Improving Map Accuracy: Correcting Photo Location Info to Centimeter-Level Precision with LRTK

Table of Contents

• Introduction: The Importance of Mapmaking and Location Accuracy

• Challenges of Mapmaking Using Photo Location Data

• How LRTK Works: Correcting Photo Location Info to cm Level Accuracy (half-inch accuracy)

• Expanded Use Cases from Higher Precision (Photogrammetry, Disaster Response, etc.)

• Benefits of Simple Surveying with LRTK

• Future Prospects: Further Evolution of Smartphone Positioning

• FAQ

Introduction: The Importance of Mapmaking and Location Accuracy

Location accuracy is critically important in mapmaking and on-site record keeping. When photos or survey data are reflected on a map, positional offsets prevent accurate situational assessment and measurement, leading to large errors. In infrastructure inspections and disaster surveys in particular, errors of several meters (several ft) can be life-threatening. Until now, cumbersome measures were often required, such as marking photo shooting locations on paper drawings or installing control points for correction. At the same time, with construction DX initiatives promoted by the Ministry of Land, Infrastructure, Transport and Tourism—such as *i-Construction*—there is increasing demand to utilize centimeter-class position data, and a solution that enables anyone on-site to easily achieve high-precision positioning is needed.

However, a recently introduced technology, LRTK, offers a major solution to this problem. By using LRTK, it becomes possible to replace a smartphone’s photo location info with cm level accuracy (half-inch accuracy), dramatically improving the accuracy of mapmaking. This article explains how it works, its benefits, and its use cases in detail. At the end of the article, we also answer common questions in an FAQ format.

Challenges of Mapmaking Using Photo Location Data

Modern smartphones and digital cameras have GPS functions that automatically record location information (geotags) in photos. However, that accuracy typically remains on the order of several meters (several ft). For example, coordinates recorded by a smartphone’s built-in GPS can often be off by 5–10 m (16.4–32.8 ft) or more from the actual shooting location. As a result, when trying later to determine exactly “where on the map the photo was taken,” the point may be shifted, and one can misinterpret the relationship between the photo and the actual location. On-site, a great deal of time has often been spent matching photos to maps to confirm “what location this photo shows.”

When creating 3D models or topographic maps by photogrammetry, low positional accuracy of each photo can cause distortions or scale mismatches in the overall model. Traditionally, to compensate for this, it was necessary to place ground control markers, survey their precise coordinates separately, and use them during photo analysis. In other words, because the photo’s own location data were inaccurate, extra time and cost were incurred.

How LRTK Works: Correcting Photo Location Info to cm Level Accuracy (half-inch accuracy)

LRTK (short for Lite Real-Time Kinematic) is a solution that enables effortless centimeter-class positioning (cm level accuracy (half-inch accuracy)) using a smartphone. Concretely, it consists of a small RTK-GNSS receiver (a high-precision GNSS device) that can be attached to a smartphone and a dedicated app, turning the smartphone into a high-precision surveying instrument. LRTK uses the RTK (Real-Time Kinematic) positioning technique: by receiving error-correction information transmitted from nearby reference stations while calculating the current position, it can reduce typical positioning errors of several meters (several ft) down to within a few centimeters (a few in).

The LRTK receiver features a multi-band GNSS chip and a high-sensitivity antenna, receiving signals not only from GPS but also from GLONASS, Galileo, and Japan’s quasi-zenith satellite system “Michibiki,” among others. Correction data from reference stations can be obtained via network RTK services (Ntrip) over the smartphone’s internet connection. In addition, in mountain areas or disaster sites where mobile communication is out of range and network access is difficult, LRTK supports Michibiki’s centimeter-class positioning augmentation service (CLAS), providing assurance in those situations. An LRTK that can directly receive CLAS signals can maintain high-precision positioning even without network connectivity, as long as satellites are visible overhead.

When RTK positioning is performed with an LRTK device attached to a smartphone, the dedicated app displays high-precision coordinates in real time. Once the positioning mode switches from “Float (approx. 1 m (3.3 ft) accuracy)” to “Fix (approx. 2–3 cm (0.8–1.2 in) accuracy),” you are ready to go. Taking photos with the smartphone camera in that state allows you to obtain the latitude, longitude, and height (elevation) of the shooting point with centimeter precision (cm level accuracy (half-inch accuracy)). LRTK apps also provide functions such as tagging photos with the measured precise coordinates, adding notes or names to shooting points, and saving them to the cloud. For example, when taking progress photos at a civil engineering site, each photo will automatically receive cm-level positional tags as you press the shutter, so the photos’ shooting locations are accurately plotted on the map later. The obtained latitude and longitude are automatically converted into Japan Geodetic Datum coordinates (JGD2011/2020) plane rectangular coordinates and geoid heights, allowing direct import of the data into drawings or GIS software.

