Correct EXIF Location Information of Construction Photos with LRTK: Achieving Reliable Location Records

Table of Contents

• What is EXIF location information?

• Problems Caused by Location Drift

• Methods to Correct EXIF Location Information

• High-Precision Positioning with RTK

• High-Precision Location Recording for Construction Photos with LRTK

• Benefits of Accurate Location Records on Site

• Simple Surveying Solutions with LRTK

• FAQ

What is EXIF location information?

Exif (Exchangeable Image File Format) is metadata contained in digital photo files that can record information such as the shooting date and camera settings, as well as the latitude and longitude of the shooting location. When you take photos with a smartphone or a GPS-equipped camera, location information (so-called geotags) is automatically embedded in the photo data. In construction photos, this location information is an important record indicating “where the photo was taken.” Especially on large construction sites or during infrastructure inspections, accurately recording the shooting location makes it clear which part of the site is shown when photos are reviewed later. For example, even on sites with many similar structures, a location tag on a photo allows you to identify the exact spot—such as “near ○○ in the north area of the site”—making it easier for all stakeholders to share the same understanding.

In recent years, including location information (GPS coordinates) in photo data has become mandatory in electronic delivery guidelines for public works. For that reason, accurately recording the EXIF location information of construction photos is a key element that supports the reliability of construction process evidence. By clearly indicating the shooting location in addition to the image itself, photos can be easily linked to drawings and survey data during quality control and report preparation. In other words, ensuring the reliability of location information in construction photos greatly improves the accuracy and trustworthiness of site records.

Problems Caused by Location Drift

However, the GPS built into smartphones has accuracy limitations. Generally, coordinates obtained by a smartphone are said to have an error of approximately 5-10 m (16.4-32.8 ft), and in some cases they can deviate even more. In places with poor satellite reception—such as the gaps between high-rise buildings or mountainous areas—positions can even shift by tens of meters. In practice, conventional smartphone or digital camera location tags often show errors of several meters, and attempts to precisely match the photo’s shooting point on a map may not align.

When the recorded location differs from the actual location, confusion arises when interpreting site conditions from the record photos. Photos taken at different locations may be misinterpreted, or you may later be unsure “where this photo was taken.” For example, on a large construction site, an error of 5 m (16.4 ft) could cause a photo to be incorrectly associated with an adjacent section or structure. Also, when compiling photos into reports, each image may need to be individually checked and corrected to compensate for location drift, creating a large workload both on site and in the office.

As a countermeasure to such location drift, it has been common to add supplementary information to photos manually. For example, notes like “near ○○” or “close to △△” may be written in a photo ledger, or photos may be cross-referenced with numbered points on a site sketch. However, these analog methods impose heavy work burdens and carry the risk of human error. If the EXIF location information embedded in photo data is itself incorrect, it must be corrected later to the true coordinates, but that correction is a specialized task and still leaves questions about the credibility of the record. In short, under the current conditions, EXIF location information in construction photos is insufficient to be considered a “reliable record,” and a fundamental improvement in accuracy is required.

Methods to Correct EXIF Location Information

If the location information recorded in Exif is incorrect, it is possible to correct (edit) it afterward. Using dedicated Exif editing software or photo management apps, you can rewrite the latitude and longitude values embedded in an image file and update them to the correct coordinates. For example, you can identify the accurate position on a map, manually enter those coordinates into the photo data, and apply the changes. There are also tools that automatically reassign location tags by matching the photo’s timestamp with GPS logger track data. Using these methods, it is theoretically possible to correct Exif location information to the correct values after noticing location drift.

However, manual correction of Exif location information has many drawbacks. First, determining the correct coordinates in the first place is not easy. To precisely identify the shooting location you ultimately need coordinates measured with separate surveying equipment or careful positioning on a map. Even correcting errors of a few meters by eye or estimation can introduce new inaccuracies. Second, the workload of correcting multiple photos is very large. While one or two images may be manageable, it is unrealistic to check and overwrite coordinates on dozens of photos one by one on a map. Additionally, rewriting Exif data with common image editors may be regarded as data tampering. For photos submitted for public works, altering metadata after shooting can undermine credibility, so casual corrections are not recommended.

For these reasons, retrospectively correcting the location information of construction photos is inefficient and undesirable from the perspective of record reliability. It is far more effective to introduce a system that can record high-precision location information from the outset and eliminate drift at the time of shooting. The key to this is the high-precision positioning technology called RTK (Real Time Kinematic), which is discussed next.

High-Precision Positioning with RTK

RTK (Real Time Kinematic) is a method that enables high-precision position measurement by correcting satellite positioning errors in real time. Normally, positioning using GNSS satellites such as GPS yields a standalone receiver accuracy of only a few meters due to small errors in the satellite signal. Factors such as satellite orbit and clock errors and signal delays in the atmosphere are the main causes, and smartphone GPS alone typically shows deviations of around 5-10 m (16.4-32.8 ft). RTK, however, uses a reference station (base station) whose exact position is known in advance; it calculates in real time the difference (error) between the position measured at the base station and the satellite-derived position. That correction data is sent to the moving receiver (rover), and by cancelling the error components in the rover’s position solution, centimeter-level accuracy is achieved. In other words, by taking the difference between a “stationary positioning point” and a “moving positioning point,” RTK drastically reduces positioning errors.

