Centimeter Surveying with Just a Smartphone! The On-site Revolution Enabled by CLAS

In recent years, an era is approaching in which centimeter-level positioning accuracy can be achieved using only a smartphone. The key to this is Japan’s satellite positioning augmentation service “CLAS”. Traditionally, centimeter-level surveying required specialized GNSS equipment or RTK base stations. However, by combining CLAS-compatible technology with smartphones, a revolution in efficiency and labor savings is beginning across field operations—from surveying, construction, and agriculture to municipal disaster response and infrastructure inspection. This article explains, in terms accessible to beginners, the potential of smartphone+CLAS positioning from the perspectives of accuracy, ease of use, cost, service coverage, and communication environment. It also covers the differences from RTK and SLAS, and finally touches on the smartphone-compatible surveying solution “LRTK” that can utilize CLAS.

What is CLAS? Centimeter Positioning Achieved with a Smartphone

CLAS (Centimeter Level Augmentation Service) is a centimeter-level positioning augmentation service provided by Japan’s Quasi-Zenith Satellite System “Michibiki” (QZSS). In simple terms, it is a technology in which high-precision correction information broadcast from satellites is received by a smartphone or other receiver and applied to GPS positioning errors, enabling the centimeter-level positioning that was previously difficult. CLAS uses a technical approach known as PPP-RTK; correction information computed from data of the national network of reference stations is broadcast nationwide via Michibiki satellites. On the user side, a CLAS-compatible receiver (a dedicated high-precision GNSS antenna/chip) receives this information and applies it to position calculations. As a result, smartphones can achieve RTK-level high-precision positioning standalone.

The greatest feature of CLAS is that it is a satellite-based correction service covering the entire country. With conventional GPS, a smartphone’s built-in GPS accuracy was at best about 5–10 meters, but by using CLAS, single-receiver positioning can dramatically improve to a few centimeters. Moreover, this service is basically free (no fee is required to receive the satellite signals themselves) and can be used even in areas where terrestrial communication infrastructure does not reach, such as mountainous regions or at sea. However, as noted below, using CLAS requires a compatible receiver device, and current general-purpose GPS chips built into smartphones cannot directly receive CLAS signals (the L6 band). Therefore, the mainstream approach is to use a small CLAS-compatible GNSS receiver connected to the smartphone. With that in hand, a new field workflow of centimeter-level positioning using just a smartphone becomes a reality.

Differences Between RTK, SLAS, and CLAS

For high-precision positioning, long-established methods include RTK positioning (Real-Time Kinematic) and another Michibiki augmentation service, SLAS (Sub-meter Level Augmentation Service). Let’s outline the main differences between CLAS and these conventional methods.

• RTK positioning: This method provides horizontal accuracy of several centimeters and vertical accuracy of several centimeters by real-time correction of GNSS errors between a reference station and a rover. In local RTK, users set up a base station at a known point and send correction data by radio; in network RTK (VRS and similar), correction information is obtained from a distribution service via cellular communication. RTK delivers a fast initial fixed solution (often within seconds) and high accuracy, but base stations and communication lines are indispensable, and accuracy degrades as you move away from the base station. Additionally, the need for dedicated equipment and service fees means higher upfront costs.

• SLAS (sub-meter augmentation): A free positioning augmentation service by Michibiki that enhances standard GPS accuracy (several meters) to sub-meter level (on the order of several tens of centimeters). SLAS is mainly provided on the L1 frequency and can be used with compatible receivers or some commercial GPS devices. If a smartphone is compatible, positional accuracy improves somewhat, but it does not reach the level required for surveying (centimeters). It is intended for applications like car navigation or basic positioning improvements and is insufficient where centimeter accuracy is needed.

• CLAS: As mentioned above, CLAS enables centimeter-level accuracy without a reference station and without communication. Unlike RTK, there is no need for the user to set up a base station or receive corrections via the Internet; receiving the correction signal directly from Michibiki satellites is sufficient. Within Japan, the same quality of corrections can be obtained anywhere, making CLAS suitable for mobile surveying over wide areas and for use on remote islands or in mountains. The decisive differences from RTK are that no private base station is required and it does not depend on communication infrastructure. Note, however, that CLAS requires an initial convergence time of several tens of seconds to about one minute, so positioning accuracy is somewhat lower immediately after startup (decimeter-level), and you need to wait a short time for it to stabilize to a few centimeters. Also, absolute precision may be slightly inferior to an RTK fixed solution, sometimes reaching around 5–6 cm horizontally. Even so, it provides sufficient accuracy for many field tasks, and the elimination of base station setup and communications contracts is a major advantage. Compared to SLAS, CLAS requires dedicated equipment but offers dramatically higher accuracy (an improvement of about tenfold or more), so the two services occupy different use niches. Overall, RTK offers the highest precision and immediacy but at higher cost and environmental constraints; SLAS is easy to use but lacks precision; CLAS fills the gap as a new option. Tasks that previously required RTK can now be replaced by CLAS in some cases, and an era is beginning in which the two approaches are used complementarily depending on the situation.

Five Benefits of Smartphone × CLAS

Combining a smartphone with CLAS yields the following significant on-site benefits.

• High positioning accuracy: Even with a smartphone, positioning can be done with errors within a few centimeters, making it easy to perform precision surveying and stakeout tasks that used to require total stations or conventional RTK equipment. Vertical measurements become possible as well, meeting the accuracy requirements for as-built control, batter board placement, boundary point measurement, and a wide range of applications.

• Ease of use and rapid measurement: Positioning is done with intuitive smartphone apps, without complicated setup or specialist knowledge. There is no need to carry heavy tripods or large equipment, and you can start surveying immediately upon arrival. Eliminating equipment setup time and base station installation makes it efficient to visit multiple points in a short time. With a lightweight smartphone system, a single person can complete surveying tasks, helping to alleviate labor shortages and improve safety (assistants do not need to enter hazardous areas).

