Table of Contents

Translation target:

• Mechanism and background of RTK surveying

• Emergence of smartphone RTK surveying and the technical characteristics of LRTK

• The basis for centimeter-level accuracy achievable with smartphone RTK

• Applications to point cloud measurement, as-built management, CAD integration, AR guidance, and cloud sharing

• Field deployment case studies and their effects (reduced manpower, time savings, improved safety)

• Barriers to adoption and tips for field utilization

• Introduction of simplified surveying using LRTK

• FAQ

Mechanism and Background of RTK Surveying

A technology indispensable for conducting precise surveying is RTK surveying. RTK stands for *Real Time Kinematic（リアルタイムキネマティック）* and refers to a real-time, high-precision positioning method based on satellite positioning (GNSS). Position measurements using ordinary GPS or GNSS typically have errors of several meters on common smartphones. This is because various factors such as atmospheric effects, satellite orbit errors, and receiver clock errors cause positioning deviations. However, in RTK surveying, the differences between GNSS signals received by a base station (a receiver placed at a known accurate position) and a rover (the device whose position is to be determined) are used in real time to correct these error sources. As a result, one can determine their position on site with centimeter-level accuracy.

In Japan, the Geospatial Information Authority of Japan (GSI) has established a nationwide electronic reference point network (GEONET) that supports network RTK, and correction information (differential data) can be received over the Internet via a system called Ntrip. If the positioning terminal is an RTK-capable receiver and it connects to a correction information service via a smartphone, a high-precision solution called a "fixed solution" can be obtained in about one minute, after which position information continues to be updated with an astonishing accuracy of approximately 1–2 cm. More recently, a centimeter-level augmentation service (CLAS) using the Quasi-Zenith Satellite System (QZSS, nickname: Michibiki) operated by the Cabinet Office has emerged; with a compatible receiver, correction information can be obtained directly from the satellite even in mountainous areas where cellular signals do not reach, allowing high-precision positioning to be maintained. Even at disaster sites or in regions with unstable communication infrastructure, as long as equipment supports CLAS, stable centimeter-level positioning becomes possible, and the usefulness of RTK technology continues to grow.

Thus, through the mechanism of RTK surveying, even GNSS satellite-based positioning can have errors thoroughly eliminated and absolute positions determined with centimeter-level accuracy. Traditionally, achieving this level of precision required optical surveying instruments such as total stations or expensive GNSS receiver sets, and assumed operation by skilled surveyors. However, recent technological advances have brought new solutions that make this much easier to achieve. A representative example is smartphone-based RTK surveying, and among these, the groundbreaking product attracting attention is LRTK.

The Emergence of Smartphone RTK Surveying and the Technical Features of LRTK

スマホRTK測量とは、その名の通りスマートフォンを使ってRTK測位を行う手法です。高精度な測位と聞くと特殊な機器を思い浮かべる方も多いでしょうが、近年はスマホに外付けデバイスを装着するだけでセンチ級測位が可能になりました。その先駆けとなったのが東京工業大学発のスタートアップ企業Lefixea（レフィクシア）社が開発したLRTK Phoneです。これはポケットに収まる超小型のRTK-GNSS受信機で、専用ケースを介してiPhoneやiPadにワンタッチで取り付けられるよう設計されています。重量は約125g、厚さ約13mmという薄型軽量ながら内部に高性能アンテナとバッテリーを搭載しており、スマホの背面に付けてもかさばりません。Bluetooth接続またはLightningケーブル経由でスマホと通信し、前述したネットワーク型RTK（Ntrip方式）および準天頂衛星からのCLAS補正受信に両対応しているため、日本全国どこでもリアルタイムに高精度測位が行えます。内蔵バッテリーで連続約6時間駆動し、USB給電による延長も可能です。防塵・防水性能も備わっているため、屋外の過酷な現場環境でも安心して使用できます。

