New Techniques and Methods for Slope Surveying: Improving Accuracy and Greatly Increasing Efficiency Using Point Clouds

Purpose and Importance of Slope Surveying

Along roadsides and developed sites in Japan, there are many man-made slopes known as "slopes". These slopes are shaped to prevent landslides and collapses and are crucial structures that support the safety and long-term stability of infrastructure. To ensure that such slopes are constructed with the proper gradient and shape, and to check for deformation over time or after heavy rainfall, slope surveying is indispensable. Surveyors and civil construction managers measure slope geometry accurately during construction and after completion to verify compliance with design and to detect any abnormalities.

The objectives of slope surveying can be broadly divided into two categories. One is quality control during construction. This verifies whether slopes created by fill or cut are finished to the gradients and heights shown in the design documents (as-built). The other is maintenance and disaster response. Completed slopes are periodically inspected to check for deterioration or deformation over time, and repair plans are developed as needed. In cases of slope collapse caused by earthquakes or heavy rain (disaster response), it is necessary to quickly determine the extent of the damage and the volume of collapsed soil to plan safe restoration work. Accurate slope survey data in these situations directly supports site safety and efficient construction and maintenance.

Challenges of Conventional Methods (Labor, Time, Safety)

Traditional slope surveying has mainly been performed through manual observation. Survey crews use instruments such as total stations and levels to measure key points on the slope (crest, toe, and a number of points on the slope) one by one. In some cases, staff physically enter the slope area and measure angles and distances with tape measures or inclinometers. However, these conventional methods present several challenges.

First, they require substantial labor and time. Surveying a large slope requires multiple staff moving measurement points repeatedly, often taking a long time to complete. Because the number of measurable points is limited, the overall slope shape must be inferred by plotting cross-sections, which risks overlooking detailed terrain changes. Second, there are safety issues. Working on steep slopes always carries the risk of slips and falling rocks. At disaster sites where collapse is a concern, sending personnel in using traditional methods poses significant risk. Setting up equipment and conducting surveys in areas with poor footing is also a heavy burden on workers.

Furthermore, conventional methods require time-consuming steps to record and share measurement data. Field values must be compiled into drawings and reports by hand, and reusing survey results for other purposes later is not easy. Given these challenges, there has been growing demand for new technologies that can measure slopes more efficiently and safely.

Technical Background and Procedure of New Slope Surveying Using Point Cloud Data

A recently developed approach is slope surveying that leverages three-dimensional point cloud data. Point cloud data are 3D measurement data representing an object's surface with numerous points, each including X, Y, and Z coordinate information. By scanning the entire slope with laser scanners or photogrammetry, it is possible to acquire high-density point clouds comprising tens of thousands to millions of points. Whereas conventional surveys could only measure some points on a slope, point clouds enable digitally "measuring the entire slope".

The new survey procedure typically begins by preparing appropriate equipment such as laser measurement devices or drones. For example, a terrestrial laser scanner mounted on a tripod can emit 360° laser pulses toward the slope and capture point cloud data for a wide area in a single scan. Alternatively, small unmanned aerial vehicles (drones) can take multiple photos from above the slope, and dedicated software can generate high-density point clouds via photogrammetry. The method is selected according to site conditions and scale to collect data across the entire slope.

After obtaining point clouds, the next processing step is to assign survey control coordinates to each dataset. By combining RTK-GNSS (real-time kinematic positioning) for high-precision coordinate acquisition, the acquired point clouds can be directly tied to public coordinate systems. This allows slope point cloud data to be accurately georeferenced and analyzed in overlay with design drawings and other terrain data. When multiple scans are performed, point clouds are aligned and integrated through registration processing.

From the completed 3D point cloud, the required survey outputs can be generated. For example, transverse and longitudinal sections can be cut at arbitrary locations to measure slope gradients and heights, or the surface area of the slope can be calculated—these analyses can be performed freely in software. Because the entire slope is digitized, if a location was missed in the field, additional measurements can be taken on the point cloud afterward. This new method is revolutionary in that it achieves both improved accuracy and significant efficiency gains simultaneously. Reliance on a veteran worker's intuition is reduced, and the slope geometry can be understood and managed with objective measurement-based data.

Concrete ICT Applications: Smartphone Surveying and AR Visualization

Slope surveying using 3D point clouds aligns with the Ministry of Land, Infrastructure, Transport and Tourism's (MLIT) promoted ICT construction (i-Construction). Two ICT applications gaining attention in recent years are smartphone surveying and on-site AR visualization.

