The Mechanism and Features of RTK

RTK (Real-Time Kinematic positioning) is a technology that uses GNSS (Global Navigation Satellite System) to measure high-precision positions in real time. Specifically, it involves receiving satellite signals, such as GPS, simultaneously at both a known base station (reference station) and a moving station (rover). The error correction data from the base station is applied to the moving station, allowing the relative position of the moving station to be calculated with centimeter-level accuracy. The shorter the distance between the base station and the moving station, the more effectively the error can be eliminated, enabling high-precision positioning. In recent years, network-based RTK, which delivers correction data from electronic reference point networks set up by governments and private entities, has become more widespread. This has made it easier to perform high-precision positioning over wide areas. For example, in Japan, RTK positioning can be done without setting up a base station on-site by using correction services based on electronic reference points and the VRS method.

The strength of RTK lies in its positioning accuracy. The error is confined to just a few centimeters horizontally and vertically, setting it apart from traditional standalone positioning, which typically has errors of several meters. Additionally, the positioning results obtained are expressed in absolute coordinates, such as latitude and longitude or the World Geodetic System, which can be directly linked to the public coordinate systems used in design plans and maps. For example, if RTK is used in a reference point survey, the coordinates can be directly used as public coordinates in various drawings. Moreover, since measurements are taken in real time, positions can be immediately verified and recorded, making it ideal for tasks that require guidance of construction machinery or real-time positioning, such as the automatic control of unmanned construction machines.

In the construction industry, network-based RTK-GNSS has streamlined the automatic control of ICT construction machinery and the measurement of as-built conditions, contributing to improved efficiency and addressing labor shortages.

Additionally, satellite positioning can be used regardless of day or night or weather conditions (even during heavy rain, the accuracy remains stable and less prone to large deviations), making it notable for its ability to provide stable positioning.

On the other hand, the weakness of RTK lies in its limitations regarding the positioning environment. Since receiving signals from GNSS satellites is essential, the accuracy may decrease, or positioning may become impossible in environments where there are obstacles that block the signals.

For example, in environments such as forests, under overpasses, in the gaps between buildings (the so-called "urban canyons"), tunnels, or indoors, satellite signals can be blocked or reflected, making it difficult to obtain a fixed RTK solution. Additionally, high accuracy requires communication with the base station, so in areas with unstable communication, operation can become challenging. In network-based RTK, being outside of the communication range means correction data cannot be received, which leads to a decrease in accuracy. Furthermore, initial setup requires specialized high-precision GNSS receivers and data communication environments, and traditionally, the cost of equipment was high (although prices have been decreasing in recent years). For these reasons, RTK is a technology that excels in "open outdoor areas," but it has limitations depending on the environment.

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The Mechanism and Features of LiDAR

LiDAR (Light Detection and Ranging) is a remote sensing technology that uses laser light to measure the distance to an object and capture the surrounding shape in 3D. The basic mechanism is simple: a laser pulse emitted from the light source hits the target object and reflects back. The time it takes for the pulse to return is measured, and the distance is calculated based on the speed of light. This distance measurement is performed at a high frequency, more than several hundred thousand times per second, and by combining the resulting vast amount of point data, a detailed 3D model (point cloud) of the target area is created. LiDAR sensors use a rotating mechanism or scanner mirrors to sweep 360 degrees or a specific range around them, enabling high-precision measurement of shapes such as buildings and terrain in a short amount of time. For example, drone-mounted laser surveying equipment can acquire point cloud data of surface elevations and structures from the air, which is useful for creating highly accurate topographic maps.

The strength of LiDAR lies in its ability to capture the shape of objects and environments with high resolution. The point cloud data, consisting of numerous distance measurements, allows for the detection of even millimeter-level surface irregularities in terrain and objects, and it is especially effective for recording the dimensions and shapes of complex structures. Additionally, one of its advantages is that it is less affected by the environment. Since laser light is an active sensor that emits its own light for measurement, it can operate without issues in dark or low-light environments, such as at night or in tunnels with no lighting. In fact, LiDAR can operate 24/7, unaffected by surrounding light, making it ideal for nighttime surveys and measurements in areas like tunnels where cameras would face difficulties. Unlike GNSS, which requires an unobstructed view of the sky, LiDAR can be used in forests covered with trees or indoors in buildings, allowing for relative positioning and scanning. Furthermore, the point cloud data it produces is three-dimensional, making it flexible for post-processing, such as creating cross-sections or measuring distances and volumes. Recently, LiDAR has been integrated into autonomous vehicles, where it is used to detect the surrounding environment in real-time to avoid obstacles, showcasing its high-speed, high-precision spatial awareness capabilities.