LRTKが提供するのはハードウェアの受信機だけではありません。LRTK systemは大きく3つのコンポーネントから構成され、現場での測量をトータルにサポートします。一つ目は先述の専用GNSS端末「LRTK Phone」です。二つ目がiPhone/iPad用のLRTK appです。この専用アプリを使うことで、測位の開始からデータ記録、AR表示やナビゲーションまで、必要な作業をすべてスマホ上で完結できます。例えば単発のポイント測定（シングル測点）から毎秒最大10点の連続測位までボタン一つで切り替え可能で、複数点の平均化機能も備わっているため精度の向上にも寄与します。取得した測点はリアルタイムに地図上へプロットされ、日本の平面直角座標系への換算やジオイド高の自動計算もバックグラウンドで行われます。さらにスマホのカメラで撮影した写真には高精度な位置情報と撮影方位が自動的にタグ付けされ、一枚撮影するごとにワンタップでクラウドへアップロードすることもできます。また記録したポイントを目標地点として設定し、地図やAR上でナビゲーションする機能も搭載しています。ARを活用して仮想的な杭やマーキングを表示することで、杭打ちや丁張り位置への誘導をサポートしたり、遠方の目標物を非接触で測位したりと、測る・示す・記録するといった現場作業がスマホ一つで可能になります。

The third component is the web-based LRTK Cloud. Survey data collected on site (survey point coordinates, point clouds, photographs, etc.) can be synced to the cloud with a single click and immediately shared and saved without returning to the office. As long as there is an internet connection, there is no need to install dedicated software; data can be viewed on maps and a 3D viewer in the browser. By sending a shared link to stakeholders, external partner companies or clients who do not hold a license can view the data without needing a high-spec PC or a specialized point cloud viewer. It is also easy to track survey points and photos organized in time series, and to download CSV coordinate data or point cloud data for use in your company's CAD software.

Thus, LRTK is offered as a comprehensive platform of dedicated devices + app + cloud, and it not only measures high-precision coordinates with GNSS but also supports the entire workflow, from subsequent data processing to utilization and sharing. The approach of turning a handheld smartphone into a surveying instrument makes it easier to achieve accuracy and functionality comparable to conventional, costly surveying equipment, which can be considered innovative.

The Basis for Achieving Centimeter-Level Accuracy with Smartphone RTK

When it comes to smartphone RTK surveying, some people may be skeptical and wonder, "Can a smartphone really achieve that level of accuracy?" However, the basis for centimeter-level accuracy lies in the use of the RTK technology described above. The GPS chips built into typical smartphones are not intended for high-precision positioning and therefore carry errors of several meters; however, if you connect an RTK-compatible dedicated receiver such as LRTK, you can utilize a professional-grade GNSS antenna and positioning engine, which dramatically improves positioning accuracy. Furthermore, by receiving correction information via Ntrip from the Geospatial Information Authority of Japan’s electronic reference station network or by directly receiving CLAS satellite signals to augment positioning, most of the error is eliminated. As a result, even when using a smartphone, in principle it becomes possible to achieve positioning at a level equivalent to conventional surveying GNSS equipment.

In fact, the positioning accuracy of LRTK has been confirmed to rival that of conventional fixed receivers. For example, when a stationary LRTK unit was used for static positioning and coordinates were calculated using an averaging function, the standard deviation in the horizontal direction—about 12 mm during standalone positioning—was reduced to roughly 8 mm by averaging 60 observations. This means the position could be determined with accuracy under 10 mm, a result not inferior to full-scale surveying instruments. Of course, accuracy is influenced by the positioning environment (such as sky openness and surrounding obstructions), but generally outdoors in clear weather one can expect horizontal accuracy of about 1–2 cm and vertical accuracy of about 3–5 cm. That such precision can be achieved on a smartphone represents a technological leap that would have been unthinkable not long ago.

Various measures implemented in the LRTK app also contribute to maintaining accuracy. For example, multiple sampling of positioning points with automatic averaging and tilt correction using the smartphone’s attitude sensors (applying vertical correction even when mounted on a pole and tilted) address common on-site situations and prevent degradation of accuracy. Furthermore, as mentioned above, CLAS support allows positioning to continue even outside cellular coverage, so centimeter-level positioning can be maintained regardless of distance from reference stations, even in areas with no signal such as forests or under overpasses. These elements combine to create a system that ensures centimeter-level reliability at all times even with smartphone RTK.

Applications for Point Cloud Measurement, As-built Management, CAD Integration, AR Guidance, and Cloud Sharing

LRTK-enabled smartphone RTK surveying goes beyond merely measuring point coordinates and holds potential for a wide range of field operations. Here we introduce the representative 5 application areas.