For example, drone photogrammetry is already widely used on many sites, but even more accessible methods using smartphones are emerging. Modern smartphones are equipped with LiDAR sensors and high-performance cameras, and combined with dedicated apps they can perform 3D scans and high-precision positioning. Attaching a small GNSS receiver to a smartphone and using RTK positioning can achieve centimeter-level accuracy from a single device. Smartphone surveying, which enables tasks that previously required expensive survey instruments to be done with just a phone, is spreading as an easily adoptable approach for small contractors and municipal staff.

Augmented reality (AR) is also useful for slope management. If a 3D model or survey results can be overlaid on the real-world view on a smartphone or tablet screen via AR visualization, site conditions that are hard to grasp from plans alone can be understood intuitively. For instance, point cloud data of an as-built slope can be overlaid with the design model on site and mismatched areas color-coded for immediate inspection. Alternatively, virtual design structures (retaining walls, slope reinforcement, etc.) can be displayed on the slope before construction so stakeholders can share the expected completed appearance. These ICT techniques support surveying and construction management in ways not previously possible and accelerate DX (digital transformation) of field operations.

Applications to As-Built and Quantity Control and Cloud Integration

Point cloud-based slope surveying brings significant advantages to construction management tasks such as as-built control and work quantity (progress) measurement.

First, regarding as-built control: if the completed slope is fully measured with point cloud data, deviations from the design model can be comprehensively checked. Traditional inspections typically measured a few cross-sections after completion and compared heights against drawings. However, with point cloud measurement of the entire slope, any portion from the toe to the crest can be verified to ensure the design gradient and thickness are maintained. MLIT has prepared guidance such as the "Guidelines for As-Built Control Using 3D Measurement Techniques (draft)", promoting the use of point cloud data for quality inspection in works like tunnels, dams, and slope construction. For example, in tunnel excavation, laser scanning of the inner cross-section and visualizing differences from the design with color maps is a common approach. Using point clouds for slopes allows inspection of complex terrain that is difficult to measure manually, preventing omissions and improving the reliability of quality control.

Point clouds are also powerful for quantity control. By acquiring pre- and post-construction terrain as point clouds and comparing them, excavated and filled volumes can be accurately calculated. Previously, quantity calculation required hand computations of cross-sectional areas from survey data and multiplying by length. With point clouds, software can automatically aggregate cut and fill volumes from 3D differences, greatly streamlining computation. Because measured data that justify the numbers are retained, confirmation of quantities with the client becomes smoother.

Managing these point cloud datasets in conjunction with cloud services further expands their usability. Large survey datasets collected on site can be stored securely in the cloud and shared immediately within or outside the organization. For example, uploading a scanned slope to the cloud right after scanning allows remote supervisors or clients to check the data in real time. Accumulated 3D data in the cloud become an asset that can be reused for future maintenance or other projects. Comparing newly acquired point clouds against previous ones years later, or using them as baseline data in disaster response, increases the value of survey data for secondary use. Through point cloud measurement and cloud integration, as-built and quantity control for slopes evolves into a more precise, efficient, and reliable process.

Adoption Trends and Case Examples in National and Local Governments (Public Works and Disaster Recovery)

MLIT and local governments are advancing the adoption of point cloud technologies for slope surveying. As part of i-Construction, MLIT promotes ICT construction and has published trial guidelines that incorporate 3D surveying techniques into slope works (e.g., "Guidelines for Implementing ICT Use in Slope Works (draft)"). These guidelines describe methods for measuring the whole slope surface and using software to calculate and inspect slope length and height. Rather than partial measurements, inspections using point clouds from drone aerial photography or terrestrial laser scanning aim to enhance and streamline quality control. Several municipalities, including Kumamoto City and Saga Prefecture, have conducted trials of ICT slope works and implemented as-built control and quantity calculations using point cloud data.

Point cloud surveying also plays a key role in disaster recovery. As mentioned earlier, photographing a damaged slope with a drone immediately after a disaster and creating a 3D model allows assessment of collapse conditions without risking secondary disasters. For example, during the large-scale debris flow in Atami City, Shizuoka Prefecture in July 2021, comparison of pre-disaster baseline terrain point clouds and post-disaster point clouds enabled rapid estimation of the volume of displaced material. Differences in point clouds also revealed the presence of improper fill that became the collapse initiation point. This case demonstrates how point cloud measurement significantly contributed to cause analysis and restoration planning in disaster response.

Public agencies are also moving to open their point cloud datasets. Shizuoka Prefecture, for example, publishes terrain point cloud data from laser surveys as "Virtual Shizuoka" to promote use in disaster prevention and infrastructure management. The Geospatial Information Authority of Japan is also advancing nationwide high-resolution terrain data via airborne laser scanning. Such government initiatives encourage private surveying and design work. As the utility of point cloud data becomes widely recognized, more public projects are expected to adopt 3D surveying technologies, contributing to slope safety management and disaster preparedness.

