Coordinate-tagged point clouds usable directly in the public coordinate system: Streamline public surveying with LRTK

Table of Contents

• What is a georeferenced point cloud?

• Use of public coordinate systems in surveying

• Challenges of traditional point cloud surveying and coordinate alignment

• Benefits of georeferenced point clouds

• What is LRTK?

• Efficient public surveying enabled by LRTK

• Frequently Asked Questions (FAQ)

What is a georeferenced point cloud?

Georeferenced point cloud refers to a three-dimensional collection of points in which each point of the acquired point cloud data is assigned precise real-world coordinates (latitude, longitude, elevation, etc.). A point cloud is a collection of countless measured points acquired by laser scanners or LiDAR sensors and is digital data that represents the shape of objects or terrain in detail. Typically, point cloud data are recorded in a coordinate system relative to the sensor, but in a georeferenced point cloud each point has absolute coordinate values on a map. In short, it is point cloud data that can be used in map coordinates from the start, so there is no need for coordinate transformation or alignment when overlaying the point cloud on maps or CAD drawings.

By obtaining such coordinate-attached point clouds, the 3D data captured at surveying sites can be used directly in design drawings and geographic information systems (GIS). Because each point holds coordinates in a public coordinate system (described below), the point cloud can be handled in a way that is consistent with other surveying results and map data. Traditionally, point cloud data were aligned afterwards by matching them to reference points, but with coordinate-attached point clouds they can be utilized as 3D data positioned correctly from the moment they are captured on site.

Public Coordinate Systems and Their Use in Surveying

Public coordinate system refers to a coordinate reference system that is standardly used in public surveying (such as control point surveys and land surveys conducted by the national government or local authorities). In Japan this typically corresponds to the "plane rectangular coordinate system," a two-dimensional coordinate system that divides the country into several blocks and sets an origin for each region. For example, in the plane rectangular coordinate system, the whole of Japan is divided into systems numbered 1–19, and within each area positions are represented by their own X-Y coordinates. Public surveying requires submission of results with coordinate values based on this plane rectangular coordinate system (Japan Geodetic Datum/JGD2011), and "data that can be used directly in the public coordinate system" therefore refers to survey data that contain plane rectangular coordinate values.

If point cloud data acquired by surveying are provided with coordinates in a public coordinate system, they can be directly overlaid with the Geospatial Information Authority of Japan (GSI) topographic maps and various infrastructure design drawings. Conversely, for point cloud data that lack coordinates (or use a proprietary local coordinate system), matching with known control points and performing coordinate transformation work are required before they can be used in public-surveying contexts. If point clouds are georeferenced in a public coordinate system, creating drawings for submission to government offices and importing into GIS can be done smoothly, greatly reducing the workload for data preparation. Therefore, when using point cloud data for infrastructure maintenance and management, urban planning, disaster response, and so on, it is highly valuable that the data be acquired in a public coordinate system from the start.

Challenges of Traditional Point Cloud Surveying and Coordinate Alignment

With traditional point cloud surveying, sites are scanned using laser scanners, drone-mounted LiDAR, or the LiDAR feature of mobile devices such as the iPhone to acquire point cloud data. However, as acquired, the point cloud’s coordinate system is device-specific and local (for example, a coordinate system with the scanner as the origin), and it does not align with map coordinates. To use this point cloud in public surveying, the task of coordinate alignment (georeferencing) was indispensable.

Specifically, it is necessary to pre-install known control points and reference markers on site, then apply translation, rotation, and scaling to the post-scan point cloud data to align it with the coordinates of those points. A typical procedure is to place markers called targets at multiple locations on site and laser-scan them, separately measure the targets' positions (coordinates) with a total station or GNSS surveying, and convert the point cloud to real-world coordinates during data processing. This method requires advanced surveying knowledge and considerable effort, and had the following challenges:

• Requires significant effort and time: After point cloud acquisition, coordinate transformations and alignment of point clouds must be performed in data processing software, requiring time for post-processing work both in the field and in the office. On large sites, point clouds are often acquired by changing scan positions multiple times and each must be merged, which further increases the labor involved.

• Additional surveying work is required: For high-precision alignment, it is necessary to prepare known points on site (points with known coordinates) or to install and measure targets. Because the control point surveying process is added separately from point cloud measurement, it becomes a burden including arrangements for personnel and equipment.

• Risk of errors and misalignment: Manual point cloud merging and coordinate transformations mean that slight measurement errors or processing mistakes affect the accuracy of the final deliverable. Especially when stitching together point clouds acquired separately multiple times, misalignments at each step can accumulate, potentially causing reduced positional accuracy and shape distortion.

• Dependence on expensive equipment: To place point clouds immediately into map coordinates, very costly specialized equipment such as high-performance 3D laser scanners with GPS functionality or mobile mapping systems was required. For small and medium surveying firms and local governments, assembling such equipment is a significant economic hurdle.

In this way, assigning coordinates to point clouds by conventional methods required considerable effort and cost. As a result, even when point clouds were acquired, they were often not fully utilized, or in some cases people avoided point cloud measurement from the outset, saying "coordinate alignment is too difficult."