In traditional RTK surveying, the user needed to set up a base station on-site and transmit correction information via radio while working. Recently, however, networked RTK services provided by government and private organizations make high-precision positioning possible with only a rover unit at hand. For example, Japan’s Geospatial Information Authority maintains a GNSS reference station network called GEONET with approximately 1,300 locations, and many telecom and surveying equipment manufacturers offer paid correction services. By using these services, RTK positioning can be performed without deploying a base station at each site. Also, systems that enhance positioning accuracy by receiving correction signals directly from satellites—such as the centimeter-level augmentation service (CLAS) provided by Japan’s Quasi-Zenith Satellite System “Michibiki”—are emerging. Receivers compatible with CLAS can achieve centimeter-level positioning without internet access, even in mountainous or maritime environments.

By using RTK technology and correction information, positioning errors that used to be on the order of meters can be reduced to centimeters. This accuracy rivals that of conventional high-end surveying equipment and is increasingly used in fields such as civil engineering surveying and agricultural autonomous driving. Notably, solutions that enable this RTK high-precision positioning to be used easily with smartphones have recently appeared. The next section looks in detail at LRTK systems that use a smartphone and a dedicated device to correct the location information of construction photos in real time.

High-Precision Location Recording for Construction Photos with LRTK

LRTK is a high-precision location recording system that makes RTK-style centimeter-level positioning easy to use with a smartphone. It consists of a pocket-sized dedicated RTK-GNSS receiver (LRTK device) that attaches to a smartphone and an app, allowing you to obtain your current position with accuracy far beyond the smartphone’s built-in GPS. With LRTK, errors that were previously over 5 m (16.4 ft) with smartphone GPS can be reduced to within a few centimeters (within a few inches), rivaling professional surveying instruments. When you power on the dedicated device on site and connect it to your smartphone, real-time corrections to GNSS satellite data begin, and high-precision positioning is achieved in a short time. The app displays the RTK reception status (satellite count and correction status), and once a Fix solution (integer solution) is obtained and accuracy stabilizes, it’s time to shoot. Once ready, simply take photos with the smartphone camera and the centimeter-precision coordinates measured at that moment are automatically recorded in the photo file’s Exif metadata.

With LRTK, taking a construction photo and obtaining a high-precision position are accomplished in one action. Previously, you might have needed to obtain GPS coordinates separately after taking a photo or to reconcile photos with survey data, but with LRTK a single shutter press automatically records everything about “when, where, and in which direction the photo was taken.” In addition to latitude, longitude, and altitude of the shooting point, the camera’s orientation (bearing) from the smartphone’s orientation sensor is also saved in the metadata, so you can know “which direction the photo was taken” for each image. Site personnel do not need to switch to specialized surveying equipment; they can capture the situation in front of them with a smartphone and obtain photo data with reliable positioning information.

Captured photo data can be immediately reviewed or shared on site. The LRTK app can plot each photo’s shooting location on a map so you can visually confirm “which point the photo corresponds to” right after shooting. Because the photo files include height information as well as latitude and longitude, they can easily be imported into other GIS software or CAD drawings as needed. Furthermore, LRTK systems often include a function to upload photos and location data to the cloud in real time. When photos are synchronized to the cloud, each shooting point is automatically shown as a pin on a map, making it obvious where photos were taken even from the office. In this way, LRTK dramatically improves the accuracy and convenience of recording location information for construction photos and greatly streamlines site record workflows. It is an intuitive tool that does not require surveying expertise, allowing anyone to leave reliable location-attached photos on site.

Benefits of Accurate Location Records on Site

When LRTK allows construction photos to be recorded with accurate location information, various positive effects appear in site record management and work efficiency. The main benefits are summarized below.

• Prevention of recording errors: Mistakes in location assignment or transcription are eliminated, and the correspondence between photos and site locations can be securely understood. This reduces the risk of confusing which photo belongs to which area, even on sites with multiple construction zones.

• Streamlined reporting and organization: With coordinates automatically attached to photos, post-shooting organization work is greatly simplified. There is no need to manually cross-check photos with drawings or add explanatory notes, shortening report preparation time. Because location data is standardized, daily reports and inspection reports can be automatically generated in the cloud, reducing administrative workload.

• Real-time information sharing: Accurate location-attached photos can be shared with stakeholders via the cloud immediately after shooting. By viewing photos plotted on a map, managers in the office can intuitively understand site conditions and make rapid decisions or issue instructions. On wide-area equipment inspections, seeing which point a photo corresponds to at a glance prevents reporting omissions and communication errors.

• Time-series comparison and verification: Regularly photographing the same point allows precise tracking of changes over time. Because positions are recorded consistently, photos taken before and after construction or at different inspection times can be strictly compared, making it easy to assess deterioration or verify construction outcomes. Fixed-point observation that was previously difficult becomes possible, contributing to more accurate maintenance planning.

