What Is an Inflection Point?

An inflection point (henkyokuten) generally refers to a point where the curvature of a line or curve changes. Mathematically it is defined as the point where the concavity of a curve switches (the sign of the second derivative changes), but in civil engineering and surveying the term is used in a more practical sense. For example, in longitudinal road design the crest where an uphill slope changes to a downhill slope or the trough where downhill changes to uphill are inflection points. On plan views, locations where a straight line switches to a curve (near the curve’s start or end) or corners where property boundary lines bend can also be broadly called inflection points.

On-site, inflection points serve as important reference points. For road alignment change points they act as markers to correctly form curves, and for land boundaries they mark where boundary monuments should be installed. However, these inflection point positions are not visible on the ground. Accurately locating the points shown on drawings in the field has traditionally required time-consuming tasks such as staking out by surveying or setting batter boards. If an inflection point is placed incorrectly, road alignment can be disturbed or structures can be misaligned, so reliable identification is essential.

Conventional Surveying Methods and On-Site Challenges

To accurately display important points like inflection points on the ground, conventional methods relied on careful staking by skilled surveyors. For example, using a total station to calculate site positions from design coordinates, driving stakes for marking, or using levels and marking instruments to set reference lines. Such conventional surveying typically requires two or more personnel and takes time to transport and set up equipment. Additionally, surveying often did not finish in a single operation; a surveying crew would return after construction to perform as-built verification to confirm the work matched the drawings.

However, with a worsening shortage of surveyors and tighter time constraints, these conventional methods face efficiency and cost challenges. Missing required points or misplacing marks leads to rework, causing delays and additional costs. Translating a point on a drawing into a real-world location often relies heavily on experience, making it dependent on a veteran’s intuition. There is a demand for surveying methods that allow everyone to share the same accurate spatial image.

Benefits AR Technology Brings to the Field

AR technology brings various benefits to on-site work that address these challenges. By overlaying lines and points from drawings onto the real world through a smartphone or tablet camera, AR display enables intuitive understanding of inflection point positions. Workers can see on the screen where a curve’s transition point will be or the extent of the design area, allowing everyone on site to share a common spatial image.

AR is also effective for early detection of positional deviations. If the design model or reference lines are continually projected in AR during construction, you can verify in real time whether completed structures or terrain deviate from the drawings. For example, in embankment work, when soil reaches the specified height the design elevation line on the screen will be occluded by the terrain, making it immediately apparent that the required height has been reached. As-built verification that used to be performed after completion can be done on the spot, reducing rework.

Furthermore, following virtual AR guides can contribute to labor savings and improved safety. Even without relying on an expert’s experience, anyone can reach the correct points by following on-screen instructions, enabling newcomers to perform multiple stake-driving or setting tasks by themselves. If locations of buried pipes or structures are pre-registered, they can be displayed in AR as through-vision during excavation to reduce the risk of damaging buried assets. In these ways AR contributes to efficiency and quality assurance in construction management, aligning with the Ministry of Land, Infrastructure, Transport and Tourism’s i-Construction (*i-Construction*) initiative promoting productivity improvement through ICT.

How High-Precision Coordinate Navigation Works

For AR to be practical on site, highly accurate positioning is essential. Built-in smartphone GPS typically has errors of several meters or more, making it difficult to project drawing points precisely onto the site. Recently, RTK-GNSS (real-time kinematic) technology, which enhances satellite positioning, has drawn attention. RTK uses correction information from a base station to improve positional accuracy to within several centimeters (a few in). This enables centimeter-level positioning (half-inch accuracy) even with a smartphone, dramatically improving AR object alignment.

High-precision coordinate navigation is the technology that uses this centimeter-accuracy positioning to guide users to specified coordinates or display real-time coordinate information of the current location. Specifically, arrows or guide lines to the target point are shown on the map or AR screen on the smartphone, navigating users to a pinpoint location. Tasks that previously involved estimating approximate locations from paper drawings or handheld GPS units can be completed without confusion using this high-precision navigation.