Benefits of georeferenced point clouds

Given the challenges above, the advantages of obtaining point cloud data with accurate coordinates from the outset are immeasurable. By leveraging georeferenced point clouds, surveying and civil engineering sites can gain the following benefits.

• Ready to use immediately without alignment: Because the point cloud is placed in the correct position on a public coordinate system at the same time as data acquisition, post-processing coordinate alignment is unnecessary. You can overlay the point cloud on topographic maps and design data immediately after scanning on site. This dramatically increases the speed of delivering surveying results.

• Ensured surveying accuracy: Coordinates are assigned by high-precision GNSS positioning, so the positional accuracy of each point becomes very high — on the order of several centimeters (a few inches). This provides more stable accuracy than manual alignment and makes it easier to produce deliverables that meet public surveying standards.

• Reduced labor and manpower: Because the workflow is completed one-stop from point cloud acquisition to position identification, the number of work steps is greatly reduced. A single operator can survey a wide area in a short time, helping to alleviate labor shortages and reduce workload.

• Expanded data usability: Coordinate-tagged point clouds can be integrated immediately with other GIS data and CAD drawings, allowing them to be used as-is for as-built management, quantity measurement, and maintenance tasks. For example, you can create cross-sections from the acquired point cloud or measure the distance and elevation difference between any two points on the spot. They are also powerful for comparing terrain changes over time or recording conditions immediately after a disaster.

• Promotion of digitization: Because field measurements become 3D digital data immediately, paperless workflows and real-time sharing are promoted. If point cloud data are shared on the cloud among teams, stakeholders can check the current situation within the same day. This in turn contributes to advancing construction DX (digital transformation) and realizing initiatives advocated by the Ministry of Land, Infrastructure, Transport and Tourism, such as i-Construction.

Thus, georeferenced point clouds take conventional surveying methods a step further in terms of accuracy, efficiency, and range of applications. So how can such ideal point cloud data be obtained? The technology that has emerged as the solution is called LRTK.

What is LRTK?

LRTK (pronounced “L-R-T-K”) is a high-precision GNSS solution developed by our company that brings Real-Time Kinematic (RTK) satellite positioning to smartphones. Normally, the positioning accuracy of a smartphone’s built-in GPS has errors on the order of several meters (several ft), but by using LRTK it becomes possible to achieve precise positioning with errors of several centimeters (a few inches). Specifically, an ultra-compact GNSS receiver device attached to the smartphone (product name: LRTK Phone) and a dedicated app use, in addition to satellite signals, correction information from the electronic reference point network and the Quasi-Zenith Satellite “Michibiki”’s centimeter-class augmentation service (CLAS) to correct positioning errors in real time. As a result, your current position can be determined with astonishing accuracy of about ±1–2 cm (±0.4–0.8 in) horizontally and ±2–3 cm (±0.8–1.2 in) vertically.

By attaching an LRTK device to your smartphone and launching the dedicated app, your phone instantly becomes a high-precision positioning device. A major feature is that a single pocket-sized unit weighing only about 125 g can substitute for surveying tasks that previously required stationary high-end GNSS receivers or total stations. LRTK also supports network RTK (obtaining correction data via cellular communications), but even if you are outside of cellular coverage you can continue positioning using only augmentation signals from the MICHIBIKI satellites, so you can maintain high accuracy even in environments where cellular signals do not reach, such as mountainous areas (excluding places where GNSS satellite signals themselves cannot be received, such as underground or inside tunnels).

Efficient Public Surveying Enabled by LRTK

The true value of LRTK is realized when combined with a smartphone's sensors. Among these, notable is high-precision coordinate-tagged point cloud measurement through integration with smartphone built-in LiDAR. By scanning the surroundings with the LiDAR sensor built into iPhone and iPad Pro models and fusing positioning data from LRTK in real time, anyone can easily acquire 3D point clouds with coordinates in the public coordinate system. Because the smartphone's own position and orientation are continuously determined and corrected at centimeter-level accuracy (half-inch accuracy) during scanning, distortions and positional shifts in point clouds that commonly occur with typical mobile-device LiDAR are minimized. The resulting point cloud data are placed and saved on Japan's surveying coordinate system (the Plane Rectangular Coordinate System) on site, making them immediately usable as surveying deliverables (if necessary, conversion to latitude/longitude in the world geodetic system or to a custom local coordinate system is also possible with one touch).

This enables dramatic efficiency improvements in public surveying fieldwork. By simply walking the site alone with a smartphone in hand, topographic surveys and as-built measurements that previously required several people can be completed. For example, when surveying the ground elevation of a site, conventionally a surveyor had to read dozens of height points with a total station, but with LRTK a single person can simply scan the site and obtain elevation information for countless points. On that data, you can instantly check elevation differences at any desired point and perform analyses on-site, such as automatically calculating fill and excavation volumes. There is no need to carry heavy tripods, prisms, or laptop PCs; because surveying can be completed with only a smartphone and a compact device, the convenience of fieldwork is greatly improved.

