New Technologies and Methods for Slope Surveying: Improving Accuracy and Dramatically Increasing Efficiency by Utilizing Point Clouds

Purpose and Importance of Slope Surveying

Along roads and on developed land in Japan, there are many artificial slopes called “slope faces” (hereafter, slopes). Slopes are shaped inclines constructed to prevent landslides and collapses, and they are important structures that support infrastructure safety and long-term stability. To ensure that these slopes are constructed with the correct gradients and shapes and to check for deformation over time or after heavy rainfall, slope surveying is indispensable. Surveying technicians and civil engineering construction managers accurately measure slope geometry during construction and after completion to verify that the works conform to design and that no abnormalities have occurred.

The objectives of slope surveying can be broadly divided into two. One is quality control during construction. It checks whether slopes formed by embankment or cutting are finished with the slopes and heights indicated on the design drawings (as-built condition). The other is maintenance management and disaster response. Completed slopes are periodically inspected to determine whether they are deteriorating or deforming over time, and repair plans are made as needed. In the event of slope failure caused by earthquakes or heavy rain (disaster response), it is necessary to quickly grasp the extent of damage and the volume of collapsed soil to plan safe restoration work. Accurate slope survey data in these situations directly contributes to site safety and efficient construction and maintenance management.

Problems with Conventional Methods (Labor, Time, Safety)

Traditional slope surveying has mainly been conducted by manual observation. Survey staff use surveying instruments such as total stations and levels to measure key points on the slope (slope crest, slope toe, and several points on the slope) one by one. In some cases, staff enter the slope itself to measure angles and distances with tape measures or inclinometers. However, these conventional methods have several issues.

First, they require significant manpower and time. Surveying a wide slope requires multiple staff moving between measurement points repeatedly, taking a long time to complete. Because the number of measurable points is limited, understanding the entire slope shape often relies on extrapolating sectional measurements onto drawings, which can miss detailed terrain changes. Second, there are safety issues. Working on steep slopes always carries the risk of slipping or falling rocks. At disaster sites where collapse is likely, having people enter the area with conventional surveying methods is a major risk. Setting up equipment and conducting surveys in areas with poor footing also places a heavy burden on workers.

Additionally, conventional methods are time-consuming for recording and sharing measured data. Field-acquired values must be manually compiled into drawings and reports, making it difficult to reuse surveying results later for other purposes. Amid these challenges, there has been growing demand in recent years for new technologies that can measure slopes more efficiently and safely.

Technical Background and Procedures of a New Slope Surveying Method Using Point Cloud Data

A recent development is a slope surveying method that utilizes 3D point cloud data. Point cloud data are 3D measurement data that represent the surface of an object with a large number of points, each containing X, Y, and Z coordinate (position) information. By scanning an entire slope with laser scanners or photogrammetry, a high-density point cloud consisting of tens of thousands to millions of measured points can be obtained. While conventional methods could only measure some points on the slope, using point clouds makes it possible to digitally “measure the entire slope” as a complete geometry.

The new surveying procedure begins with preparing appropriate equipment such as laser scanners or drones. For example, a terrestrial laser scanner set on a tripod and directed at a slope can emit a 360° laser to acquire point cloud data over a wide area in a single scan. Alternatively, a small unmanned aerial vehicle (drone) can capture multiple photos from above the slope, and specialized software can generate a high-density point cloud via photogrammetry. Depending on site conditions and scale, the most suitable method is chosen to collect data across the entire slope.

After point cloud acquisition, the processing steps include assigning survey control coordinates to each measurement dataset. By combining RTK-GNSS (real-time kinematic positioning) for high-precision coordinate acquisition, the acquired point cloud can be directly tied to the public coordinate system. This enables accurate geolocation of the slope point cloud on maps for overlay and analysis with design drawings and other terrain data. When multiple scans are performed, the point clouds are integrated through registration processing.

From the completed 3D point cloud data, necessary surveying deliverables can be obtained. For example, longitudinal and transverse sections can be extracted 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 it can later be measured on the point cloud. This new approach is groundbreaking in that it simultaneously achieves improved accuracy and dramatic efficiency gains. Even without relying on the intuition of veteran craftsmen, objective, measurement-based data now allow accurate understanding and management of slope geometry.

Concrete Examples of ICT Use such as Smartphone Surveying and AR Display

Slope surveying using 3D point clouds aligns with the Ministry of Land, Infrastructure, Transport and Tourism’s promotion of ICT construction (i-Construction). Notable ICT applications include smartphone surveying and on-site AR display.

