What is Centimeter-Level Accuracy? Difference Between Standalone Positioning and RTK Positioning

"Centimeter-level accuracy" literally means that the positioning error is within a few centimeters. In typical standalone positioning using a smartphone’s GPS (GNSS), positioning is done with just one receiver, making it convenient, but errors of several meters are common. For example, consumer GPS devices typically have an error of about 5 to 10 meters. On the other hand, RTK positioning can reduce this error to approximately 1/100th (around 5 cm). In practice, the accuracy of network-based RTK (VRS system), which is widely used on-site, is reported to be around 3-4 cm horizontally.

The high accuracy of RTK is due to its use of error correction data. While standalone positioning uses only one receiver, RTK uses two receivers (a base station and a rover) simultaneously. The base station, which has a known position, is fixed in place, while the rover, which is the receiver on the moving object, moves freely. The base station calculates the error information from the satellite data it receives and transmits this correction data to the rover in real-time, allowing the rover’s position to be corrected to centimeter-level accuracy. Simply put, "using two receivers instead of one allows errors to cancel each other out, improving accuracy."

The table below summarizes the differences between standalone positioning and RTK positioning:

Positioning Method | Number of Receivers | Use of Correction Data | Estimated Horizontal Accuracy

Standalone Positioning | 1 (Rover only) | None | A few meters
RTK Positioning | 2 (Base station + Rover) | Yes | A few centimeters

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Standalone positioning can be easily done with a smartphone GPS or handheld GNSS receiver, but due to errors of several meters, it is unsuitable for terrain surveying or construction. On the other hand, RTK requires setting up a base station, but it provides exceptional high precision. As a result, the use of RTK has rapidly spread in the construction and surveying fields in recent years, driven by the need for quality management and efficiency improvements.

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How RTK Positioning Works – Base Station and Rover, and the Error Correction Process

The key to understanding RTK positioning lies in the coordination between the base station (fixed station) and the rover. First, the base station is assigned accurate coordinate values (latitude and longitude). On construction sites, the antenna is placed at a known point, or information is obtained from a public electronic reference point (GNSS reference station). When both the base station and rover receive signals from the same GNSS satellites simultaneously, their observation data will contain common error components (such as atmospheric effects or satellite clock errors). The base station compares its actual position with the positioning results derived from satellite measurements to calculate the positioning error at that moment. This error information is then transmitted to the rover via radio or the internet. The rover applies this correction to its own positioning results, allowing it to calculate high-precision location data.

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The key point is that this correction process is carried out in real-time every second, allowing the rover’s position to be dynamically and highly accurately determined (hence the name "Real-Time Kinematic"). The closer the distance between the base station and rover, the more similar the error factors between the two, leading to higher accuracy. Generally, within a few kilometers, centimeter-level accuracy can be maintained. Additionally, with the increasingly popular network-based RTK (VRS system), correction data can be obtained from nearby electronic reference point networks via the internet, allowing RTK positioning to be performed with just a single rover.

In Japan, the Geospatial Information Authority's electronic reference points and correction services provided by private companies (e.g., mobile carrier's "Ichimill") are well-established, and by using these, centimeter-level positioning can be achieved without the need to set up a base station on-site.

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The Value of Centimeter-Level Accuracy – Impact on Construction Management, Surveying, and Infrastructure Maintenance

The value that centimeter-level positioning information from RTK brings to the site is immeasurable. First, in construction management, it allows structures to be built at the correct position and height according to the design, improving quality.

For example, in road and bridge construction, significant surveying errors could lead to discrepancies in the as-built conditions. However, by using RTK, accurate positioning (staking, pile driving) can be achieved from the start. Additionally, in ICT construction, machine guidance/machine control (MG/MC) is widely used, where GNSS receivers are installed on construction machinery to monitor the position of the blade in real time. RTK-GNSS supports this high-precision positioning.

Heavy machinery operators can always check the difference between the current height of their blade and the design surface on a monitor, allowing for efficient embankment and excavation work. This reduces human error and prevents rework, leading to a significant improvement in construction productivity and accuracy.

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RTK is revolutionary in the field of surveying. Traditionally, as-built surveys and staking tasks were carried out by two-person teams using a total station. However, with RTK-compatible surveying equipment, a single person can walk around with a GNSS rover and measure multiple points in a short time. Additionally, since real-time planimetric coordinates and elevations can be obtained, it becomes easy to verify data and perform additional measurements on-site.

While ensuring accuracy in the vertical direction requires some adjustments, horizontal positioning already achieves precision comparable to that of a total station. For example, in the validation of a certain RTK system, the standard deviation of single-point positioning was improved from around 12mm to 8mm by using positioning averaging.