• Point cloud measurement: You can obtain 3D point cloud data of the surroundings using the LiDAR scanner and camera on iPhone or iPad. By combining with LRTK, high‑precision geographic coordinates (global coordinates) can be assigned to every scanned point, eliminating the “distortion” and “scale instability” that are common with scans performed by a smartphone alone. For example, if you walk around a development site scanning terrain and structures, you can quickly obtain an accurate 3D point cloud model without positional drift. In addition to the smartphone LiDAR’s effective range (about 5 m), combining photogrammetry techniques can cover targets up to 50–60 m away. This makes it easy for anyone to create point cloud data with absolute coordinates over wide areas, which is also useful for large‑scale earthwork calculations and terrain analysis.

• As-built management: Smartphone RTK also proves powerful for as-built management, which verifies post-construction shapes in civil and building works. Traditionally, after construction a surveying team would measure numerous point elevations and then return to the office to compare them with design drawings and reference elevations to evaluate excesses or shortages of fill and excavation. Using LRTK, this process can be performed on-site in real time. For example, immediately after leveling a development site, you can scan a point cloud of the ground surface with the smartphone’s LiDAR and simultaneously assign absolute coordinates to each point with RTK. The acquired as-built point cloud can then be overlaid on the design completion model (3D design data) or the specified design elevations on site, allowing you to intuitively identify areas of excessive fill and areas where excavation is insufficient via color-coded displays. A cloud-based point cloud viewer lets you check the elevation or cross-section at any location and includes automatic calculations of fill and cut volumes, dramatically shortening the time required for as-built inspections. Being able to perform quality control immediately also helps prevent rework, enabling safe and efficient construction management.

• CAD Integration: Point cloud data and survey point coordinates acquired with LRTK can be seamlessly utilized for subsequent CAD drawing creation and BIM model construction. From the LRTK cloud, you can export them as CSV-formatted coordinate data or point cloud files such as LAS, and import them into common civil engineering CAD or 3D software. Conversely, it is also possible to upload 3D models or drawing data from the design phase to the LRTK cloud and display them overlaid with the point clouds acquired on site. This allows you to detect discrepancies between the design model and actual conditions in advance, or use them as material for considering design changes. As data flows between the field and the design office and the cycle of survey→design→construction is digitally linked, tasks that were previously separate processes are integrated and sped up.

• AR guidance: AR (augmented reality) technology that overlays information onto the real world through a smartphone screen is also becoming a more practical tool thanks to LRTK. For example, virtual stakes or markings can be displayed in AR at pre-set coordinate positions to support locating positions for staking and batter boards (establishing reference lines). Tasks that traditionally required using tape measures and surveying equipment on site based on dimensions on drawings can now be guided to precise points simply by following arrows on the screen while holding a smartphone. Even in places where it's physically difficult to drive stakes, such as on slopes or bedrock, AR markers can indicate positions, reducing the workload. Also, if the positions of underground pipes and cables are pre-scanned and recorded as point clouds, that data can be projected in AR during future excavations to visualize areas that must not be dug. AR guidance is expected to help prevent mistakes on site and improve efficiency.

• Cloud Sharing: Any data acquired using LRTK (control points, point clouds, photos, notes, etc.) can be shared instantly via the cloud. Because information measured on-site can be shared on the spot with internal stakeholders and clients, real-time information sharing becomes possible. For example, if a site representative uploads points or photos measured with LRTK to the cloud, designers and supervisors located remotely can immediately review them and issue instructions as needed. This greatly reduces the traditional time lag of "measuring on-site, bringing data back, reporting/consulting, and then making decisions." Also, because data is centrally managed in the cloud, it's easy to review daily survey results and construction progress on a timeline. There is no longer a need to transfer data via paper field notebooks or USB memory sticks, dramatically improving the speed and accuracy of information sharing.

On-site Implementation Case Studies and Their Effects (Labor Reduction · Time Savings · Improved Safety)

Case studies of actual on-site implementations of smartphone RTK have reported effects such as labor savings, time savings, and improved safety compared with conventional methods. Let's look at some specific cases.

Case 1: Stake-driving work at a road construction site – At a paving project site, a two-person team used to spend more than half a day laying out positions by stretching a string line from reference points. After introducing LRTK, a single worker, smartphone in hand, could identify stake positions with the AR guidance feature and simply mark them on the spot. The same task was completed in a matter of tens of minutes, and with the workforce halved it led to a significant efficiency improvement. "Because one person can do it, we were able to assign other personnel to different tasks, increasing productivity," the site supervisor said, also surprised.