Using RTK-GNSS with a smartphone generally involves combining it with a dedicated small receiver. Attachable GNSS modules for smartphones allow existing phones to be used as high-precision positioning terminals (this approach is sometimes called “smartphone RTK”). As long as GNSS positioning is stable, AR-inflected-point markers remain fixed at the correct coordinates, and the display stays stable without shifting even when the user moves around and views from different angles. This “non-shifting AR” is the foundation of AR visualization usable on site.

How to Visualize Inflection Points with AR

To AR-display inflection points on site, several preparations and steps are required. First, prepare digital data of the design drawings and points. CAD data (DXF/DWG, etc.) of the road centerline or boundary lines and a coordinate list of inflection points are best. If you only have paper drawings, scan or trace them beforehand to digitize them for AR import.

Next, confirm the coordinate system of the drawing data. If the data are designed in an absolute coordinate system such as the Geodetic System of Japan plane rectangular coordinates or the World Geodetic System (WGS84), they can be directly matched to the site positioning coordinates. If the drawing uses a local arbitrary coordinate system, alignment with the site is needed. For example, measure a field point corresponding to a drawing reference point with GNSS, then translate and rotate the drawing data appropriately. If this coordinate alignment is done beforehand, loading the data into the AR app will display virtual objects in the correct positions.

Once preparations are complete, start the smartphone and high-precision RTK-capable GNSS receiver on site and use the app to AR-display the data. If the device position is accurately determined by corrected GNSS, virtual inflection point markers and design lines should appear to align perfectly with the real landscape. If necessary, you can measure one known point on site and use fine-tuning functions to align the AR display with the physical object. After this calibration, simply holding up the smartphone and walking will let you always know exactly where you are relative to the design. As you approach an inflection point, distance displays or arrows will indicate the way, and when you reach the point a marker will appear over the ground on the screen. You can then drive a stake or mark with paint at that spot to complete the physical staking efficiently.

Use Cases for AR

• Efficient stake-driving and setting-out: By AR-displaying points and lines based on design drawings, stake-driving and marking tasks can be performed intuitively. Workers need only follow virtual markers on the smartphone screen to reach precise positions, reducing the number of surveying assistants required and enabling a single person to perform positioning tasks efficiently. Survey points on distant slopes can be “placed” virtually in AR to confirm and share positions, minimizing the need to enter hazardous locations.

• Boundary verification and visualization of design areas: If property boundaries and construction area data are pre-registered, they can be visualized on site. Invisible boundaries can be shown as lines or fences in AR, helping to prevent encroachment on adjacent land. Building placement and heights from the design can be overlaid on the spot to check harmony with the surrounding landscape in advance.

• Sharing construction images and forming consensus: Overlaying a completed 3D model (BIM/CIM data, etc.) on the existing terrain before construction provides a powerful communication tool for meetings with clients and nearby residents. The completed appearance, which is hard to convey with drawings or words alone, can be experienced at actual scale via AR, making explanations and persuasion easier. This helps smooth pre-construction consultations and consensus-building and reduces the risk of later changes.

• As-built management and quality checks: Immediately scanning the site after construction and overlaying the acquired point cloud data or as-built measurement results on the design model in AR allows visual checking of finish errors. It is possible to color-code deviations where the measured embankment or structure differs from the design, aiding quality control. Tasks that used to require separate PC-based postprocessing for as-built inspection and calculations can now be performed on site, yielding significant labor savings.

Simple Surveying with LRTK

Finally, as a solution that makes AR and high-precision positioning easy to implement, we introduce LRTK. LRTK is a small GNSS receiver that attaches to a smartphone and enables the phone to perform centimeter-level positioning (half-inch accuracy). Although lightweight at about 150 grams and pocket-sized, it features a high-sensitivity antenna and RTK engine, and integrates with a dedicated app to provide an all-in-one set of functions from positioning to point cloud measurement, photography, and AR display.

Previously, introducing high-precision AR systems on site required significant investment in dedicated equipment and software. With LRTK, however, you only need your smartphone combined with a small device, making it low-cost and easy to bring to the site. Complex setup is unnecessary; anyone can begin surveying and AR navigation by following in-app guidance. There are reports that even novice engineers can handle it with a game-like ease, making it a true embodiment of simple surveying.

