Replacing Photo Location Data with cm-Level Accuracy Using LRTK: Achieving High-Precision Positioning on Construction Sites

Table of Contents

• The importance and challenges of photo location data on construction sites

• Precision limits of conventional GPS positioning

• Why centimeter-level accuracy is required

• What is RTK (Real Time Kinematic)?

• How to replace photo location data with cm-level accuracy

• Use cases for high-precision photo location data

• Simple surveying achievable with LRTK

• FAQ

The importance and challenges of photo location data on construction sites

On construction and civil engineering sites, photos are taken routinely for recording construction progress and inspection reports. Photos are important as “evidence” showing the construction process and defect locations, and information about when and where they were taken is indispensable. In recent years, it has become common to add location coordinates (geotags) at the time of shooting with smartphones and digital cameras, enabling visualization of shooting locations on maps and automated incorporation into reports.

However, conventional methods have had issues with the accuracy and reliability of photo location data. Built-in GPS in smartphones has limited accuracy, and on large sites there are cases where it is impossible to precisely determine “where in the site this photo was taken.” As a result, people end up marking locations on drawings by hand or organizing photos based on the memory or intuition of staff, undermining the objectivity of records and operational efficiency. Without accurate location information embedded in photo data, valuable digital records cannot be fully utilized. Conversely, if images are linked with high-precision location information along with timestamps, they become powerful objective evidence of construction events, dramatically improving the reliability of site management.

Precision limits of conventional GPS positioning

Positioning errors from standard GPS (GNSS) are on the order of several meters for the built-in receivers in typical smartphones and consumer cameras. Under poor conditions, errors of 10 m (32.8 ft) or more can occur, and accuracy degradation is especially pronounced around buildings and under overpasses due to satellite signal reflection and blockage. In addition, GPS vertical (height) errors tend to be larger than horizontal errors and can be off by several meters from the actual elevation.

For example, a 5 m (16.4 ft) position offset corresponds roughly to the size of a house or the width of a road, which is far too coarse to indicate exact photo locations on-site. If the coordinates included in a photo have that level of error, plotting them on a map will not match the real positions, causing discrepancies when analyzing spatial relationships or reflecting them on drawings later. In short, conventional GPS accuracy is insufficient to meet the needs of detailed construction management and infrastructure inspection.

Why centimeter-level accuracy is required

Why is centimeter-level position information necessary for site photos? One reason is to pinpoint the location of objects or defects on-site. Even for small items like cracks or bolt positions, if you tag photos with coordinates accurate to a few centimeters, you can indicate exact positions on drawings or CAD. This enables spatially precise sharing of “which component and which part has what kind of defect,” allowing accurate prioritization for repair planning. Also, accumulating high-precision geotagged photos allows accurate comparison of changes over time. Because past inspection photos can be overlaid without positional offsets, you can quantitatively assess the progression of deterioration.

Centimeter-level position information also directly improves operational efficiency. If accurate coordinates are obtained at the time of shooting, photos can be plotted to a map system and shared on the spot. This eliminates the verification work of “which photo corresponds to which location” that was previously necessary during photo organization and report creation. Even large numbers of photos taken by multiple people can be automatically organized and placed on a map, greatly reducing the time required for report creation and information sharing. Moreover, improving the accuracy of location information is a key to on-site DX (digital transformation). As promoted by the Ministry of Land, Infrastructure, Transport and Tourism (MLIT) through initiatives like i-Construction, linking construction data to accurate spatial information is expected to advance site management and productivity. Replacing photo location data with centimeter-level accuracy is therefore an important step supporting this digital construction trend.

What is RTK (Real Time Kinematic)?

A representative technology for achieving centimeter-level positioning is the RTK (Real Time Kinematic) method. RTK uses two GNSS receivers (a base station and a rover) to correct satellite positioning errors in real time, enabling position determination within an error range of a few centimeters. While standalone positioning typically has errors of several meters to over ten meters, RTK reduces the error to around a few centimeters. In principle, the base station sends error information from the received satellite signals to the rover (the field device) via communication, and by taking the difference between their observation data, a high-precision relative position is calculated. A major feature is that real-time error correction provides high-precision coordinates on the spot.

RTK operation generally requires a GNSS receiver acting as the base station and a communication method, but network RTK services that utilize the Geospatial Information Authority of Japan’s (GSI) reference station network (e.g., VRS) have become widespread, making high-precision positioning available without setting up your own base station. In Japan, the Quasi-Zenith Satellite System “Michibiki” provides a centimeter-class augmentation service (CLAS), and using a compatible receiver makes cm-level positioning possible from satellite augmentation signals alone even in areas without communication coverage. Furthermore, the emergence of high-performance chips that can simultaneously use multiple constellations such as GPS, GLONASS, Galileo, and Michibiki has made it possible to achieve stable centimeter accuracy even in urban canyons where accuracy was previously difficult to maintain. In this way, RTK—combined with the latest technologies—is making “cm accuracy anywhere” a reality, and is becoming accessible not only to surveyors but also to site technicians.