Expanded Use Cases from Higher Precision (Photogrammetry, Disaster Response, etc.)

When LRTK elevates photo location accuracy dramatically, mapmaking and on-site recording methods are transformed across various fields. Below are major use cases and their benefits.

• Photogrammetry: Photogrammetry from drones or ground-based photography generates 3D models and topographic maps from large numbers of photos. If each photo is tagged with centimeter-level positioning using LRTK, both the coordinate accuracy and scale accuracy of the overall model are dramatically improved. Models at true scale, which previously required physical markers measured as known points, can now be generated using only LRTK-enabled photos. For instance, when measuring dimensions of structures contained in the images later, measurements based on high-precision photo coordinates can achieve measurement-level errors within a few centimeters (a few in), reducing the need for additional field measurement and improving efficiency.

• Disaster Site Documentation: In earthquake, landslide, or other disaster sites, rapid situational awareness and accurate records are essential. Photos taken with an LRTK-equipped smartphone can be instantly shared to the cloud as precise, non-shifted, coordinate-tagged images. Even where communication infrastructure is severed, CLAS compatibility enables standalone centimeter-class positioning, so position information can be reliably obtained in mountainous areas or immediately after a disaster. Precisely geotagged photos feed directly into disaster response headquarters for decision-making and damage map creation, speeding up planning of relief activities. Previously, matching photos to maps for documentation took time; with LRTK, photos are plotted on the map at the moment of capture, preventing recording errors or omissions and enabling rapid situation sharing.

• As-Built Management and Infrastructure Inspection: High-precision photo location data are powerful for as-built checks (comparing to design shapes) and regular inspections of bridges, roads, and other infrastructure. For example, if progress photos taken across a large construction site are recorded with LRTK, you can later pinpoint “which location a photo corresponds to” accurately on a map. Before-and-after comparisons and remeasurements at the same location become easy, improving construction management accuracy. In infrastructure inspections, even slight offsets in shooting locations can be detected, allowing precise marking and sharing of repair-needed spots with stakeholders. Even new staff are less likely to make location-recording mistakes, and veteran engineers benefit from automated matching of photos to drawings and GIS data, reducing workload.

• 360° Site Documentation: LRTK can also be combined with 360° cameras. By recording all-around images in a single shot while simultaneously logging a high-precision position, full directional site documentation and positioning are completed in one action. Traditionally, photos for each direction and positioning were taken separately and later correlated, but with RTK-linked cameras, pressing the shutter yields a panoramic image with precise positional metadata. The acquired images are automatically plotted on cloud maps, with each photo’s latitude, longitude, and altitude stored as metadata, eliminating the need to trace where each image was taken later. For large-scale progress records and structural inspections, the combination of 360° photography plus high-precision position data creates new value.

Benefits of Simple Surveying with LRTK

Because LRTK can make photo location info centimeter-precise, it is effectively a simple surveying system anyone can use. Previously, centimeter-level positioning required expensive specialized equipment and expertise, but LRTK has changed that landscape. The benefits are summarized below.

• Low-cost deployment: LRTK can be used by simply adding a small device to an existing smartphone, avoiding large initial investments. High-precision positioning that once needed equipment costing millions of yen can now be obtained at an affordable cost per person on-site, making a one-device-per-person setup feasible.

• High precision comparable to professional gear: Cost savings do not mean sacrificing accuracy. Under appropriate conditions, RTK positioning can yield planar accuracy of about 2–3 cm (0.8–1.2 in) and vertical errors of a few centimeters (a few in). Using averaging functions, errors below 1 cm (below 0.4 in) can even be targeted, approaching the precision of traditional total stations and high-end GNSS receivers. The ability for non-surveyor construction management staff to instantly obtain such precision on-site is revolutionary.

• Real-time data sharing: Position and photo data acquired with a smartphone plus LRTK can be uploaded to the cloud and shared on the spot. There is no need to transcribe to paper notebooks or drawings, and everyone involved can view and confirm the latest data before returning to the office. For example, you can share results online with the client immediately after measurement and receive further instructions. Because data are digital from the outset, post-processing to CAD drawings or GIS integration is straightforward.

• Ease of use for anyone: LRTK is designed so that non-specialist users can operate it. High-level correction calculations are automated, and users only need to tap “Start Positioning” or “Record” on the smartphone screen, so even those unfamiliar with the equipment find it intuitive. Lightweight and pocketable, it removes the need to carry heavy tripods and can be quickly deployed when needed. This enables fast measurements between tasks and flexible response to minor checks or emergency surveying needs. Even with shortages of experienced personnel, on-site staff can perform high-precision positioning themselves, improving personnel allocation efficiency.