• Low-cost deployment: Compared with conventional high-precision GNSS equipment and surveying instruments, the smartphone plus small receiver combination substantially reduces initial deployment costs. Because CLAS correction signals are free, there is no monthly fee like private VRS services. Small and medium-sized companies or municipalities that previously gave up on high-precision positioning for cost reasons can now adopt it at a more accessible price point.

• Consistent accuracy nationwide: Since CLAS covers all of Japan, uniform accuracy is maintained across wide work areas. For example, during long road or railway construction where surveying is done while moving, there is no need to worry about variations in correction accuracy by area. There is no need to consider distance limits from base stations or VRS area boundaries, enabling smooth wide-area surveying and patrols. If the sky is open, continuous positioning for mobile platforms (vehicle-mounted MMS, drones, etc.) is also consistently supported.

• Positioning even outside communication coverage: Although smartphones are typically associated with cellular connectivity, CLAS operation does not depend on communication networks at all. Because no Internet connection is required to receive correction data, centimeter-level positioning can continue even in mountains, forests, remote islands, or offshore sites where cellular service is unavailable. This makes it highly reliable as a backup means of positioning when communication infrastructure is down during disasters. This peace of mind of being able to measure anytime, anywhere is a major advantage for field personnel.

CLAS Use Cases Advancing in the Field

Smartphone+CLAS positioning technology is already being utilized across many sectors. Here are some concrete examples and their effects.

• Surveying and construction sites: In civil surveying and construction, many tasks require centimeter-level accuracy, such as control point surveying, as-built verification, and ICT construction (machine guidance). Using CLAS-compatible equipment makes it possible to perform high-precision stakeout and surveying even in locations with unstable communications, such as mountain tunnel portals or dam construction sites. Machine-operation guidance can also be executed accurately, contributing to improved construction quality and operational efficiency. Field surveying has become more labor-saving; there are reports of tasks that previously required two people now being safely completed by one person for batter board placement and cross-sectional surveys.

• Infrastructure inspection and maintenance: CLAS is proving effective for inspecting roads, railways, bridges, and other infrastructure. Small receivers carried during patrols can monitor track or pavement displacements to the centimeter level, and during bridge or tunnel inspections, high-precision position tags can be attached to photos for management. Where position records were previously ambiguous, precise latitude, longitude, and elevation embedded in photos taken with a smartphone make post-repair planning and monitoring of long-term changes dramatically easier. For long infrastructure, the advantage of CLAS—no need to relocate base stations—comes into play, and there are cases of continuous precision surveying while patrolling highways.

• Agriculture (smart agriculture): High-precision positioning is indispensable in agriculture for autonomous tractors and drone spraying—core elements of smart agriculture. With CLAS-compatible tractor steering systems, automatic driving with straightness errors within a few centimeters is possible across large fields, enabling overlap-free and even cultivation and seeding. Demonstrations using Michibiki CLAS for autonomous agricultural machinery have reported results that help alleviate labor shortages and boost work efficiency. In mountainous farmlands where communications are unstable, guidance based solely on satellite augmentation is a major advantage. If smartphones or tablets become common operator terminals and autonomous machines usable by anyone spread, this will revolutionize labor-saving in farming.

• Municipal disaster response: Smartphone surveying + CLAS is also effective for damage assessment and recovery planning after major disasters. For example, one municipality introduced a CLAS-compatible device attached to an iPhone to survey landslide sites and other disaster areas. As a result, they could measure the affected areas faster and more accurately than before, significantly reducing the time and cost for recovery planning. Because measurements can continue even when mobile networks are down, the system proved powerful in immediate emergency response. There are also cases where slope surveys of dangerous terrain were performed remotely and safely by a single worker, contributing to labor-saving and safety at disaster sites. Municipalities and government agencies are increasingly adopting such smartphone surveying systems as disaster-preparedness tools, expecting them to enable quicker recovery and more accurate support.

As shown, CLAS-enabled centimeter positioning is beginning to deliver results across a wide range of fields—from surveying and construction to infrastructure maintenance, agriculture, and disaster response. For field personnel, the ability to perform “precision surveying with just a smartphone” has the potential to significantly change work processes and can be considered revolutionary in both labor savings and enhancement of capabilities.

The Emergence of Smartphone Surveying Solutions with LRTK

A concrete solution for using CLAS with smartphones is the system called LRTK. LRTK is a surveying solution consisting of a compact, smartphone-compatible high-precision GNSS receiver and a dedicated app, and it supports Michibiki’s CLAS signals. By simply attaching a small wireless device to a smartphone, centimeter-level positioning becomes easily available to anyone. The dedicated app allows one-touch recording of survey points and photo capture, and photos are automatically tagged with high-precision position and orientation information. By leveraging the smartphone camera, tasks that formerly required expensive surveying equipment—such as 3D point cloud scanning and AR-based stakeout verification—can now be performed. Acquired data can be integrated with cloud services so that point clouds and photos measured on site can be shared and used immediately.

With such smartphone-compatible high-precision surveying systems, non-specialist field personnel can perform surveying and record-keeping themselves. The convenience of a “surveying instrument that fits in your pocket,” requiring no base stations or communications, will strongly drive on-site DX (digital transformation). Municipalities that have adopted LRTK report dramatic efficiency gains in surveying tasks during disaster response, evaluating that necessary data collection is completed simply by “walking the site with surveying equipment.” Centimeter surveying enabled by the smartphone+CLAS combination is changing on-site norms right now. Make high-precision positioning more accessible—why not try using this on-site revolution in your own work?