Smartphone-Compatible High-Precision Positioning Leveraging Position Correction Data: LRTK's Labor- and Cabling-Saving Solution

By utilizing position correction information, centimeter-level high-precision positioning has become possible even with smartphones. This is driving reduced manpower and reduced wiring at construction sites and in surveying work, and on-site work efficiency is rapidly improving.

In this article, we carefully explain why position correction information is necessary for high-precision positioning and provide a technical overview (such as RTK and network-based corrections). We also describe concretely how to realize RTK surveying using smartphones and the on-site benefits (reduced manpower, reduced cabling, fast startup, cloud integration). Comparing with conventional surveying equipment, we introduce practical use cases of smartphone RTK, including coordinate acquisition, point cloud measurement, as-built recording, and AR-guided navigation. At the end of the article, we touch on the simple surveying solution provided by LRTK that utilizes position correction information and present the benefits of its implementation.

Table of Contents

• What is position correction information? Why it is necessary for high-precision positioning

• The mechanism of RTK positioning and network-based corrections

• How smartphone-compatible RTK surveying is implemented and its benefits

• Comparison with conventional surveying equipment: What changes with smartphone RTK

• Field use cases of smartphone RTK: from coordinate measurement to point clouds and AR

• Potential for applications across many fields

• LRTK-based simple surveying solution leveraging position correction information

• FAQ

What is position correction information? Why it's necessary for high-precision positioning

First, let's clarify what "position correction information" is. In positioning using satellites (GNSS satellites such as GPS, GLONASS, and Michibiki), small errors in the satellite signals typically cause position offsets of several meters. For example, errors in satellite orbits and clocks, and signal delays caused by the atmosphere (ionosphere and troposphere) are among the causes, and the GPS built into smartphones typically has a position accuracy of around 5–10 meters. To reduce these errors to a few centimeters, it is necessary to apply correction information to the raw data received from GNSS and refine the positioning calculations.

Position correction information is, simply put, "data used to compensate for errors in satellite positioning relative to an accurate reference position." On the ground, reference stations (GNSS receiving stations that serve as reference points with pre-known accurate coordinates) are installed. The correction information service is a system that calculates in real time the difference between the position measured at a reference station and the true accurate position, and transmits that difference to receivers in motion (rovers). By receiving this correction data, the rover (the operator's GNSS terminal) can cancel the error components in its positioning results and determine its current position with centimeter-level accuracy.

Conventional methods known as DGPS (augmentation via a Wide-Area Positioning System) and RTK (Real-Time Kinematic) have used such correction information to improve positioning accuracy. In particular, RTK’s high accuracy has led to its adoption across a wide range of fields, such as civil surveying and autonomous operation in agriculture. In Japan, the Quasi-Zenith Satellite System "Michibiki" also provides a centimeter-class positioning augmentation service (CLAS), making it possible to improve positioning accuracy by receiving correction information directly from the satellites.

Mechanism of RTK Positioning and Network-Based Corrections

Now, let's take a closer look at the technical mechanisms of RTK and network-based corrections.

RTK (Real-Time Kinematic) positioning is a method in which a reference station and a rover perform GNSS positioning simultaneously and use the difference in their observation data to correct errors on the rover side. Because the reference station knows the "true" position without error, it can compute in real time how the satellite signals are offset as seen from that location. That information is transmitted to the rover, which applies it to the satellite signals it receives, reducing errors that would be several meters when operating alone to just a few centimeters. RTK achieves centimeter-level high accuracy by using the satellite carrier phase, a highly precise measurement.

In conventional RTK operations, you had to provide your own reference station (base station) with known coordinates and wirelessly transmit its observation data to the rover. Installing a reference station at each site every time is time-consuming and costly. This led to the emergence of an approach called network RTK. It uses correction information generated from a network of multiple reference stations maintained nationwide by governments and companies, delivered via the Internet. Users do not need to set up their own base station; a single rover (+ a communications link) is sufficient to perform RTK positioning.