By adopting simple surveying with LRTK, one device per person enables on-site situational awareness, as-built checks, and setting-out tasks. Even on sites with personnel shortages in the surveying team, construction managers can perform necessary measurements on the spot, reducing wait times and enabling faster progress. With a high-precision coordinate framework always at hand, on-site decision-making becomes quicker and rework due to mistakes is greatly reduced. LRTK is a device that can be the key to realizing a “site anyone can survey” through cutting-edge technology. If you are considering AR surveying and navigation as part of your on-site DX, LRTK can make a significant first contribution.

FAQ

Q1. What equipment and preparations are required to AR-display inflection points? A. Basically, you need an AR-capable smartphone (or tablet), a GNSS receiver capable of centimeter-level positioning, and a dedicated app that links them and performs AR display. While basic AR display is possible with a phone alone, RTK-capable GNSS is essential for high-precision positioning. For example, attaching an LRTK device to an iPhone or Android device turns it into a high-precision positioning and AR device. It is also best to have network access for correction information and reception environment for domestic augmentation services (e.g., QZSS CLAS signals) if available.

Q2. I only have a PDF or paper drawing. Can I still use AR? A. Direct AR display from a PDF or paper drawing is difficult, but there are several approaches. Ideally, obtain the original CAD drawings or coordinate tables. If that is not possible, import the PDF drawing into CAD software and convert it to DXF/DWG, or import it as an image and georeference and scale it to paste onto a virtual plane. Although this takes some effort, digitizing the drawing enables AR overlay. LRTK apps can directly handle DXF/DWG data, so preparing CAD data is recommended when possible.

Q3. Is AR positioning accuracy really within a few centimeters? How reliable is it? A. If RTK positioning is performed properly and coordinate alignment with the drawing data is achieved, horizontal position errors generally fall within ± several centimeters (± a few in). This is comparable to conventional surveying with total stations. Note, however, that smartphone AR can produce height-direction errors of several centimeters depending on device orientation and tilt. Height projection errors tend to increase with distance, but for typical stake-driving and reference setting tasks the accuracy is sufficient. In short, horizontal accuracy is reliable, but for critical elevation checks it is advisable to allow a safety margin and verify.

Q4. Can AR surveying be used in places where GNSS reception is not possible (e.g., inside forests or tunnels)? A. In environments where satellite signals cannot reach, the high-precision AR described here is unfortunately limited. Without RTK positioning, the phone must rely on its camera and sensors for relative positioning, and errors will gradually accumulate over long durations or distances. However, for short periods IMU-based dead reckoning inside a smartphone can maintain position to some extent, allowing limited AR continuity. In tunnels, placing QR code markers and scanning them intermittently to reset position can help supplement accuracy. In summary, although you cannot expect outdoor-level precision, with appropriate measures some AR functionality is possible in indoor or underground spaces.

Q5. Compared to conventional methods, what are the benefits of introducing AR on site? A. The biggest benefits are efficiency and labor savings. Surveying and inspection tasks that used to require multiple people and time can be carried out quickly with fewer personnel using AR navigation. Digitally displaying information that previously relied on an expert’s intuition makes it easier for anyone to confirm and reduces mistakes, leading to more consistent quality. Additionally, easier information sharing on site improves communication and helps visualize unseen hazards, enhancing safety. Overall, AR deployment boosts productivity and safety on site, contributing to cost reduction and shorter schedules.

Q6. Can beginners unfamiliar with the technology use it easily? Is special training required? A. Basic operations are completed within smartphone apps with intuitive GUIs, so many field newcomers become proficient quickly. Even without specialist knowledge, you can follow on-screen instructions to take measurements and check AR displays. However, stabilizing high-precision positioning does require some understanding of RTK communication settings and coordinate systems. Knowing why satellite corrections improve accuracy or what a reference-point coordinate is can help in troubleshooting. LRTK provides support sites and manuals that explain these points clearly, and users can learn while using the system. With field practice and experience, anyone can become competent in a short time.