Because of these features, LRTK can serve as a trump card that simultaneously achieves “higher accuracy” and “greater efficiency” on mapping and surveying sites. Easy acquisition of high-precision photo data and positional information improves situational awareness and the quality of deliverables, increasing the reliability of inspections and reports. Take advantage of LRTK as a simple surveying tool to elevate time- and labor-consuming mapmaking and recording tasks to the next level. Adopt this new mapmaking approach that balances accuracy and efficiency to further improve on-site productivity and quality.

Future Prospects: Further Evolution of Smartphone Positioning

Centimeter-precision positioning using smartphones and LRTK is currently entering a diffusion phase. As technology advances further, this method could well become the industry standard. For example, improvements in smartphone GNSS performance are expected, and some latest phones already support dual-frequency GNSS. In the future, smartphones may be able to perform RTK positioning on their own, making centimeter-level accuracy commonplace even without external devices. Japan’s Michibiki satellite constellation is also scheduled for further augmentation, which should expand coverage and improve accuracy of augmentation signals. Overseas, high-precision satellite services such as Europe’s Galileo are also advancing, and a world where centimeter-level positioning becomes widely accessible is imminent.

In this context, field-oriented solutions like LRTK play a bridging role. By packaging cutting-edge technologies into forms that are easy to use on-site so everyone can benefit, the adoption base for high-precision positioning widens. Eventually, surveying with smartphones will become commonplace, and high-precision mapmaking and data recording will no longer be exceptional but routine. Starting now to use LRTK and accumulate know-how will provide significant first-mover advantages heading into that era.

FAQ (Frequently Asked Questions)

Q. Can a smartphone really obtain centimeter-level position information? A. Yes. With LRTK and under appropriate conditions, positioning including photo location info can achieve high accuracy of about 2–3 cm (0.8–1.2 in) horizontally. If an RTK fixed solution (Fix) is achieved in an open-sky outdoor environment, the recorded position accuracy is incomparably better than standalone positioning. However, accuracy can degrade in environments where satellites are hard to see, such as under elevated structures or in urban canyons; in such cases, averaging functions can reduce error. With careful operation, position information can be obtained at accuracies comparable to traditional surveying equipment.

Q. Which smartphones are supported by LRTK? Is a special device required? A. Currently, LRTK-compatible products are primarily available for *iPhone and iPad* iOS devices (connected via Lightning connector). The key feature is that high-precision positioning becomes possible by attaching to commercially available smartphones, so you don’t need a specially customized surveying terminal. That said, it is recommended to use relatively recent smartphone models (with sufficient processing power and battery capacity) for high-precision processing. Android-compatible versions are expected to be released in the future.

Q. Can it be used in remote mountain areas or immediately after a disaster when there is no network? A. Yes. LRTK can operate even where communication infrastructure is absent. Because it can directly receive Michibiki’s CLAS signals as correction data, centimeter-class positioning can be maintained even when network-based corrections (Ntrip) are unavailable. In practice, LRTK can perform positioning standalone at mountain sites or construction sites outside mobile coverage and later sync data to the cloud. Note, however, that positioning is not possible in environments where GNSS signals cannot reach at all, such as inside tunnels or deep indoors.

Q. How can the acquired positioning data and photos be used? A. Through the LRTK app and cloud services, recorded coordinate data and photos can be exported in common formats. For example, coordinate lists can be output as CSV or SIMA format (the Geospatial Information Authority of Japan’s standard) for import into CAD software or insertion into reports. Photos also contain high-precision latitude, longitude, and altitude embedded in Exif metadata, so they can be imported into commercial GIS software or map services as geotagged photos. Furthermore, LRTK cloud services store uploaded photos linked to coordinates and offer functions to plot shooting points on web maps in a list view. This makes it easy for team members to intuitively share and manage on-site photos and their location information.

Q. Can smartphone LRTK surveying completely replace traditional surveying equipment? A. It depends on the application; replacing everything may still be difficult at present. LRTK covers many surveying and measurement tasks, but for work requiring millimeter-level precision (millimeter-level precision (about 0.04 in))—such as precise layout of structures or precision control in very small areas—dedicated instruments like total stations and levels remain advantageous. Therefore, practical operation typically involves selective use and combination: using LRTK for routine position checks and as-built verification, and complementing it with optical surveying instruments for final inspections or parts requiring very high precision. However, as smartphone positioning reliability continues to improve, occasions requiring dedicated equipment are expected to decrease.