A representative example of network RTK is the Geospatial Information Authority of Japan's electronic reference points (GEONET), which has about 1,300 GNSS reference stations nationwide. Based on data from these observation networks, correction information is distributed using virtual reference stations (VRS method), enabling accurate positioning to be maintained even at remote locations. In the private sector as well, telecommunications companies and surveying equipment manufacturers provide correction-data distribution services over the Internet. By simply accessing a contracted distribution service from a smartphone or receiver to receive the correction information, high-precision positioning can be started in almost any location across Japan.

On the other hand, as a correction method that does not rely on networks, there is the aforementioned Michibiki CLAS. CLAS (Centimeter-Level Augmentation Service) is a service that directly provides error correction information via L6-band radio signals from Japan’s Quasi-Zenith Satellite “Michibiki.” Satellite orbit and atmospheric errors are corrected based on data from a national reference point network maintained by the government, and a major advantage is that, with a compatible receiver, positioning accuracy of several centimeters can be obtained even in mountainous areas or outside communication coverage.

However, using CLAS requires a dedicated CLAS-compatible GNSS receiver. Standard smartphones and in-dash car navigation GPS chips cannot decode the L6 signal, so investment in compatible equipment is necessary. Also, because the correction information comes from satellites, a slight time lag has been reported when using it while moving. Even so, CLAS—capable of ensuring stable accuracy across wide areas without relying on communications infrastructure—is a technology expected to see increasing use in various fields such as surveying, construction, and agriculture.

Implementation and Benefits of Smartphone-Compatible RTK Surveying

High-precision positioning that leverages the position correction information described above has become achievable on smartphones in recent years. Specifically, it involves preparing a compact RTK-capable GNSS receiver that can pair with a smartphone and using the smartphone’s communication functions to obtain correction information for positioning. With a smartphone, there is no need to carry dedicated large controllers or surveying instruments, and a single person can easily perform centimeter-precision surveying.

Two main things are required to realize smartphone RTK. One is a high-precision GNSS antenna receiver that supports multi-GNSS and multi-frequency, and the other is a means of accessing services that provide correction information. Regarding the former, conventional equipment has mainly been stationary surveying instruments, but recently pocket-sized GNSS devices that can be attached to smartphones have appeared. For example, by using a small receiver such as the LRTK that mounts on an iPhone/iPad, you can attach a device with a built-in antenna and battery to your smartphone and easily begin high-precision positioning. As for the latter correction information services, the smartphone either connects over the Internet to an Ntrip distribution server to receive data, or, if the device supports CLAS, obtains correction signals directly from satellites.

The advantages of smartphone RTK surveying are significantly greater compared with conventional methods. First, reduced manpower. Traditionally, surveying that involved handling heavy equipment required multiple workers or specialized operators, but with smartphone RTK, field workers can complete surveys single‑handedly, enabling a substantial reduction in dedicated personnel.

Next is Reduced cabling. Since the connection between the smartphone and the GNSS device can be made wirelessly via Bluetooth and the like, cumbersome cable wiring is unnecessary. Furthermore, wiring to external power supplies and radios that was required when installing base stations is also eliminated, shortening equipment setup time and allowing high-precision positioning to begin immediately upon arrival on site. Fast startup is also a major advantage. For example, with network RTK, after powering on the equipment the receiver’s solution switches from a "Float" solution to a "Fix" solution in roughly several tens of seconds to one or two minutes, at which point centimeter-level positioning becomes possible. If base station information and coordinate system settings are preset in the dedicated app, measurements can be started with the push of a single button, providing added convenience.

Furthermore, an additional advantage of using smartphones is cloud integration. Data obtained through positioning can be synchronized to the cloud on-site and shared in real time with the office and stakeholders located remotely. Tasks that traditionally involved saving data to USB drives or SD cards and bringing them back can, with smartphone RTK, be sent directly from the field, dramatically improving efficiency and speed. In addition, dedicated apps on the smartphone integrate functions such as map display, importing drawing data, taking photos, and entering notes, allowing workflows to be completed digitally without carrying paper field notebooks or drawings. By synchronizing data from the field to the cloud, checks of as-built conditions and creation of daily reports can be performed the same day, enabling a significant transformation of the overall workflow.