Benefits for Practitioners (Efficiency, Safety, Reporting)

The many advantages of new slope surveying technologies are attractive to practitioners responsible for field work. The main benefits of adopting these methods are summarized below.

• Improved efficiency and labor savings: Introducing point cloud measurement dramatically speeds up surveying. Tasks that once took half a day for cross-section surveys can be completed in tens of minutes by drone or in minutes with smartphone surveying. Surveys can often be performed by a single person, reducing staffing needs and allowing freed personnel to work on other tasks.

• Enhanced safety: Remote surveying minimizes the number of times people must enter hazardous slopes. For slopes with unstable footing or risk of collapse, relying on machines and drones reduces the risk of secondary disasters. There is also reassurance in being able to perform measurements from safe locations for high or poor-weather sites.

• Improved accuracy and reliability: Digital 3D-based measurements reduce human error and oversights. Data can be validated from multiple angles, increasing trust in deliverables. For example, saving point cloud data as records of as-built inspections provides objective evidence that supervisors or third parties can verify later.

• Streamlined report preparation: Using point cloud data makes it easier to prepare reports that were previously time-consuming. Cross-sections and longitudinal/transverse profiles can be generated automatically from 3D models, and displacement maps can be output with color coding, enabling clear, fast explanatory materials for clients. Information that is hard to convey with photos or drawings alone can be presented via 3D views or AR for deeper stakeholder understanding.

• Accumulation and use of data assets: Once point cloud survey data are stored in the cloud, they can be used over the long term. Comparing historical data during periodic inspections quantifies degradation trends, and baseline data enable immediate damage assessment after disasters. Accumulating survey data as corporate assets supports long-term cost reduction and knowledge sharing.

As described above, point cloud-based slope surveying offers significant benefits for field personnel and has become an indispensable consideration for surveyors and construction managers. Because it can improve efficiency, safety, and quality all at once, the method is well worth evaluating for site adoption.

How to Use LRTK for Simple Surveys

Tools have also appeared that make the point cloud and ICT technologies introduced so far easily accessible to anyone. One example is the versatile smartphone surveying system LRTK. LRTK consists of a small GNSS receiver and a dedicated app, and attaching and running it on an everyday smartphone enables high-precision simple surveying. It is intuitive to operate without special surveying qualifications and excellently supports an integrated workflow from field surveying to point cloud acquisition and cloud storage.

Key features of LRTK's simple surveying functions include:

• Photo geotagging: High-precision coordinate tags can be attached to photos taken with a smartphone. When you photograph a slope condition, the coordinates of the photo location are automatically recorded, allowing later measurement on the photo or precise plotting of photo positions on a map.

• AR visualization: Design models or point cloud data obtained from surveys can be overlaid on the real view on site. Through the smartphone screen, virtual lines or structures can be projected onto the slope, and measured point positions can be visualized, enabling intuitive on-site checks and instructions.

• Cloud synchronization: Survey data syncs to the cloud in real time. Coordinates and point clouds recorded on site are immediately saved and shared in the cloud, eliminating worries about data loss and removing the need for post-field transfer. Multiple people can access the same data to smoothly collaborate and check progress.

• Monopod measurement: By attaching the smartphone and LRTK device to a dedicated pole (monopod), a single person can easily measure points. Place the pole tip at the desired point and press the phone button to get coordinates; there is no longer the need for two people to operate a pole and instrument, enabling agile coordinate acquisition even in confined or high locations.

• Point cloud generation: Using the smartphone's built-in LiDAR or camera, the surrounding environment can be scanned to generate point cloud data. Short scans of parts of a slope can produce 3D models for recording deformed areas or simple as-built checks. The acquired point clouds include RTK-based positioning, so they can be treated as georeferenced 3D data without post-processing.

• Volume calculation: The LRTK app can compute volumes from point cloud data. For example, the volume of collapsed slope material or fill can be calculated on the spot from scanned point clouds, aiding rapid initial disaster response and quick assessment of construction progress.

By using LRTK in this way, advanced 3D surveying becomes easily achievable and strongly drives DX at the site. Because initial investment is small and existing smartphones can be used, adoption is spreading from small and medium enterprises to local governments. LRTK is also a tool for responding to MLIT's i-Construction initiative and is expected to become one of the standard surveying methods in the future. Consider adopting modern tools like LRTK in slope management to experience their accuracy and efficiency on site.