Case 2: As-built inspection in land development work – In verifying the as-built condition of embankments and cuts in earthworks, the conventional method involved measuring hundreds of survey points and performing volume calculations in the office. At one site, they used LRTK to acquire the current point cloud immediately after construction and instantly calculated the volume differences from the design model on the cloud. As a result, the as-built verification work that had previously taken a full day was completed in a few hours, and identification of corrective areas and instructions for additional embankment could be issued the same day. Not only did this prevent situations of "discovering mistakes later and having to rework," but because progress could be quantified daily, reporting to the client also became faster.

Case 3: Safe surveying at disaster sites – A certain municipal government used LRTK for the restoration design of a slope (norimen) that had collapsed in a heavy rain disaster. Surveying a landslide site carries the risk of secondary disasters, so conventionally it was difficult to obtain detailed topography until repair work began. Immediately after the damage, a staff member used a smartphone equipped with LRTK to perform a point-cloud scan of the collapsed site from a safe, distant location, instantly estimating the volume of collapsed soil and calculating the slope angle of the cliff face. A single person completed the task in tens of minutes without entering dangerous areas, greatly contributing to faster initial response and ensuring staff safety. The municipality plans to continue using smartphone surveying as a disaster response tool, saying that “surveying is completed just by carrying it around.”

As described above, the on-site introduction of smartphone RTK has demonstrated effects such as resolving labor shortages, shortening work time, and ensuring worker safety. In terms of equipment costs as well, solutions like LRTK are more affordable than traditional surveying instruments, so if they become widespread across many sites, efficient work in a one-device-per-person setup will become possible.

Hurdles to Implementation and Practical Tips for On-site Use

Smartphone RTK is innovative, but there are several hurdles to consider when introducing it on site. First, among field staff there may be anxieties and resistance, such as "Is it really OK to use a smartphone?" The more experienced surveyors and technicians are, and the more they are accustomed to conventional equipment, the more likely they are to be skeptical of new technology. This can only be dispelled by actually using it and feeling its effectiveness, so it's best to begin by using it gradually in auxiliary roles. For example, continue to use a total station for establishing important control points while trying smartphone RTK for other simple surveys and progress checks; the key is to build trust step by step.

Next is consideration of costs. There are upfront costs for devices and services for smartphone RTK, but they are considerably less expensive than costly total stations or 3D laser scanners. For organizations that already use iPhones or iPads on site, in many cases you can get started simply by purchasing an additional receiver. Also, because they may qualify for national or municipal subsidies promoting ICT-based construction, the financial barriers have been falling year by year. From the perspective of return on investment, compare the labor and time savings that can be achieved with the equipment costs and use that as material to persuade management.

As a technical hurdle, attention must also be paid to the constraints specific to GNSS positioning. Even though it offers high accuracy, because it is satellite-based positioning, positioning can become unstable or impossible in places where the sky is not open (gaps among high-rise buildings, wooded areas, tunnels, etc.). In such environments, avoid forcing it and address the issue by combining conventional methods or, later, by using the remote positioning function (a function that measures distant targets with a camera) from an open location. For workers unfamiliar with smartphone operation, it is also important to provide sufficient initial operational training and prepare manuals. Fortunately, the LRTK app is designed with an intuitive UI, and basic usage can be learned in a short training session, but it is reassuring to practice in advance so you won’t be confused on site.

As a tip for on-site use, designate an internal champion to share know-how. Staff who are well-versed in digital surveying should take the lead in mastering the tools, and by sharing and rolling out the collected data internally, other members will more easily understand that "results can be achieved this easily." It is also effective to start with small-scale sites or pilot implementations and build a track record of success. Doing so will help the benefits of smartphone RTK permeate the entire site and lead to smooth adoption.

Introduction to Simplified Surveying Using LRTK

As we have seen, the combination of smartphones and RTK technology is poised to bring major changes to the world of surveying. By leveraging solutions like LRTK, tasks that previously required specialized survey teams are being transformed into simple surveys that anyone can easily perform. Free from complex equipment operation and cumbersome procedures, field personnel themselves can acquire the necessary data on the spot and put it to use immediately.