Comparison with Traditional Surveying Instruments: What Changes with Smartphone RTK

How and in what ways do smartphone-compatible RTK positioning solutions change compared to traditional surveying equipment? We'll compare them from several perspectives.

• Portability and ease of handling of equipment: Conventional GNSS surveying equipment required many peripheral devices such as large receivers mounted on tripods, long poles, external batteries, and radios. In contrast, smartphone RTK can be completed with only a small device that attaches to a smartphone. It is lightweight—only a few hundred grams—and can be carried in a pocket, so it can be taken out whenever needed.

• Initial acquisition cost: High-precision GNSS surveying instruments are very expensive and can cost several million yen for a base station plus rover set. With smartphone RTK solutions, you can make use of a smartphone you already own, and the additional GNSS devices are offered relatively inexpensively. Therefore, deploying one per person is realistic, and organization-wide adoption can significantly reduce costs.

• Ease of operation: With conventional equipment, specialized knowledge was required for on-site equipment setup and calibration and for configuring positioning software. There was also a time lag between obtaining measurement results and importing them into office PCs to reflect them in drawings. With smartphone RTK, intuitive operation via dedicated apps enables one-stop workflows from starting positioning to data sharing. Workers without special training can handle it easily, and responsiveness is improved so measurements can be taken immediately when needed.

• Function integration and scalability: Smartphones are equipped with a variety of functions beyond positioning, such as cameras, accelerometers, and AR displays. With smartphone RTK, you can not only obtain position coordinates but also add high-precision location tags to photos, measure point-cloud data on site, and verify construction accuracy with AR overlays of design drawings—tasks that previously required separate devices can be performed with a single unit. Data can also be centrally managed in the cloud, and new features can be added easily through app updates.

As described above, smartphone RTK has the potential to fundamentally change traditional surveying methods. However, conventional equipment also has advantages, such as stable operation in all-weather conditions and robust durability. By using both according to on-site needs and proactively incorporating smartphone RTK into daily surveying and measurement tasks, improvements in efficiency and productivity can be expected.

Examples of On-site Use of Smartphone RTK: From Coordinate Measurement to Point Clouds and AR

What becomes possible when smartphone RTK is actually used on-site? By combining a smartphone with high-precision positioning, a single device can handle various tasks that previously required specialized equipment. Here are the main use cases.

• High-precision coordinate acquisition: On surveying sites, high-precision GNSS has been used for setting control points and measuring the coordinates of features. With smartphone RTK, you can simply hold the device over any point and press a button to obtain latitude, longitude, and height with centimeter-level accuracy. Recorded coordinates are automatically converted not only to the World Geodetic System (WGS84) but also to Japan’s plane rectangular coordinate system and elevation (geoid height). Point names and notes can be entered on the spot, eliminating the need to transcribe into paper field notebooks and simplifying site records.

• Point cloud data measurement: By leveraging a smartphone camera or LiDAR sensor, it is also possible to capture on-site 3D point cloud data. For example, by combining the LiDAR on the latest iPhone with the high-precision positioning information from LRTK, you can scan terrain and structures while walking to generate high-precision point clouds. Because each point is assigned absolute coordinates, these can be used for as-built management and displacement measurement. A major benefit is that tasks that previously required expensive 3D laser scanners can now be accomplished with a single smartphone. The acquired point clouds can be displayed in a cloud-based 3D viewer or overlaid with design data to analyze differences.

• As-built records and quality control: Smartphone RTK is also useful for recording and inspecting as-built conditions in civil engineering works. If post-construction terrain and structures are uploaded to the cloud as point clouds or coordinate sets, the office can immediately create as-built drawings and display deviations from the design drawings as a heat map to check for excesses or deficiencies. High-precision location information is also tagged to photos taken on-site, allowing you to instantly identify on a map where each photo was taken. Because distances between survey points and enclosed area and volume calculations can be performed within the app, the speed and accuracy of quality control tasks improve.

