Active in Disaster Recovery Too! 3D Point Cloud Data Supporting Slope Greening

Japan is hit by landslide disasters caused by heavy rains and earthquakes almost every year, and slope and hillside collapses occur across the country. For example, the 2018 Western Japan heavy rains triggered more than 2,000 landslide incidents, causing numerous slope failures nationwide. To restore collapsed slopes safely, covering the surface with vegetation—known as "slope greening"—is indispensable. Slope greening is a technique that plants grasses and trees on inclined ground to fix the soil and prevent erosion and collapse due to rain and wind. In disaster recovery, quickly covering exposed soil with vegetation reduces the risk of secondary disasters and helps preserve the surrounding environment.

At the same time, applying the latest digital technologies has become important to carry out slope greening work quickly and reliably. Particularly noteworthy is 3D point cloud data obtained by drones and LiDAR. Point cloud data that records the current condition of a slope in detail provides great support in every phase from disaster recovery planning to construction. This article explains how 3D point cloud data supports the field, starting from the relationship between slope greening and disaster recovery. We will look step by step at surveying methods that overcome conventional challenges, improved safety, advanced design, AR-assisted construction, cloud-based information sharing, and even mobility for small-scale sites. At the end, we will introduce a new technology, LRTK, that enables simple point cloud measurement and AR display with a smartphone, offering tips for improving disaster response capabilities.

The Relationship and Necessity of Slope Greening in Disaster Recovery

Slope greening involves covering artificial slopes such as those along roads or on development sites with vegetation to prevent soil collapse and washout. While it is valued in normal times for aesthetic improvement and ecosystem conservation, its importance increases significantly in disaster recovery. After landslides caused by heavy rain or earthquakes, if the slope surface remains exposed, each rainfall can wash away topsoil and trigger further collapse. Applying emergency slope greening and letting plant root systems hold the soil reduces the risk of secondary disasters.

In actual disaster recovery work, early greening and prevention of topsoil erosion are key. Techniques such as spraying seeds onto collapsed slopes or installing vegetation mats and vegetation sandbags make it possible to cover slopes with greenery in a short period even on sites where heavy machinery cannot access. This helps stabilize slopes during rainfall that occurs while recovery work is ongoing, protecting nearby residents and infrastructure. Restoring a green landscape also contributes to the psychological stability of affected communities. In addition, attention is being paid to the concept of "green infrastructure," which leverages vegetation rather than relying solely on artificial structures like concrete retaining walls or sprayed concrete. In this regard, slope greening plays an important role in both disaster prevention and environmental protection. Slope greening is therefore an indispensable measure that supports both aspects.

Importance of As-Built Surveying of Damaged Slopes and Challenges of Conventional Methods

At a site where slope collapse has occurred, an initial as-built survey is indispensable. Accurately understanding the terrain of the collapsed area allows judgment of how much soil has shifted and how much earthfill or reinforcement is needed for restoration. When planning slope greening, knowing the slope angle and the volume of collapsed soil enables selection of appropriate methods and materials (such as seed types and quantities, or the scale of vegetation mats). If recovery work proceeds based on an incorrect understanding of the as-built condition, the finished slope may not achieve the designed gradient or height, and insufficient strength could lead to further collapse. However, conventional surveying methods for investigating damaged slopes carried many challenges.

Conventional general surveying uses optical instruments like total stations and levels, measuring points one by one manually on site. This method had the following problems:

• Time-consuming: For wide slopes the number of survey points becomes enormous. In conventional methods, each point must be measured in sequence, so covering the entire collapse area required a tremendous amount of time. Surveying work could become a bottleneck and compress the construction schedule even when quick recovery planning was needed.

• Labor-intensive: Total station surveys typically require two-person teams, with one operator and another person holding the prism or rod at the target point. In disasters with multiple scattered slope collapses, it was difficult for limited surveying staff to cover everything, raising the risk that investigations would fall behind due to staff shortages.