The weaknesses of LiDAR include the high cost of the equipment and its unsuitability for direct use in positioning. High-precision laser scanners are generally expensive, with small LiDAR units for drones costing several hundred thousand yen, and ground-based long-range LiDAR systems or mobile mapping systems sometimes reaching tens of millions of yen. Additionally, LiDAR alone does not include absolute position coordinates in the point cloud data it captures, so combining it with positioning data is necessary to determine its location on a map. In other words, while LiDAR excels at relative shape measurements, other reference systems such as GNSS or known points are essential to overlay the measurement results onto Earth coordinates.

In fact, even in the field of autonomous driving, LiDAR alone has limitations in self-position estimation for vehicles. It is only when combined with GNSS/RTK that the vehicle can accurately determine its absolute position on a map.

Additionally, LiDAR is affected by weather conditions. Laser light can be scattered by airborne particles such as raindrops or fog, which causes a reduction in measurement range and accuracy during heavy fog or rainfall.

Furthermore, it is important to note that the volume of data obtained by LiDAR is very large, requiring significant processing time and advanced software. In this way, LiDAR is a "technology specialized in shape measurement," and "location information" is often complemented by combining it with other methods.

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Division of Roles Between RTK and LiDAR

As outlined above, RTK and LiDAR are technologies with different areas of expertise. RTK excels at accurately determining the position coordinates of a specific point in space, while LiDAR excels at capturing the detailed shape of surfaces and spaces. In construction and surveying, the optimal approach is to use RTK for positioning and LiDAR for shape measurement, effectively dividing their roles.

RTK and LiDAR have a complementary relationship. RTK excels at determining the position on global coordinates, while LiDAR is skilled at capturing the shape of objects at that location. By combining these technologies, their full potential is realized.

For example, in the field of autonomous driving, RTK-GNSS is used to accurately determine the vehicle's current location on the map, while LiDAR simultaneously scans the surrounding obstacles and road shapes in real time to enable safe route selection. By combining these two technologies, it becomes possible to achieve safe and precise navigation, something that would be difficult with each technology used alone.

Similarly, the use of both RTK and LiDAR is becoming more common in drone surveying and construction surveying. By equipping drones with RTK-GNSS receivers, it becomes possible to add position coordinates to aerial photos or LiDAR point clouds, allowing for the generation of high-precision orthoimages and terrain models without the need to set up reference points. In practice, methods for obtaining 3D survey results with centimeter-level accuracy from photo data captured by RTK-enabled drones like the Phantom 4 RTK have been put to use. Additionally, on construction sites, RTK is commonly used for reference point surveys and as-built management, while ground LiDAR scanners or mobile mapping systems are used to capture detailed terrain and as-built information. For example, in volume calculations, the height of known points is measured with RTK in advance, and the height of the LiDAR point cloud is adjusted to this reference to precisely calculate the cut and fill volumes. In this way, using "RTK for positioning" and "LiDAR for shape measurement" in the right places is essential for efficient and high-precision surveying.

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Relation to LRTK

The recently introduced LRTK is a group of innovative devices and services that greatly enhance the usability of RTK positioning.

LRTK is a compact, integrated RTK-GNSS receiver, about the size of the palm of your hand, which incorporates an antenna, battery, and communication module. It can be attached to a smartphone or similar devices for use. Traditionally, RTK equipment was often set up on tripods or poles, but by making it small and lightweight enough to integrate with a smartphone, the portability and mobility in the field have been greatly improved. For example, the LRTK Phone device is designed to be attached to an iPhone, and since all the equipment fits in one hand, it allows you to perform positioning tasks while using the other hand for additional work. Bluetooth connectivity eliminates the need for cumbersome cable connections, significantly reducing setup time on site.