The advantage of being able to perform RTK surveying with a smartphone is not only that it increases work efficiency, but that it has the potential to change the way work is done on site. For example, processes that used to have to wait for a surveyor's arrival can now be carried out swiftly at the discretion of field technicians themselves. In addition, because surveying results are shared digitally in real time, collaborative work—where multiple people participate remotely and provide instant feedback—is made easy. True to the slogan "One surveying instrument per person", smartphone RTK is poised to become the standard on job sites.

Adopting new technology inevitably brings concerns, but with LRTK its appeal also lies in the simplicity—you can get started with nothing more than a smartphone. If you’re interested, why not try this simple smartphone-based surveying even on a small site or as a trial run? Once you can freely handle centimeter-precision location information, on-site productivity and creativity should improve dramatically. As a step toward pioneering the future style of surveying, please consider using LRTK.

FAQ

Q: Can smartphone RTK really achieve centimeter-level accuracy? A: Yes, it is possible if the appropriate equipment and conditions are in place. By using an RTK-capable receiver such as LRTK and correction information (Ntrip services or CLAS satellites), high-precision positioning within about 1–2 cm horizontally can be achieved. Of course there are some conditions, such as ensuring a clear view of the sky, but it has been confirmed that in clear outdoor conditions accuracy comparable to conventional high-precision GNSS devices can be obtained.

Q: What equipment and preparations are needed to start using smartphone RTK? A: Basically, you can get started simply by attaching a dedicated RTK receiver to your smartphone (currently iPhone and iPad are supported) and installing the compatible app. In the case of LRTK, the dedicated receiver "LRTK Phone" comes as a set with a smartphone case and can be attached with one touch. You also need an environment to receive correction data via the internet (using the smartphone's 4G/5G connection on site), but if CLAS is supported you can receive corrections directly from satellites even in areas without mobile signal. There is no need to set up a special base station yourself, and it can be deployed with relatively simple preparations.

Q: Can it be used in mountainous areas or sites outside communication coverage? A: Yes. LRTK receivers support three-frequency GNSS and can be augmented by the CLAS signal from the Quasi-Zenith Satellite System (Michibiki). Therefore, even in remote mountain interiors or on isolated islands where mobile signals do not reach, centimeter-level positioning can be maintained as long as the sky is open. There have been actual cases of use in forest surveying and disaster sites, making it a useful surveying tool that does not depend on communications infrastructure. However, in environments where satellites cannot be acquired—such as inside tunnels or in deep urban canyons behind tall buildings—use is difficult, so in those cases combining with conventional methods or adopting other measures is necessary.

Q: Is it difficult to operate? Can people who are not good with machines use it? A: If you're accustomed to using smartphone apps, it's not difficult. The LRTK app has an intuitive user interface and is designed so you can perform positioning and recording simply by tapping buttons while looking at the map. For example, "record a survey point" is a one-tap action and "take a photo and save it" is also one tap, so no complex configuration is required. Even people who aren't good with machines should be able to start surveying immediately after a basic briefing. Also, the in-app help and support system are available if you run into trouble, so you can be reassured.

Q: Can smartphone RTK replace conventional surveying equipment (total stations and GNSS receivers)? A: It depends on the application. For many situations—general topographic surveys, as‑built measurements, and stakeout for pile positions—smartphone RTK can be used as a substitute. It is especially advantageous for wide‑area surveys, sequential point recording, and point‑cloud measurements, where a single operator can work nimbly with a smartphone RTK. On the other hand, for precise displacement measurements requiring millimeter‑level accuracy, or in cases that require positioning indoors or underground, conventional total stations or specialized instruments are still necessary. Therefore, smartphone RTK should be regarded as one tool among others, and it is desirable to use it alongside conventional equipment according to site needs. At present, it is often employed more as a new option that complements and improves the efficiency of conventional instruments than as a single device that replaces them all.

Q: Can I use it on an Android smartphone? A: As of 2024, LRTK is provided for iOS (iPhone/iPad). Because Apple devices offer access to high-precision GNSS and have stable sensor performance, the product was initially made available for iOS. However, demand for an Android version is expected to be high in the future, so development may be underway (please check official information). At present, Android users can use LRTK by pairing an iOS device with an LRTK receiver. It is also possible to use a tablet device instead of a smartphone, and there are cases where an iPad has been introduced and used on site.