With the introduction of high-precision GNSS surveying, the number of situations where not only surveyors but also construction managers and workers can take measurements themselves has increased. The widespread use of affordable RTK positioning systems that are easy to use even for non-professionals will significantly change surveying tasks on construction sites in the future.

In infrastructure maintenance, centimeter-level precision is also creating new solutions. This enables continuous verification of the positions of work vehicles and track closure conditions, contributing to improved site safety. Additionally, in civil engineering structure inspections, AR technology is emerging that overlays map information of pre-registered buried utilities with live camera footage on a tablet.

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This allows for the intuitive understanding of the location of buried pipes before excavation, leading to accident prevention. In road maintenance, RTK can be used to record the locations of cracks and subsidence or to detect small displacements by comparing regular survey data, enabling preventive maintenance based on high-precision positioning data. Additionally, in the growing field of drone-based infrastructure inspections and terrain surveying, RTK-equipped drones can capture high-precision orthophotos and point cloud data with errors reduced to a few centimeters.

This leads to improved quality in as-built management and increased accuracy in 3D models, thereby enhancing the reliability of maintenance and planning.

In this way, centimeter-level positioning accuracy provides value in various areas such as construction quality management, surveying efficiency, safety management, and advanced maintenance. By obtaining high-precision location data, on-site information that was previously only handled in two dimensions can now be accurately recreated as a digital twin, contributing to the promotion of DX in civil engineering and construction.

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Use Cases of RTK – Benefits in Map Creation, Construction Management, Staking, and Infrastructure Maintenance

Let’s explore the value discussed in the previous chapter through several concrete use cases.

• High-Precision Map Creation (Surveying and GIS): In traditional terrain surveying, it was necessary to measure offsets from reference points and create drawings. With RTK-GNSS, it’s possible to directly acquire absolute coordinates in the global coordinate system on-site, allowing the data to be immediately integrated into electronic maps or GIS. For example, in public surveying, surveying companies use RTK to perform reference point measurements, with network RTK (receiving correction data from electronic reference points) enabling on-site surveying to be nearly completed. The resulting point cloud data, with an accuracy of a few centimeters, forms a highly accurate base for creating 2D maps or 3D terrain models. Additionally, in drone-based aerial surveying, using RTK-equipped drones eliminates the need for numerous ground reference points, greatly improving workflow efficiency. High-precision maps created from point clouds are powerful tools for land surveying, urban planning, and damage mapping in disaster situations.

• Construction Site Management: On a road construction site, a construction manager carrying an RTK-equipped tablet can walk around and measure embankment as-built conditions in real-time. This allows for immediate verification of the height discrepancies and adjustments to be made on-site. Moreover, even in manual construction work (e.g., placing box culverts), RTK rovers are used to continuously check the installation positions of structures, preventing any misplacement. This has reduced rework and extra excavation, leading to shorter construction times and improved quality. Furthermore, in sites with ICT machinery (machine guidance/machine control), bulldozer and backhoe operators can work with 3D models of the design surface displayed on their on-board monitors, enabling precise finishes even by a single operator. These solutions are all made possible by RTK’s centimeter-level positioning.

• Staking and Marking (Positioning Work): RTK is also used for staking foundation piles for buildings and bridge piers, as well as for positioning structures (staking). Instead of manually aligning positions using surveying instruments, workers carrying GNSS rovers simply stand at the designated positions and move to the required coordinates, following guidance from the receiver. Today, there are RTK-compatible guiding apps that direct workers to specific coordinates, for example, by saying, "Move 5 cm east, 2 cm north."

• For example, by marking the center of a buried pipe detected using radar, or the location of a structure foundation defined in the design drawings, RTK ensures that later construction work is accurate. On one construction site, RTK was used to align the positions of foundation piles, resulting in the position deviation of all piles staying within 1 cm, eliminating the need for additional corrections. In staking, the labor-intensive process of marking rectangular areas on large sites has been eliminated, allowing workers to quickly draw reference lines on their own.

• Infrastructure Maintenance and Inspection: In railways, RTK is used in maintenance vehicle positioning systems to enhance safety management.

• In the highway sector, systems are being developed to track the current location of inspection workers using RTK and send alerts if they enter hazardous areas. For regular inspections of bridges and tunnels, high-precision 3D point cloud data is compared with previous data to accurately pinpoint cracks and structural deformations. For example, in evaluating the structural health of concrete structures, it is crucial to detect even small displacements of components, which has been made possible by combining ground-based RTK-GNSS with high-precision IMU (Inertial Measurement Unit) systems.