How to replace photo location data with cm-level accuracy

So how do you specifically replace photo location data with centimeter-level accuracy? The key is to obtain RTK high-precision coordinates at the moment of shooting and link them to the photo data. Traditionally, obtaining high-precision positions required specialized surveying equipment and manually writing coordinates on photos afterward. Today, however, the process has been dramatically simplified by RTK-compatible devices attachable to smartphones and dedicated apps.

For example, RTK-equipped drones automatically record cm-level positioning data for each aerial photo, enabling the creation of high-precision orthophotos and 3D models without post-processing. Similarly on the ground, by connecting an external RTK-GNSS receiver (antenna) to a smartphone or camera, you can add real-time high-precision location tags to every photo shot.

Specifically, the smartphone and RTK device are linked via Bluetooth or a dedicated cable, and correction information is received while taking photos. When the shutter is pressed, the centimeter-precision latitude and longitude obtained on the spot are written into the photo’s Exif metadata. In other words, a photo whose location accuracy was at best around 5 m with just a smartphone can, simply by combining with an RTK device, improve to a few centimeters—effectively replacing the photo’s location data automatically with cm-level accuracy.

The advantage of this method is that you can dramatically increase accuracy without substantially changing on-site workflows. Photographers simply take photos with their usual smartphones or cameras while RTK positioning runs in the background and attaches precise coordinates. There is no need to consciously perform special surveying steps, and the captured photo data can immediately be shared internally via cloud services or plotted on GIS maps. As a result, integrating high-precision positioning with photography improves both the accuracy and efficiency of site records.

Use cases for high-precision photo location data

Photos with cm-level location data can be utilized in various ways in site management. One example is improving the efficiency of infrastructure inspections. In routine inspections of bridges and tunnels, damage points are photographed, but traditionally each photo had to be cross-referenced with drawings to determine “which part it was.” With high-precision geotagged photos, the shooting coordinates are accurately placed on maps or 3D models, so the position identification work for reports is effectively completed at the time of shooting. In one case, during a routine inspection of a road bridge, an RTK unit attached to a smartphone automatically added coordinates to photos, which significantly reduced the time required for photo organization and report generation while improving record accuracy.

Applications in construction management are also notable. For example, as-built verification for paving work (measuring the finished surface) used to require considerable time to acquire as-built data, but high-precision GNSS equipment has made this more efficient. At one paving site, using RTK-capable devices to measure finished elevations reduced the required time by more than 50% compared to conventional methods. Combining photo records with these measurements allows immediate cloud sharing of photo-attached survey data of completed areas, enabling stakeholders to share progress and quality in real time. High-precision photos thus serve not merely as images but as measured data with spatial coordinates useful for construction management and quality verification.

Furthermore, there are extensions to safety management and work analysis. If high-precision devices like LRTK are mounted on workers’ helmets, not only are photos taken during patrol inspections recorded with precise coordinates, but the workers’ movement paths can also be logged accurately. In a large plant demonstration, helmet-mounted devices were used to analyze workers’ location histories to check for inspection omissions and to manage entry into hazardous areas. Enhancing photo location accuracy to cm-level therefore not only improves the reliability of inspection records but also provides a foundation for various on-site data utilizations.

Simple surveying achievable with LRTK

A notable solution for easily achieving the high-precision positioning described above is the LRTK series. LRTK is a lineup of RTK-GNSS products offered by Refluxia Co., Ltd., characterized by a design focused on “compact, lightweight, and high precision” and ease of use in the field. RTK surveying, which was traditionally dominated by stationary or large equipment, has been developed to be introduced more easily and smartly. One flagship model, the “LRTK Pro2,” is a rugged compact unit integrating antenna, receiver, battery, and radio, capable of stable centimeter-level positioning even under harsh construction site conditions. Its compact housing of about 20 cm (7.9 in) in diameter can be mounted on the tip of a survey pole without getting in the way of work, and because it has tilt compensation, it can obtain accurate coordinates even if the pole is slightly tilted.

Main features of the LRTK series:

• Compact and lightweight: A compact design with excellent portability makes it easy to carry and set up on site. For example, the LRTK Pro2 is a size that can be held in one hand while maintaining high robustness, and because it integrates the necessary equipment, you can start field surveying without complicated wiring.

• High-precision positioning: Multi-GNSS support with RTK, and compatibility with Japan’s Michibiki centimeter-level augmentation service (CLAS). It can achieve cm-level positioning from satellite augmentation signals alone even in locations with limited communication. When stationary, errors are under several centimeters; even while moving, it maintains stable accuracy.

• Immediate use and connectivity: Connects to a dedicated smartphone app (LRTK app) via Bluetooth and begins RTK positioning within just a few dozen seconds after power-up. Positioning data can be sent to the cloud via the smartphone or plotted on a map on the spot. It omits complicated initial setup and features an intuitive user interface that site staff can operate without specialized knowledge.