Additionally, there is a CLAS-compatible model available that can be used even outside of communication coverage. By receiving the centimeter-level augmentation service (CLAS) transmitted from Japan's Quasi-Zenith Satellite "Michibiki," high-precision positioning is possible even in areas without mobile signal coverage, such as mountainous regions or remote islands. This helps address issues such as "RTK cannot be used due to lack of communication coverage at tunnel construction sites," providing users with a high level of confidence.

In fact, during the 2023 Noto Peninsula earthquake, a technician who happened to have an LRTK model with the capability to operate outside communication coverage was able to use it for photogrammetry at the disaster site. Despite the paralysis of communication infrastructure, they were able to leave highly accurate records.

In this way, LRTK is a solution that significantly improves the "portability" and "dependence on communication environments," which were weaknesses of RTK positioning.

LRTK also enables new surveying methods by integrating with smartphones and the cloud. Using the dedicated LRTK app, various features combining the smartphone's camera and sensors with high-precision positioning data can be utilized.

For example, there is a feature that automatically adds positioning information (latitude, longitude, elevation, and orientation) to photos taken with the smartphone and records them, as well as a function that uses the smartphone's built-in LiDAR scanner to capture point cloud data with high-precision positioning.

In the latter case, the point cloud scanned with the iPhone's LiDAR is immediately assigned RTK coordinates, allowing for the instant creation of 3D data in the global coordinate system. This means that tasks that were previously done by comparing with ground laser survey equipment or maps can now be completed with just a smartphone in hand. Additionally, the coordinate guidance feature allows users to be directed to a specified coordinate point with an arrow on the smartphone screen, making it easy to reach even survey points without ground markers. Furthermore, the AR (augmented reality) feature is particularly noteworthy, as it enables high-precision overlay of the 3D design model onto real-world imagery.

Traditional AR faced issues with misalignment between the virtual model and the real world due to GPS accuracy errors and device posture estimation inaccuracies. However, with the precise alignment provided by RTK, it is now possible to consistently display a model that perfectly matches the real-world environment.

For example, even in areas with poor visibility, such as those covered by trees, virtual structure models (such as signs or posts) can be displayed in real time at the designated location on the drawing. You can walk around the area and verify the placement without any misalignment.

By combining LRTK with LiDAR and AR technologies, the scope of surveying tasks that can be performed "by oneself" and "on-site" is expanding significantly. Measurements that traditionally required multiple people and specialized equipment can now be replaced with a simple setup of a smartphone and LRTK, greatly contributing to increased productivity on-site.

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Future Outlook

The evolution of positioning technology through the integration of RTK and LiDAR is expected to accelerate even further. As high-precision GNSS receivers and laser scanners become smaller and more affordable, it will become increasingly feasible to deploy sensors throughout construction sites to collect data in real time, which can then be integrated and analyzed in the cloud, leading to the implementation of digital twin-like operations. In fact, efforts are already underway where GNSS and LiDAR are mounted on heavy machinery and vehicles to scan the as-built conditions of construction sites in real time, with progress management done via the cloud. Additionally, as smartphones and tablets are combined with LRTK devices, the concept of "one high-precision positioning device per worker" is becoming a reality. In the future, all field workers may use their own devices for surveying, measuring, and recording. The high-precision data collected on-site will be immediately uploaded to the cloud, allowing stakeholders to share and supervise remotely, drastically improving the efficiency of remote management and inspections of construction projects.

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Advancements in communication technology will also accelerate the widespread adoption of high-precision positioning. With the establishment of high-capacity, low-latency communication networks like 5G, the delivery of RTK correction data and point cloud data transmission to the cloud can be done in near real-time. In the future, 5G base stations themselves may take on the role of RTK reference stations, providing centimeter-level positioning services over wide areas. Additionally, heavy point cloud processing and data analysis can be carried out on high-performance servers in the cloud, and the results can be fed back to on-site devices, enabling advanced real-time analysis that is not dependent on the performance of the on-site terminals. This integrated high-precision positioning, linked with communication and the cloud, will not only improve the efficiency of surveying but could also become the foundation for location-based services in autonomous driving infrastructure and smart cities, supporting society as a whole. The fusion of RTK and LiDAR technologies is expected to be applied in various fields beyond construction and surveying, and its development will dramatically transform how we handle spatial information.