• Digital integration and expanded uses: High-precision location information enhances integration with other digital data. Coordinates embedded in photos can be used to place images on GIS maps or CAD drawings, or combined with surveying results to create 3D models. Because location-attached photos themselves can become reliable parts of surveying results, site records that once relied on paper drawings or verbal explanations can evolve into data-driven workflows.

• Improved compliance and credibility: Photos with accurate Exif location and timestamp information have greater value as objective evidence. Proper Exif information is valued in inspections of public works and helps avoid errors or rejections during electronic delivery. There is also less concern about later accusations of data tampering, allowing contractors to strongly demonstrate the credibility of their construction records to clients and third parties.

Simple Surveying Solutions with LRTK

LRTK is also an all-in-one solution that powerfully supports simple surveying on site. In addition to enabling high-precision location-attached photos, it is designed so that basic surveying tasks that formerly required specialized equipment or multiple personnel can be executed easily with a single smartphone. The dedicated device is lightweight and compact at about 125 g, eliminating bulky tripods or cables. You can carry it in your pocket and, when needed, attach it to your smartphone—making it revolutionary in that anyone can start centimeter-level (half-inch accuracy) surveying alone at any time.

With LRTK you can flexibly handle a variety of on-site positioning needs. For example, tasks such as stakeout and guiding to reference points—where you search for a specified position based on preset coordinates—can be completed simply by following on-screen guidance. Tasks that previously required two survey personnel, like setting out batter boards, can be accomplished by a single worker using LRTK’s coordinate guidance. Even without surveying calculation knowledge, you can record the coordinates of arbitrary points with a tap in the LRTK app and upload measured point cloud data to the cloud on the spot. Acquired data can be exported in common formats such as GeoJSON, making it easy to incorporate into drawings or analyze in other systems later. In short, with LRTK the entire surveying workflow—“measure, record, share”—can be executed by anyone, and necessary location information can be obtained immediately without relying on surveying professionals.

The introduction of this simple surveying solution yields significant effects, simultaneously improving on-site productivity and data accuracy. In times of labor shortages, the ability for a single person to perform surveying contributes to labor savings, and the time spent transporting and setting up equipment is greatly reduced. Real-time cloud sharing of positioning data enables immediate checks in the office and smooth information sharing among stakeholders, preventing rework and enabling quick decisions. Compared to traditional methods that relied on paper field books or manual entry, on-site surveying using LRTK evolves into a far more efficient and reliable process. Without purchasing expensive surveying instruments, being able to handle centimeter-precision location information with only a smartphone and a small device strongly promotes on-site digital transformation (DX).

FAQ

Q. Can I correct the location information of photos I already took? A. It is possible, but not recommended. Using specialized software to rewrite a photo’s Exif metadata allows you to correct latitude and longitude. However, if you do not know the accurate coordinates, the correction is meaningless and you will ultimately need to re-measure on site. Also, modifying data after shooting can raise questions about the authenticity of the record. By using LRTK, accurate locations are automatically recorded at the time of shooting, avoiding the effort and risks of later corrections.

Q. What are the advantages of using LRTK instead of a smartphone’s built-in GPS? A. The greatest advantage is a dramatic improvement in positional accuracy. Smartphone GPS alone has errors of about 5-10 m (16.4-32.8 ft), whereas LRTK records positions with centimeter-level (half-inch accuracy). This difference greatly affects the reliability of photos and survey data. LRTK also records photo bearing and altitude information and enables smooth data sharing via the cloud. In short, LRTK provides reliable and rich location information that cannot be achieved with ordinary GPS.

Q. Is operating LRTK difficult? Do I need specialized knowledge? A. The operation is very simple and does not require special expertise. Attach the dedicated device to your smartphone and launch the app, then follow the on-screen prompts to shoot and measure. Complex settings and calculations are handled automatically by the system. The interface is designed to be intuitive so that site staff can become proficient with minimal training.

Q. Can high-precision positioning be performed at sites outside of mobile coverage? A. Yes. LRTK supports the centimeter-class augmentation service (CLAS) provided by Japan’s Quasi-Zenith Satellite “Michibiki,” enabling high-precision positioning from satellite correction signals even outside mobile coverage. Therefore, in open-sky areas such as mountainous sites or remote islands, centimeter-class (half-inch-class) records can be obtained. However, as with normal GPS, positioning is difficult indoors or inside tunnels where satellite signals do not reach.

Q. How can the captured photo data be used and shared? A. Photos taken with LRTK are stored on the smartphone and can also be synchronized with the cloud. On the cloud, each photo is shown as a pin on a map and can be viewed or downloaded via a web browser by location. The Exif-embedded location information can be exported in common formats, making it easy to plot photos on GIS maps or place them in CAD drawings. When sharing within an organization, showing photos with maps smooths communication of site conditions. In short, photos obtained with LRTK are easy to reuse in various ways as site records, directly improving the efficiency of reporting and verification tasks.