For instance, while drone photogrammetry is already widely used at many sites, an even more accessible method has appeared: surveying using smartphones. Modern smartphones are equipped with LiDAR sensors and high-performance cameras, and when used with dedicated apps they can perform 3D scans of their surroundings and high-precision positioning. By attaching a small GNSS receiver to a smartphone and using RTK positioning, centimeter-level positioning accuracy (half-inch accuracy) can be achieved with a single device. Smartphone surveying, which enables tasks that formerly required expensive surveying equipment to be carried out with just a smartphone, is spreading as an easily adoptable method for small and medium-sized contractors and local government staff.

Augmented reality (AR) technology is also useful for slope management. If 3D models or surveying results can be displayed as AR overlays on live site imagery via a smartphone or tablet screen, it becomes easy to intuitively understand slope conditions that are difficult to convey with drawings alone. For example, as-built point cloud data can be overlaid on the design model at the site and areas of discrepancy can be color-coded for immediate inspection. Alternatively, virtual design elements (retaining walls or slope-frame structures) can be projected onto the slope before construction to share a completion image among stakeholders. These ICT applications provide new ways to support on-site surveying and construction management and promote business DX (digital transformation).

Application to As-Built and Quantity Management and Cloud Integration

Point cloud-based slope surveying technology also offers major benefits for construction management in terms of as-built management and work quantity (progress) management.

First, for as-built management: if the completed slope geometry is fully measured with point cloud data, differences from the design model can be comprehensively inspected. Traditionally, inspections involved measuring the height at a few sections after completion and comparing them to drawings. However, if the entire slope is recorded by point cloud, any portion from the slope toe to the slope crest can be checked to ensure design gradients and thicknesses are maintained. The Ministry has prepared guidelines such as the “Guidelines for As-Built Management Using 3D Measurement Technology (draft)” to promote quality inspections using point clouds in works such as tunnels, dams, and slope projects. For example, tunnel excavation can use laser scanning to measure the inner cross-section as a point cloud and visualize differences from the design section with a color map. Using point clouds for slopes allows inspection of complex terrain that is difficult to measure manually, preventing inspection omissions and improving the reliability of quality control.

In terms of quantity management, point cloud data are powerful. By acquiring pre- and post-construction terrain point clouds and comparing them, earthwork volumes for cutting and filling can be accurately calculated. Until now, quantity calculation required hand calculations from sectional areas derived from pre- and post-survey data multiplied by lengths. With point clouds, software can automatically aggregate cut and fill volumes from 3D shape differences, greatly streamlining the calculation process. Because measured data that form the numerical basis are preserved, confirming quantities with the client becomes smoother.

These point cloud datasets can be further leveraged by managing them with cloud services. Large amounts of field survey data uploaded to the cloud are safely backed up and can be shared immediately inside and outside the organization. For example, uploading a scanned point cloud of a slope to the cloud right after scanning allows remote supervisors or clients to check the data from an office in real time. Accumulated 3D data in the cloud become an asset that can be reused for future maintenance or other projects. Previously obtained point clouds can be compared to data years later to quantify changes, or used as up-to-date data in the event of a disaster—raising the value of secondary use of survey data. Thus, point cloud measurement combined with cloud integration evolves as-built and quantity management into a more precise, efficient, and reliable process.

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

The Ministry of Land, Infrastructure, Transport and Tourism and local governments are also advancing the adoption of point cloud technologies for slope surveying. As part of i-Construction, the Ministry is promoting ICT construction and has published trial guidelines that incorporate 3D surveying technology into slope works (e.g., “Guidelines for ICT Utilization in Slope Works (draft)”). These guides describe methods for measuring the entire slope as a surface and using software to calculate and inspect slope length and slope height. Rather than partial surveys as in the past, inspections using point cloud data obtained by drone photogrammetry or terrestrial laser scanning aim to improve and streamline quality control. Trials of ICT slope works using point clouds for as-built management and quantity calculation have been conducted in multiple municipalities such as Kumamoto City and Saga Prefecture.

Point cloud surveying also plays an important role in disaster recovery. As noted above, photographing disaster-affected slopes with drones and creating 3D models immediately after a disaster allows assessment of collapse conditions without risking secondary disasters. For example, in the large-scale debris flow that occurred in Atami City, Shizuoka Prefecture, in July 2021, comparison between pre-existing baseline terrain point cloud data and point clouds acquired after the disaster enabled rapid calculation of the volume of collapsed soil. The difference in point clouds also revealed the presence of inappropriate embankment fill that likely initiated the collapse. This case demonstrates how point cloud measurement contributed significantly to cause analysis and restoration planning in disaster response.

Public organizations are also moving to open up their point cloud datasets as open data. Shizuoka Prefecture, as “Virtual Shizuoka,” has published terrain point cloud data from laser surveys within the prefecture to promote use in disaster prevention and infrastructure management. The Geospatial Information Authority of Japan is advancing the nationwide development of high-resolution terrain data from airborne laser surveys, and such government initiatives are supporting private surveying and design work. As the usefulness of point cloud data becomes more widely recognized, 3D surveying technologies will likely be adopted as a standard in more public works, contributing to slope safety management and disaster countermeasures.