Additionally, coordinate-tagged point cloud data generated by LRTK can be easily used in conjunction with cloud services. You can upload the point clouds acquired on site directly to the cloud and seamlessly share and review them immediately with colleagues in the office. This shortens lead times from surveying through drawing creation and reporting, leading to improved productivity across the entire workflow. Based on the high-precision 3D data collected on site, designers can review plans the same day and use the data in materials for explaining the project to clients, thereby facilitating smoother communication.

The efficiency improvements and higher accuracy brought by LRTK are expected to establish a new standard in the field of public surveying. In fact, local governments and construction companies are increasingly adopting LRTK solutions, achieving results across a range of applications such as post-disaster damage recording, infrastructure inspections, and urban 3D modeling. Georeferenced point cloud surveying, with overwhelmingly greater speed and versatility compared to conventional methods, will become an indispensable technology in the years ahead. Its benefits in accuracy, speed, and data utilization will improve work quality and productivity, and contribute to reducing on-site burdens and alleviating labor shortages. If you are currently facing challenges in streamlining surveying operations or leveraging 3D data, consider introducing LRTK-based georeferenced point clouds. You will surely be surprised by its ease and effectiveness. Let’s leverage the latest technologies to advance public surveying to the next stage.

Frequently Asked Questions (FAQ)

Q: What is a point cloud with coordinates? A: It is three-dimensional survey data that combines point cloud data obtained by laser scanning with position information from high-precision GNSS. Because each point is assigned accurate coordinate values such as latitude, longitude, and elevation, the acquired point cloud can be used immediately on maps and drawings. The characteristic feature is that, unlike conventional point clouds which required separate georeferencing, a point cloud with coordinates can be treated directly as the result of public surveying.

Q: What is required to acquire a georeferenced point cloud? A: You need a smartphone equipped with a LiDAR sensor (e.g., iPhone or iPad Pro models), a small GNSS receiver device called the LRTK Phone, and the dedicated LRTK app. Attach the LRTK Phone to the smartphone and simply tap "Start Positioning" or "Start Scan" in the app; point cloud acquisition and coordinate calculation will be performed automatically. Positioning and scanning are generally possible even without an Internet connection, but having network access is convenient for cloud synchronization of collected data and displaying basemaps. Also, for stable high-precision measurement it is ideal to have a monopod or pole to secure the smartphone, but for simple tasks handheld operation is also possible.

Q: Can it be used in places where mobile phone signals do not reach? A: Yes, it can be used. LRTK supports the centimeter-level (half-inch accuracy) positioning augmentation service (CLAS) provided by Japan’s Quasi-Zenith Satellite "Michibiki", and can maintain high-precision positioning using only augmentation signals from satellites even in areas outside mobile communication coverage, such as mountainous regions. However, as with standard GNSS receivers, positioning is not possible in underground spaces or tunnels where the signals from GPS/GLONASS satellites themselves cannot be received.

Q: Can it be used without surveying expertise? A: Yes, it is designed for intuitive operation, so beginners will have no problem. Difficult settings are handled automatically by the system, so users only need to follow the app’s prompts and press buttons to obtain high-precision point cloud data. For example, after positioning starts, coordinate corrections are performed in the background without any special actions, and once you start scanning, coordinates are assigned to the point cloud in real time. Additionally, manuals and support systems are in place for first-time users, so you can adopt it with confidence even without specialized knowledge.

Q: What kinds of applications can it be used for? A: It can be used for a wide range of applications, including topographic surveying, as-built management, setting out for foundation construction, earthwork quantity calculation (embankment and excavation volume calculations), damage recording during disasters, maintenance and management of infrastructure such as roads and bridges, and forest surveying and monitoring. In short, whenever you want to accurately record and share on-site shapes and positions, point cloud data with coordinates is highly useful. High-precision point cloud measurement using LRTK has already begun in various fields such as surveying, civil engineering, architecture, urban planning, and disaster prevention.

Q: How does it differ from conventional surveying methods? A: Conventionally, measurements were taken carefully point by point with a total station, or even with GNSS the accuracy remained on the order of several meters (several ft). However, by using LRTK you can acquire countless points at once in a short time and assign centimeter-level coordinates (cm level accuracy; half-inch accuracy) to all of them. Laborious tasks like carrying a tripod and moving a prism are unnecessary, and field surveying can be completed with just a smartphone and a compact device. Also, because the results immediately become 3D data, post-processing effort is greatly reduced and the data can be used on site as reporting material for stakeholders. In other words, even for the same surveying work, you can achieve a fundamentally different level of efficiency in terms of "speed," "accuracy," and "the amount of information obtained."

Q: Can you create drawings from the acquired point cloud data? A: Yes, it is possible. Point cloud data acquired with LRTK can be exported in common point cloud file formats (e.g., LAS or PLY formats) and imported into major CAD/BIM software to create drawings and 3D models. LRTK's cloud service also provides features such as extracting cross-sections from point clouds and automatically extracting terrain contour lines to generate DXF drawings. Once the point cloud is obtained, traditional plans and cross-sectional drawings can be created smoothly from it, streamlining the preparation of design and construction planning documents.