• Safety risks: It is extremely dangerous for people to enter slopes with loosened ground. Approaching a collapse site carrying heavy surveying equipment in rainy, slippery conditions risks secondary collapse or falls. In large-scale disasters there may also be aftershocks or heavy rain, increasing the accident risk during surveying. Additionally, in emergencies benchmark points (known survey control points) may have been washed away or power outages may render equipment unusable, meaning conventional tools may not function adequately in urgent situations.

With these issues of "time," "labor," and "safety," conventional surveying methods sometimes could not adequately meet the initial demands of disaster recovery. For speedy restoration, a new approach that can measure damaged conditions more efficiently and safely was needed.

Area-based Understanding and Safe Remote Surveying Enabled by Point Cloud Measurement

Enter 3D point cloud surveying. Point cloud data is a collection of countless 3D coordinate points obtained by laser or photogrammetry that records terrain shape at high density. Point clouds can be generated from aerial photos taken by drone-mounted cameras, captured by ground-based LiDAR scanners, or more recently obtained with smartphone-embedded LiDAR sensors. Using point cloud measurement makes possible the area-based as-built understanding and safe remote surveying that were difficult with conventional methods.

For example, photographing the entire collapsed slope from the air with a drone can generate a detailed 3D model of a wide area in a short time. There are cases where surveying that used to take three days was completed in less than half a day using drone aerial photography. Also, with LiDAR-capable smartphones like iPhones, the terrain can be recorded by scanning from below without people climbing hazardous slopes. With short time and few personnel, the entire site shape can be digitized, and because there is no need to remain on site for extended periods for surveying, safety is dramatically improved.

The greatest advantage of point cloud measurement is that it can record the slope surface exhaustively. The number of points that can be measured manually is limited, but point clouds can acquire hundreds of thousands of measurement points down to every nook and cranny of the slope. This allows identification of bumps or unstable blocks that might be missed by the naked eye. Because surveying can be done remotely or from safe positions using equipment, worker burden and risk can be greatly reduced. Obtaining a digital surface model means all information needed for recovery planning can be collected without omission.

Advanced and Faster Design Planning Using Acquired 3D Data

The detailed as-built models obtained from point cloud data are extremely powerful for recovery design planning. Since the data are digital, designers can analyze slopes freely on a computer. For example, accurately calculating the volume of collapsed soil from point clouds allows numerical consideration of where and how to backfill and how much earthfill is required. What was traditionally estimated from plan and cross-sectional drawings can now be achieved with rapid and accurate quantity calculation using point clouds.

Moreover, overlaying the as-built point cloud and the design plan (the finished model) for comparison streamlines the evaluation of optimal recovery plans. For instance, by comparing pre-disaster slope shape data or design drawings with the current collapsed terrain point cloud, one can instantly see which parts have collapsed and by how much. Calculating the volume to be backfilled or the number of rock masses to be removed from that difference immediately clarifies the required work. It is also easy to try multiple recovery plans and compare earthwork volumes and slope gradients for each plan on the point cloud.

Using 3D data in the design phase enables advanced planning aligned with actual site conditions. Design based on precise terrain data reduces the risk of over- or under-provisioning materials and prevents rework during construction. Digitalized information is also easy to share among stakeholders, so when design changes occur the point cloud can be used to quickly verify and develop countermeasures. As a result, higher-quality recovery plans can be compiled in a shorter period than before, leading to earlier construction start and completion.

Visual Construction Support Combining Point Clouds and AR

Even in the construction phase after the design is finalized, 3D point cloud data provides various supports. Particularly useful is combining it with AR (augmented reality) technology. By holding an AR-capable smartphone or tablet on site, you can overlay the design model or lines on the camera view. This makes it possible to intuitively confirm the post-construction slope image and the work area composited into the actual scenery.

For example, when laying vegetation mats for slope greening, highlighting the mat area in AR at the designed positions makes it immediately clear to workers how far to install them. Being able to share the final greening completion image on site facilitates explanations to clients and local residents. Images that are hard to convey on drawings alone can be shown together with the actual scene using AR, smoothing consensus building. During construction, you can also overlay the design model and the as-built point cloud to compare and confirm the result in real time. This enables on-the-spot checks that grades and thicknesses match the design, preventing mistakes and rework. As a form of visual construction support, point clouds × AR deepen site understanding and greatly reduce communication losses.