• Furthermore, in road infrastructure management, assets like guardrails and signposts are now recorded using RTK-positioned photos, which are then managed as images with centimeter-level positioning data on GIS systems.

• In this way, the increased reliability of position data through RTK in infrastructure maintenance contributes to the maintenance of safe and secure public infrastructure.

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From the above examples, it is clear that centimeter-level high-precision positioning is strongly driving the digital transformation (DX) on construction sites. From map creation to construction, inspection, and maintenance, RTK has made possible the efficiency and advancement in various scenarios that were previously impossible.

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The Evolution of RTK Positioning with LRTK – Differences and Strengths Compared to Traditional RTK

RTK technology is highly accurate, but traditionally, there was the hurdle of "large equipment and the need for specialized knowledge." Equipment such as antennas, GNSS receivers, batteries, and radio modems had to be brought to the site, set up, and connected by cables.

However, recently, all-in-one RTK devices have emerged that break this conventional norm. A prime example is the LRTK series. Developed by Lefixea, LRTK is an innovative GNSS receiver that integrates all the necessary equipment for RTK positioning, successfully wireless and miniaturized.

Designed with rugged, dustproof, and waterproof features for on-site use, LRTK can be used safely even in environments with rain or heavy dust.

Among the LRTK devices, the pocket-sized LRTK Phone has gained particular attention. This smartphone-integrated RTK receiver allows you to attach a dedicated ultra-small GNSS module (weighing 125g and 13mm thick) to an iPhone or Android device, turning the smartphone into a versatile surveying tool with centimeter-level positioning accuracy.

Designed to overturn the traditional image of RTK equipment, LRTK is compact enough to literally be "carried in your pocket and used instantly when needed."

The smartphone screen allows you to check positioning results and maps, perform surveying, point cloud measurements, staking, and even AR visualizations. The data collected can be shared on the cloud immediately, and with its very affordable price compared to traditional surveying equipment, the "one device per person" setup is now realistic.

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One of the key differences between LRTK and typical RTK equipment is that LRTK only requires a smartphone, making it very easy to use. It also has a built-in battery, allowing for continuous positioning for over 13 hours, which is enough to cover a full day's work.

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Additionally, communication is supported via Bluetooth and Wi-Fi, eliminating the need for cumbersome wiring. It can be linked with a smartphone using NMEA output, and with the dedicated app, positioning and recording can be done with a single tap. Notably, it is also compatible with Japan's Quasi-Zenith Satellite System "Michibiki" and the CLAS (Centimeter-Level Augmentation Service), which provides high-precision positioning. Unlike traditional network-based RTK systems, LRTK can receive correction signals from satellites even in areas without mobile network coverage, such as mountainous regions, allowing for high-precision positioning. It can also switch to the VRS system via mobile networks.

The flexibility to switch to "out-of-coverage mode" by simply changing the antenna is highly effective in situations like infrastructure inspections, where positioning inside tunnels or in remote mountainous areas is required.

LRTK is well-equipped not only in hardware but also in software and cloud integration. The dedicated "LRTK app" automatically converts the coordinates of measured points into a planimetric coordinate system and includes convenient features to meet on-site needs, such as calculating distances, areas, and volumes between multiple points. For example, with the positioning photo feature, photos taken with the smartphone can be tagged with high-precision coordinates and orientation, allowing users to accurately identify which point and direction the photo was taken from, even when shared via the cloud.

Moreover, the point cloud scanning feature allows for 3D scanning directly on-site in collaboration with the smartphone’s camera or LiDAR, enabling the collection of point cloud data with high-precision location information. You can instantly calculate soil volume from this point cloud or measure cross-sections as needed.

Additionally, the coordinate guidance feature enables workers to be directed to pre-set coordinates. Even in situations where "the exact location is unknown" during routine inspections, LRTK ensures pinpoint navigation without confusion. All of these features are realized with just one smartphone and LRTK device, making it a highly effective digital tool for the site, addressing all the specific needs.

In this way, LRTK can be considered the next-generation RTK solution that allows anyone, anywhere, and easily to utilize centimeter-level positioning. What was once dependent on experts and expensive equipment for high-precision positioning is now accessible to everyone on-site with a smartphone, significantly improving productivity. In fact, in one construction site, after introducing LRTK, waiting time for surveying was drastically reduced, and construction managers themselves were able to perform surveying and as-built checks. By lowering the barriers to incorporating centimeter-level accuracy into daily tasks, LRTK represents a new evolution of RTK technology.