Improving the Positional Accuracy of Construction Photos: EXIF Location Correction with LRTK

Table of Contents

• Background: Why improved positional accuracy for construction photos is needed

• What is Exif? Location data contained in photos

• Smartphone GPS errors and challenges in recording location data

• High-precision location correction using RTK

• Features of LRTK and its application to EXIF location correction

• Benefits of photos with high-precision location data

• What is simple surveying with LRTK?

• FAQ

Background: Why improved positional accuracy for construction photos is needed

On construction sites, work record photos serve as evidence of progress, and accurate recording of the shooting date/time and shooting location (positional information) is required. Especially for public works, the Ministry of Land, Infrastructure, Transport and Tourism’s "Digital Photo Management Information Standards" and "Electronic Submission Guidelines" require that photo data (JPEG, etc.) include correct date/time and location information in the Exif metadata. Therefore, the reliability of location information indicating when and where a photo was taken is extremely important.

Methods for recording location information in work photos include using the GPS data in Exif, automatically adding location along with site information via electronic blackboard apps, or photographing an analog blackboard (photo slate) and later entering the location manually. These methods vary in efficiency and reliability for data management; leaving GPS info in Exif is convenient because it requires no special equipment, but it has issues regarding positional accuracy and tamper prevention.

In practice, GPS location data provided by ordinary digital cameras or smartphones often include errors on the order of several meters. Even if location information is recorded on site, large errors make it difficult to determine exactly which point the photo represents, causing extra work in preparing reports or verifying as-built conditions. Also, if location data are substantially off and later corrected, there is a risk of being regarded as data tampering, which could be flagged during electronic submission inspections. Against this background, improving the positional accuracy of construction photos has become a major issue.

What is Exif? Location data contained in photos

Exif is a standard for recording shooting information inside digital photo files. It contains various metadata such as camera model, shooting date/time, and lens settings; when shot with a smartphone or a camera with built-in GPS, location information (latitude and longitude) is also automatically written into the Exif data. Exif location data are generally recorded in latitude and longitude coordinates of the World Geodetic System (WGS84).

Exif information is also used in construction industry photo management. If Exif contains location information, each photo’s shooting location can be checked later on a PC or in the cloud, streamlining photo organization and report preparation. As noted above, for electronic submission it is important that Exif contains correct location information, so keeping GPS recording enabled in device settings is essential.

Smartphone GPS errors and challenges in recording location data

Location data recorded by smartphones or ordinary GPS cameras inevitably include some error, due to the characteristics of satellite positioning and the influence of radio wave conditions and the atmosphere. Even outdoors in clear conditions with good visibility, typical GPS positional accuracy is said to be on the order of a few meters in radius; in urban areas surrounded by tall buildings or in mountainous regions, errors of tens of meters or more can occur. Indoors or inside tunnels, GPS signals may not be received at all, preventing acquisition of location information.

Such GPS errors are a challenge in managing construction photos. If photo positions are off by a few meters, you can generally grasp the approximate place, but when precise identification of the construction location is required, this can be problematic. For example, it may become unclear which of adjacent structures was photographed, or photos used to confirm site boundaries may lack precise location information, causing confusion when later comparing with drawings. Significant positional shifts may also lead to queries from the client, requiring re-photography or additional surveying. Correcting Exif location information is therefore important to reduce these risks and increase the reliability of photo data.

High-precision location correction using RTK

RTK (Real-Time Kinematic) is a technique that corrects GNSS (such as GPS) errors. Error information measured at a reference station is sent in real time to a mobile receiver (rover) to compute high-precision positions. Errors of several meters in standalone positioning can be reduced to centimeter-level accuracy with RTK (horizontal position at about ±2–3 cm (±0.8–1.2 in)). RTK is a reliable method also used in public surveying, and with the recent spread of compact GNSS receivers it has become more accessible for field use.

Applying RTK-based high-precision positioning to construction photos allows positional information of the shooting point to be corrected on site to centimeter-level and recorded in Exif. For example, with an RTK-enabled camera (RTK-integrated camera), the high-precision latitude and longitude are tagged to the photo at the moment the shutter is pressed. Alternatively, an external RTK-GNSS receiver can be connected to a smartphone or camera to obtain high-precision coordinate data at shooting time and later reflect it in the photos. In Japan, by using satellite positioning augmentation services (for example, the CLAS signal from the Quasi-Zenith Satellite Michibiki), centimeter-class positioning can be achieved in real time even at sites without Internet connectivity, such as mountainous areas. By leveraging RTK in this way, conventional Exif location information with large errors can be dramatically improved, greatly enhancing the reliability of construction photos.