Benefits of Adoption for Practitioners (Work Efficiency, Safety, Reporting)

The many advantages of new slope surveying technologies are attractive to the practitioners responsible for sites. Below are the main benefits of adopting this approach.

• Improved work efficiency and labor savings: Introducing point cloud measurement dramatically speeds up surveying. A slope sectional survey that used to take half a day can in some cases be completed in tens of minutes with a drone or in minutes with smartphone surveying. A single person can perform surveys, reducing personnel coordination burdens and allowing freed personnel to be assigned to other tasks.

• Improved safety: Because surveys can be performed remotely, the number of times people must enter hazardous slopes can be minimized. By entrusting risky slopes or slopes prone to collapse to machines or drones, the risk of secondary disasters can be reduced. There is also reassurance in being able to measure from safe locations for high-elevation work or under adverse weather conditions.

• Improved accuracy and reliability: Surveys based on digital 3D data reduce human error and oversight. Data can be verified from multiple angles, increasing confidence in the quality of deliverables. For example, saving point cloud data as inspection records for as-built management provides objective backing for later checks by supervisors or third parties.

• Streamlined report preparation: Using point cloud data makes the previously labor-intensive preparation of reports and drawings easier. Software can automatically generate sectional and longitudinal/transverse drawings from 3D models, output color-mapped displacement diagrams of slopes, and more—enabling quick preparation of clear explanatory materials for clients. Information that is difficult to convey with photos or drawings alone can be presented with 3D views or AR to improve stakeholder understanding.

• Accumulation and utilization of data assets: Point cloud survey data, once obtained and stored in the cloud, can be used over the long term. Comparing current inspections with past data quantifies degradation trends, and in disasters, comparisons with pre-disaster data enable immediate damage assessment. Accumulating survey data as company assets leads to long-term cost savings and knowledge sharing.

As described above, point cloud technology for slope surveying brings significant benefits to sites and has become an indispensable option for surveying technicians and construction managers. It is well worth considering adoption on sites as a method that can improve efficiency, safety, and quality.

How to Utilize LRTK for Simple Surveying

Tools have also appeared to make the point cloud measurement and ICT technologies introduced so far easy for anyone to use. One example is the versatile smartphone surveying system LRTK. LRTK consists of a small GNSS receiver and a dedicated app, and when attached to and started on a commonly used smartphone, it enables high-precision simple surveying. It can be operated intuitively without special surveying qualifications and is an excellent solution that supports everything from field surveying to point cloud acquisition and cloud storage.

LRTK’s simple surveying features include the following:

• Photolocation: Photos taken with a smartphone can be tagged with high-precision position coordinates. When you photograph the current condition of a slope, the coordinates of the photo location are automatically recorded, allowing dimensions to be measured on the photo later or the capture point to be precisely shown on a map.

• AR display: Design models and point cloud data obtained from surveying can be overlaid on the real scene on-site. Through a smartphone screen you can project virtual lines or structures onto a slope or visualize measured point positions, enabling intuitive on-site confirmation and instructions.

• Cloud synchronization: Survey data are synchronized with the cloud in real time. Coordinates and point clouds recorded on site are immediately saved and shared in the cloud, eliminating concerns about data loss and the need for post-site transfers. Multiple people can access the same data and check progress smoothly.

• Monopod measurement: By mounting a smartphone and the LRTK device on a dedicated pole (monopod), a single person can easily measure points. Place the tip of the pole at the point to be measured and press a button on the smartphone to capture that point’s coordinates. This enables agile coordinate acquisition even in narrow or high locations without the need for two people to operate a rod and instrument.

• Point cloud generation: Using the smartphone’s built-in LiDAR or camera, the surrounding environment can be scanned to generate point cloud data. A portion of a slope can be scanned in a short time to create a 3D model for recording deformations or for quick as-built checks. Because obtained point clouds are tagged with RTK position information, they can be used as 3D data in the local coordinate system without post-processing.

• Volume calculation: The LRTK app can calculate volumes from point cloud data. For example, the volume of collapsed slope material or embankment fill can be immediately computed from on-site scans, supporting rapid initial response at disaster sites and quick calculation of construction quantities.

By using LRTK in this way, advanced 3D surveying can be practiced easily by anyone, strongly promoting on-site DX. Initial investment is small, and because it can be introduced using existing smartphones, adoption is spreading from small and medium-sized enterprises to municipalities. LRTK is also a tool for responding to the Ministry’s i-Construction initiative, and it is attracting attention as one of the standard surveying methods of the future. Consider introducing modern tools like LRTK into slope management to experience their accuracy and efficiency on site.