• AR-based guidance and layout/setting-out: Accurate position information obtained with smartphone RTK can become an intuitive on-site support tool when combined with AR technology. For example, projecting structural models and reference lines from design drawings onto the smartphone’s AR screen allows them to be overlaid on the real-world view. A color-coded display—green when the position is correct, red when it is off—can enable immediate judgment of construction accuracy. When you point the smartphone toward a pre-registered target point, arrows or guide lines are displayed to navigate the operator to stakeout positions or hidden reference points. Stable AR displays in which virtual objects on the smartphone screen do not shift even while walking make it possible for non-experts to accurately identify positions with centimeter-level accuracy.

As described above, a single smartphone RTK can cover a wide range of tasks—from positioning and recording to analysis and guidance. By combining these functions as needed, on-site operations become markedly more efficient than before and contribute to improved quality control and safety.

Broad Applicability Across Multiple Fields

Smartphone-compatible high-precision positioning technology is expected to find applications not only in construction and surveying but across a variety of industries.

• Construction & Civil Engineering: On construction sites and civil engineering works, highly accurate positioning information is indispensable for setting out control points, as-built measurements, and machine guidance for heavy equipment. By using smartphone RTK, site supervisors and workers can perform necessary measurements even without a surveyor on site, leading to labor savings in construction management. Even in confined sites or nighttime work, a pocket-sized smartphone RTK enables agile response.

• Infrastructure Inspection & Maintenance: For inspections of bridges, tunnels, roads, and other infrastructure, it is required to embed location information in photos and record the precise positions of damage. With smartphone RTK, high-precision latitude, longitude, altitude, and camera orientation can be recorded simultaneously with photo capture, improving the accuracy and reliability of inspection reports. Advanced methods such as overlaying past inspection records with current conditions using AR for comparison are also feasible.

• Surveying & GIS: In mapping and GIS data collection, smartphone RTK is a powerful tool. Traditionally, high-precision surveying required licensed surveyors and specialized equipment, but smartphone RTK enables anyone to carry out simple field surveys and situational assessments. Tasks that used to take time—such as cadastral surveys, boundary verification, and disaster damage mapping—can be executed quickly, and importing the collected data immediately into GIS systems can greatly shorten the time to decision-making.

• Agriculture & Forestry: Centimeter-level positioning plays an important role in smart agriculture and precision forestry. Autosteering of farm machinery and drone spraying require high-precision positioning, but traditionally the need to set up proprietary base stations for RTK or to use paid correction services was a barrier. By applying smartphone RTK technology, inexpensive yet precise position data can be obtained for tasks like field partitioning and crop growth monitoring. Moreover, in situations where positioning within vast forests is difficult, CLAS-supported devices can obtain positions even outside communication coverage, making them useful for vegetation surveys and resource management.

• Disaster Prevention & Response: High-precision GNSS is useful for recording damage at disaster sites and for determining drop points for relief supplies. With smartphone RTK devices, even if cellular networks are down, accurate position coordinates can be obtained via CLAS, and damage can be recorded with photos. Even in disaster areas where the terrain has changed, reliable positioning data helps in formulating recovery plans.

In this way, high-precision smartphone positioning that leverages position correction information can be considered a foundational technology with potential new applications across a variety of fields. By flexibly implementing it to meet the needs of each industry, it can lead to the development of new workflows and the creation of new services.

Simple Surveying Solution Utilizing Position Correction Information with LRTK

As a concrete example of smartphone-compatible high-precision positioning, we introduce a solution called LRTK, developed by a startup originating from Tokyo Institute of Technology. LRTK (Eru Aru Tī Kē) is a simple surveying system composed of a compact RTK-GNSS receiver device that attaches to a smartphone, a dedicated app, and a cloud service. By using this solution, on-site operations that fully leverage the advantages of the smartphone RTK mentioned above become possible.