Features of LRTK and its application to EXIF location correction

LRTK is a compact GNSS device and system that makes the RTK technology described above easy to use. It is a product developed by Refixia Inc., a startup originating from the Tokyo Institute of Technology, and is attached to a smartphone for use. The receiver is integrated into a smartphone case, weighing about 165 g with a thickness of about 13 mm (0.51 in), making it very thin and lightweight. It can be attached to a smartphone with one touch and is powered by an internal battery, so no cable connection is required. The catchphrase is literally "a surveying instrument that fits in your pocket," emphasizing its portability and immediate usability on site.

When the LRTK device is attached to a smartphone and the dedicated app is launched, RTK positioning begins on the spot. The device receives satellite signals with high sensitivity and can achieve instant centimeter-class positioning using network-based correction data (Ntrip) or the Michibiki CLAS signal. The app displays the RTK reception status (number of satellites and Fix acquisition status), allowing users to confirm accuracy while shooting. Moreover, the app performs surveying-specific processes such as conversion to a plane rectangular coordinate system and automatic calculation of geoid height, so accurate positioning results can be obtained without specialized knowledge. When a photo is taken at the timing of an RTK fixed solution (Fix), high-precision latitude, longitude, and altitude are automatically recorded in that photo’s Exif metadata.

Photos taken with a smartphone are not only stored on the device but can also be synchronized with the cloud in real time. In the cloud, coordinates for each photo are plotted on a map, enabling the office PC to immediately check or share shooting locations. In other words, using LRTK allows the previously separate tasks of "taking photos" and "positioning" to be performed at once, and the acquired data can be used immediately.

Benefits of photos with high-precision location data

Attaching high-precision location information to site photos yields various benefits. The main advantages are as follows.

• Easier photo-to-map matching: When photo data are tied to accurate coordinates, shooting locations can be precisely plotted on a map. Even on large sites, it becomes immediately clear "which point this photo is of," helping prevent omissions in records and facilitating understanding of the management scope.

• More efficient report creation and sharing: Location-tagged photos can be used directly as materials for as-built management and reports. There is no need to search for shooting locations on drawings, and site conditions can be accurately reproduced from the office, making reporting and discussions smoother.

• Error prevention and reduced re-photography: Because even novices have accurate location recorded automatically, troubles such as "I took a photo but forgot to note the location" or "re-taking due to incorrect location recording" can be avoided. Having photo data allows later tracing of locations with confidence.

• Easier time-based comparison and verification: High-precision location tags are powerful when comparing photos of the same point over time. For example, photos taken before, during, and after construction can all be managed on the same coordinates, making it intuitive to grasp changes and verify as-built conditions. Past photos can be referenced at exact locations for future maintenance or inspection.

• Integration with surveying and design data: Coordinates included in Exif can be used in other systems. It is easy to import photos into GIS software or CAD drawings and place site photos on design drawings. In photogrammetry, which generates high-precision point clouds from photos, higher positional accuracy at shooting time improves model scale and alignment, resulting in more reliable 3D records.

• Improved on-site safety: Being able to record photos and positions simultaneously reduces the number of surveys required in hazardous areas. Also, since position information can be obtained just by taking a photo even at height or on scaffolding, workers do not need to awkwardly change posture to take notes. This reduces worker burden and enhances safety.

For example, at a large-scale earthwork site that had previously relied on smartphone GPS, confirming photo positions took a long time and errors occurred when pasting photos onto drawings. After introducing LRTK, all photo coordinates were automatically acquired at shooting, significantly reducing the time required to organize site records and streamlining report preparation. Specifically, a task that previously took over an hour each day to organize dozens of photos into a ledger became almost fully automated, requiring only minimal checks. Inexperienced staff no longer made location recording mistakes, making it possible to confidently delegate photo shooting. On-site staff reported that they were relieved not to be overwhelmed by photo organization.

Thus, when each photo carries accurate location information, the value of site records increases and both operational efficiency and quality control see substantial benefits. Using high-precision photos contributes to construction DX (digital transformation) by reducing labor for tasks that previously required specialists. It also aligns with policies proposed by the Ministry of Land, Infrastructure, Transport and Tourism, such as i-Construction, and the standardization of high-precision digital records in site DX is expected to continue. In practice, with LRTK, photo shooting itself becomes a form of simple surveying, enabling anyone to easily obtain positioning data on site. Next, let’s look at this simple surveying with LRTK.

What is simple surveying with LRTK?