Rapid Information Coordination through Cloud Sharing

Digital information such as point clouds and photos can also be shared instantly via the cloud, which is a major advantage. Recovery work involves not only contractors but also many stakeholders such as municipal disaster prevention personnel and design consultants who need information. Traditionally, compiling survey results into drawings and reports and explaining them in meetings took time. But if data are uploaded to the cloud, from the moment of measurement on site all stakeholders can check the latest conditions.

For example, by uploading point cloud data and photos acquired on site to a dedicated cloud and issuing a share link, staff at a distant head office or government office can view the 3D damage situation in a browser. The full extent of damage, which is hard to convey with text or still images alone, can be comprehended in three dimensions online, enabling quicker decision-making. Discussions on design changes or construction methods can proceed with everyone viewing the same 3D data, reducing discrepancies and shortening time required for consensus. Cloud utilization makes for rapid information coordination that eliminates the sense of distance between the recovery site and the office, ultimately speeding up and improving efficiency of the entire recovery effort.

Mobility Useful Even for Small Sites and Emergency Works

3D point cloud measurement and smartphone-based surveying technologies are powerful not only for large-scale disasters but also for small-scale sites and emergency responses. High-level surveying used to be time-consuming and costly, so small landslide locations were often handled by simple visual assessment. However, portable small devices and drones make it possible to measure any site immediately. For a one-sided collapse in mountainous areas or a small slope failure beside a road, a responsible person can go on site with a smartphone and a drone and obtain detailed terrain data and consider countermeasures within the day.

High mobility of equipment means strong initial response capabilities immediately after disaster occurrence. Drones that fit in a truck bed or surveying devices that fit in a pocket can be carried at all times, making it easy to move between multiple sites. Without arranging heavy machinery or large surveying teams, one person can quickly digitize the damaged conditions, immediately aiding decisions on emergency work and planning temporary measures. Digital surveying technology that can be flexibly used regardless of scale—from small collapses to wide-area disasters—will become standard equipment at all disaster sites in the future. In fact, the Ministry of Land, Infrastructure, Transport and Tourism is promoting the spread of such technologies for small and medium-scale works, and an era when anyone can utilize digital surveying is approaching.

Introducing the Smartphone Surveying Tool LRTK, Useful in Disaster Response

As described above, the use of 3D point cloud data and the latest technologies offers great benefits to disaster recovery works, including slope greening. One solution that can easily harness those benefits is LRTK. LRTK (LRTK) is a small surveying device attached to a smartphone that performs RTK-GNSS centimeter-level positioning and LiDAR scanning with the phone’s LiDAR camera, and even AR display, all in one revolutionary tool. RTK-GNSSによるセンチメートル精度の測位と、スマホのLiDARカメラによる点群スキャン、さらにはAR表示までを一台でこなせる画期的なツールです。

For example, simply walking around a collapsed slope with an iPhone equipped with LRTK can acquire a high-precision 3D point cloud model of the entire site within minutes. Because the obtained point cloud is automatically aligned to world coordinates, on-site earthwork volume calculations and estimates of required backfill can be performed there and then. Measurement data can be shared to the cloud with a single tap, allowing office staff to instantly grasp site conditions. Using the AR function, the design model can be overlaid on the real scene on the smartphone screen so stakeholders can easily share the completed image. The fact that this series of operations can be done with just a smartphone and without specialized expertise is a major strength in disaster response. Field personnel can complete measurement, verification, and recording themselves without relying on specialized equipment, dramatically improving the speed and quality of recovery work.

LRTK’s portability and ability to operate even at sites where power or communications infrastructure are down make it a true next-generation surveying tool that lets you “measure anytime, anywhere.” With a limited number of personnel, LRTK enables safe and rapid as-built assessment of slopes through planning and construction of greening works, making it a reassuring option for emergency preparedness. To minimize disaster damage and achieve early recovery, consider adopting such cutting-edge technologies. Through DX leveraging the latest technologies, let’s advance the creation of a disaster-resilient, safe social infrastructure.