The LRTK device is a slim unit that can be attached to an iPhone or iPad. Weighing approximately 125 g and small enough to fit in a pocket, it nevertheless houses a multi‑frequency GNSS antenna and a battery. It connects wirelessly to the smartphone via Bluetooth, so no cumbersome cables are required. Satellite data captured by the device is processed in real time through the smartphone’s dedicated "LRTK" app. Correction information can also be obtained from this app; in addition to network RTK, it supports receiving correction signals from Michibiki (CLAS) when outside cellular coverage. This provides reassurance that positioning can continue even in places without mobile signal, such as mountainous areas or underground spaces.

The dedicated LRTK app includes a variety of features such as single-point positioning, continuous positioning (track logs), capturing geotagged photos, and AR guidance display. For example, with the push of a button you can measure and record your current coordinates, or perform terrain surveying while walking using continuous logging mode. In photo mode, when you take a picture with your smartphone camera it simultaneously records the location’s latitude, longitude, elevation, and shooting direction, making it easy to later verify the photo’s position on a map. Measurement data and photos can be synchronized directly to the cloud (LRTK Cloud) and shared with stakeholders without returning to the office.

Advanced analysis features are also provided, such as visualizing 3D point cloud data captured on-site in the cloud and color-coding measurement points by comparing them with design drawings. For position guidance during pile driving and equipment installation, previously recorded points can be set as target locations, enabling arrow-guided navigation on a smartphone screen. Because a stable AR display is possible without the model shifting as you walk around, even non-experts can accurately pinpoint target positions within a few centimeters.

LRTK is, as described above, a solution that embodies the keywords labor-saving, reduced wiring, and fast startup. On actual job sites, it is increasingly used as a "one-device-per-person" surveying instrument—carried in a pocket and taken out at a moment’s notice to perform measurements and record data. In terms of price, it is easier to adopt compared with conventional surveying equipment, enabling even small companies and departments to readily implement high-precision positioning. With a smartphone and LRTK, anyone on site can measure positions like a surveyor, share data instantly, and improve the quality of construction and inspection—such an era is becoming a reality.

FAQ

Q1. What is position correction information? A1. It is differential data provided by reference stations to compensate for errors in satellite positioning. GPS-only positioning can produce errors of several meters, but using correction information can reduce those errors to around a few centimeters.

Q2. What is required to achieve centimeter-level positioning with a smartphone? A2. You need a high-precision GNSS receiver that supports multiple frequencies and a means to obtain correction information. Specifically, it can be achieved with an RTK-capable GNSS device that can connect to a smartphone (e.g., LRTK) and either a subscription to a network-based correction service or a receiving environment that supports CLAS.

Q3. What level of accuracy can be achieved? A3. It depends on the environment and satellite reception conditions, but if an RTK FIX solution (integer solution) is obtained, both horizontal and vertical accuracy is generally on the order of ± a few centimeters. If multiple measurements are taken while stationary and averaged, accuracy can sometimes be under 1 cm.

Q4. Can positioning be performed in locations without cellular signal? A4. Yes, it is possible. Network RTK requires a communication environment, but receivers compatible with QZSS CLAS can obtain correction information even without an internet connection. Devices that support both, such as LRTK, can continue positioning with CLAS even when out of coverage, so you can rest assured.

Q5. Compared with conventional surveying equipment, are there any problems with reliability or stability? A5. Smartphone RTK has ease of use as a major advantage, but compared with dedicated equipment its durability and stability during long continuous use can be inferior. Even so, it sufficiently meets the accuracy and reliability required for routine surveying and inspection work, and in fact adoption in the field is progressing. Rather, automatic backups via cloud integration and the ongoing improvement of features through software updates can be said to be major strengths.

Q6. 初期導入やランニングコストはどのくらいですか？ A6. Initial setup costs are significantly lower than those for conventional surveying equipment. Because you can start with a high-precision GNSS device and a smartphone, you can establish a centimeter-accuracy positioning setup with a budget on the order of a few hundred thousand yen per person. Fees for correction information services are generally free for public services, while private services typically charge several thousand to tens of thousands of yen per month. However, solutions that include correction information, such as LRTK, have emerged, and plans that can be used without worrying about operating costs are also available.