Simple surveying with LRTK is a new surveying method that combines a compact RTK-GNSS receiver with a smartphone and a dedicated app, allowing anyone to perform centimeter-class positioning without complex equipment or advanced setup. By attaching an LRTK receiver to a smartphone and walking the site, high-precision position data are automatically obtained and can be managed and shared in the cloud. Tasks that used to be entrusted to experienced surveyors, such as stakeout (setting out) and as-built measurement, can now be performed accurately by novices using intuitive guidance in the smartphone app (such as AR navigation). In other words, simple surveying with LRTK delivers RTK high-precision positioning with more user-friendly equipment and operation, and its effectiveness has already been demonstrated on many sites.

LRTK can also link with a smartphone’s AR (augmented reality) functions to overlay design models on site imagery or virtually indicate the locations of underground utilities. With centimeter-level positioning, AR-displayed objects have minimal positional drift, enabling more intuitive and accurate on-site verification. This allows visual confirmation of discrepancies between design and site on the spot and assists in reviewing construction plans and verifying as-built conditions.

FAQ

Q: Is the position information from a smartphone’s built-in GPS insufficient? A: A typical smartphone GPS can be useful for a rough understanding of where a construction photo was taken. In many cases, if the error is on the order of a few meters, the approximate shooting location can be determined, so this has been acceptable historically. However, large errors can cause confusion where it matters, and in cases requiring high precision, smartphone GPS is inadequate. In particular, for strict as-built verification or boundary confirmation, errors of a few meters may not be acceptable. Therefore, correction with RTK-capable equipment is desirable to reliably record correct positions, which reduces rework and improves work efficiency. Ultimately, required accuracy depends on the site, but to increase photo reliability and avoid unnecessary rework, we recommend recording with high precision from the start.

Q: What level of accuracy is required for construction photo location information? A: It depends on the client’s standards, and official numeric thresholds stating "this level of accuracy is required" are rare. What matters is that the photo location can be objectively verified. For general site records, errors of a few meters may be acceptable, but where as-built measurement or precise construction management is required, higher accuracy is demanded. If location information is significantly off, there is a risk of being flagged during inspections. In some cases, the client’s special specifications may require high-precision positioning, so check project-specific requirements. If you want to be certain, recording with RTK to achieve centimeter-level accuracy from the start is recommended.

Q: What is RTK? A: RTK (Real Time Kinematic) is a technology that corrects satellite positioning errors in real time to improve positioning accuracy. By relative positioning with a nearby reference point, RTK reduces errors of several meters in standalone GPS to on the order of a few centimeters. Originally a surveying technique, RTK has recently become smaller and cheaper, allowing typical smartphones to take advantage of RTK via external devices.

Q: Is it okay to correct Exif location information after shooting? A: Technically, it is possible to rewrite a photo’s Exif data to change location information, but caution is required. For electronic submission of construction photos, it is required that data not be tampered with after shooting. Editing location information post-shooting could cause a photo to be considered non-original. If correction is unavoidable, use dedicated software that leaves a change history or otherwise take measures to ensure reliability. Ideally, prepare to record correct location information at the time of shooting.

Q: What should be done in places where satellites cannot be received, such as inside tunnels or indoors? A: Unfortunately, GNSS positioning using LRTK cannot be performed in environments where satellite signals from outdoors cannot reach. In such cases, you can obtain coordinates for a reference point at a location where satellites can be received (for example, near the tunnel portal), and from there confirm positions inside using conventional surveying instruments (total stations, etc.) or design drawings. Record indoor point coordinates corresponding to photos and, if necessary, later add them to Exif. When supplementing or editing Exif location information afterward, it is also important to preserve change histories and otherwise ensure data reliability.

Q: What is simple surveying with LRTK? A: Simple surveying with LRTK is a new surveying method using the compact RTK-GNSS receiver "LRTK" and a smartphone app, enabling anyone to perform centimeter-class positioning easily. By attaching LRTK to a smartphone and walking while acquiring high-precision coordinates, you can sequentially record survey point coordinates. The data are automatically managed and shared in the cloud, allowing immediate comparison with drawings or team sharing of on-site positions. Tasks such as stakeout or as-built checks that previously required surveying specialists can be performed by workers following LRTK’s AR navigation. Because it requires no complicated setup or large equipment and can be learned with short training, it is easy for anyone to use. Tasks that used to take two to three people can be completed by one person, contributing to measures against labor shortages and improving safety in hazardous areas. Many projects promoting site DX have already introduced LRTK, significantly contributing to efficiency and labor reduction. SSS