• Wide variety of models: A range of devices to choose from depending on use case. Unique models such as the smartphone-mountable LRTK Phone and the helmet-integrated LRTK Helmet allow you to incorporate high-precision positioning into your work style. The LRTK Phone is a small receiver that attaches to a handheld smartphone and automatically adds cm-level location tags to photos taken with the phone camera—effective for crack surveys and disaster scene documentation where photo reliability is important. The LRTK Helmet mounts a thin antenna on a worker’s helmet so positioning occurs overhead even while both hands are occupied, allowing patrols and inspections on the move to always record accurate locations. It helps prevent gaps in location records during inspections across large facilities and is useful as a safety management tool.

These LRTK devices are already being used on construction and civil engineering sites. For example, at one paving project, the introduction of LRTK Pro2 reduced the time required for as-built measurement to less than half of the conventional time. For road bridge inspections, LRTK Phone automatically added coordinates to photos taken by inspectors, contributing to more efficient report generation and improved data accuracy. Helmet-type devices have been adopted for patrol inspections in large plants, accurately recording worker movement paths to prevent inspection omissions and to support occupational safety management.

By utilizing LRTK, site positioning accuracy and work efficiency can be dramatically improved, as demonstrated in practice. Introducing high-precision positioning technology to the field transforms traditional paper- and manual-based construction management into objective, data-driven management and supports DX of operational processes. The LRTK series is compatible with MLIT’s i-Construction initiative and is poised to become a trump card for smart construction in the industry. Replacing photo location data with cm-level accuracy—LRTK supports that step in a familiar form.

FAQ

Q: What equipment is needed to add centimeter-level location information to photos? A: Essentially, an RTK-capable GNSS receiver and a means of receiving correction information are required. Specifically, an RTK receiver that can connect to a smartphone or camera (for example, a device like the LRTK Phone) and communication to receive correction data from a base station (such as using Ntrip services over mobile networks) are typical. Products that pair with smartphones often only require a Bluetooth connection, with the smartphone app obtaining correction information to enable high-precision positioning.

Q: Is it necessary to have your own base station or internet connection to perform RTK positioning? A: It is not always necessary to prepare your own base station. Today it is common to use network RTK services provided by national or private operators (which distribute correction information using reference stations). If you connect to those services from your smartphone via the internet, you can receive high-precision correction information (such as VRS) nationwide. In Japan, using a receiver that can receive Michibiki (QZSS) CLAS signals allows obtaining corrections via satellite even outside cellular coverage. However, RTK cannot be used where GNSS signals themselves cannot be received, such as indoors or in tunnels. In such cases, alternative ground-based positioning systems or manual location recording in photos are necessary.

Q: How much more accurate is RTK positioning compared to built-in smartphone GPS? A: Typical smartphone built-in GPS accuracy is about 5–10 m horizontally, and vertical errors can exceed 10 m. In contrast, with RTK under suitable conditions, both horizontal and vertical errors can be on the order of about 2–3 cm, or 2-3 cm (0.8-1.2 in). In other words, the error range is reduced to a few hundredths of that of smartphone GPS. For example, where smartphone GPS might record a position offset by about the size of a parking space, RTK can confine the location to an area the size of a palm. This difference in accuracy dramatically improves the precision with which a photo indicates a location.

Q: Do high-precision devices like LRTK require specialized knowledge to operate? A: Recent high-precision positioning devices are designed to be usable even by those without surveying education. The LRTK series, for instance, works with a smartphone app that offers an intuitive interface where you can start positioning by turning on the power and tapping a few buttons. The app clearly indicates whether positioning is successful (e.g., whether a FIX solution is obtained), so special judgments are generally unnecessary. While basic GNSS knowledge and understanding of coordinate systems are beneficial, training materials and support systems are well provided, so there is little cause for concern. After using the device a few times on-site, most users quickly get the hang of it. Many users report that once they experience this convenience and precision, they cannot go back to conventional meter-level accuracy.

Q: How can recorded photos with high-precision location information be used? A: There are many uses. First, embedding coordinates into photos allows plotting photo icons on GIS (geographic information system) maps, creating a map-based archive. In inspection work, clicking a photo icon lets you immediately see the exact location and corresponding image, streamlining reporting. In construction management, as-built photos can be placed on drawings to visualize completed work. If connected to cloud services, photos taken on-site can be shared internally instantly, enabling high-precision situational awareness remotely. Accumulated data can also be analyzed to identify trends. By repeatedly photographing the same point and comparing images, you can quantitatively evaluate the progression of deterioration and use this for preventive maintenance planning. High-precision geotagged photos thus become valuable information assets directly supporting decision-making and asset management, not just records.

Q: I’m concerned about the cost of introducing high-precision positioning systems—can small sites adopt them? A: Although high-precision GNSS equipment once seemed expensive, advances in technology and market expansion have reduced prices. Products like LRTK are offered at accessible price points, and combining them with monthly correction services makes adoption easier for small and medium-sized firms. Considering the operational efficiencies gained—such as bringing outsourced surveying in-house and reducing the time spent organizing photos—the investment can offer significant returns. If initial costs are a concern, flexible options like renting equipment as needed or gradually scaling up devices are possible. It’s a good approach to trial on a small site to experience the benefits before full deployment. High-precision positioning is no longer limited to specialized contractors; it is becoming a tool that provides cost-effective value across a wide range of sites.