How drone surveys can identify risks before landslides occur

When landscapes are constantly changing, knowledge is essential. For municipalities, consultants, and developers, understanding the terrain is key to assessing risk, planning mitigation measures, and monitoring developments over time.

“The background to this assignment was a slope failure in a ravine near Bjørke in Ullensaker Municipality in 2025,” says Eivind Magnus Paulsen, Senior Engineer in NGI’s Remote Sensing and Geophysics Department.

A slope failure is the movement of soil or ground masses within the landscape. Even small slope failures can be important to monitor, as they may in some cases indicate an increased risk of larger landslides. A ravine is a valley formation that has been carved out over time by flowing water through a process known as erosion.

“Because the area contains significant deposits of quick clay, a decision was made to accelerate planned mitigation measures just a few months later,” Paulsen explains.

Quick clay is a special type of marine clay that can lose its strength when subjected to loading or erosion. In the worst-case scenario, this can result in large and rapidly developing landslides.

The mitigation measures were implemented to reduce the risk of further erosion and potential future landslides. Once the work had been completed, there was a need to document the results and establish a robust dataset for further assessment.

This is where drone surveying proved to be an effective tool.

Delivering data that supports decision-making

The goal of the survey was not simply to capture images of the area, but to create detailed terrain models that could be used by geotechnical specialists.

“The drone survey of the ravine provides highly detailed terrain data. These data are used to document the mitigation measures that have been carried out and to assess stability, ensuring that the measures are sufficient to prevent future slope failures,” says Paulsen.

For municipalities and project owners, this provides a stronger basis for evaluating the effectiveness of mitigation measures, documenting completed work, and planning future follow-up activities.

Surveying close to Oslo Airport required specialized expertise

The survey itself presented several challenges. The ravine was densely covered with vegetation, making it difficult to collect high-quality terrain data from the air.

In addition, the site is located just a few kilometres from Oslo Airport Gardermoen.

“The site is located approximately 2.5 kilometres from the runway at Gardermoen. This means extensive risk assessments and planning are required in advance, including approvals from Avinor and communication with the air traffic control tower before and after the flight,” explains Paulsen.

Prior to the operation, NGI conducted detailed analyses of the airspace, terrain, and vegetation to plan the flight and ensure safe execution.

“We need very detailed planning to obtain high-quality data, but also to avoid incidents and ensure that the operation is carried out safely.”

How LiDAR technology works

To map the terrain, NGI deployed a heavy-lift drone equipped with both a camera and a LiDAR sensor.

LiDAR, which stands for Light Detection and Ranging, is a technology that uses laser pulses to measure distances. By emitting hundreds of thousands of laser signals and recording their reflections, the system can build highly detailed three-dimensional models of the terrain.

“We use a combination of a camera and a laser rangefinder, known as LiDAR. The sensor emits around 350,000 laser pulses per second (350 kHz), allowing us to generate a highly detailed point cloud that describes both the vegetation and the terrain beneath it.”

A point cloud consists of hundreds of millions of measurement points with known coordinates. When combined, these points form a highly detailed digital representation of the landscape.

“This is particularly valuable in ravines and forested areas, where traditional photogrammetry often struggles to capture the terrain beneath the tree canopy.”

Photogrammetry is a method in which large numbers of overlapping photographs are used to create maps and 3D models. The method produces highly realistic models, but it relies on the camera being able to see the same point on the ground from multiple angles. It is therefore an indirect method based on image observations and geometric reconstruction in 3D. When vegetation obscures the ground surface, LiDAR offers a significant advantage because it provides direct measurements of the terrain.

“LiDAR can find openings in the vegetation that allow laser beams to reach the ground. This enables us to collect detailed data even in areas with relatively dense vegetation.”

Terrain changes can reveal risk

The value of terrain mapping lies not only in data collection itself, but also in the analyses that can be carried out afterwards.

“By comparing terrain data from different points in time, we can identify erosion and other changes in the landscape that may affect stability.”

Erosion occurs when soil and loose materials are gradually removed by water or other natural forces. Over time, this can weaken terrain stability and increase the risk of landslides.

“When monitoring erosion, we conduct multiple drone surveys and compare the terrain over time. By identifying differences and changes, we can detect both minor and more significant slope failures.”

Such analyses make it possible to identify trends at an early stage and implement measures before small issues develop into larger problems.

“We have, for example, identified areas along the River Leira where damage to bridge structures and slope failures have occurred. Findings like these trigger further geotechnical assessments to ensure that quick clay landslides cannot be initiated.”

A geotechnical assessment is a professional evaluation of the ground conditions in an area. The objective is to assess stability, risk, and the need for potential mitigation measures.

Technology contributing to safer communities

While advanced sensor technology plays a central role in the work, these projects are ultimately about people.

The data are used to protect communities, infrastructure, and buildings from natural hazards.

“It is incredibly rewarding to be out in the field carrying out this type of surveying, while knowing that it has such a significant impact on the people living nearby.”

For Paulsen, the societal value of the work is the main motivation.

“They gain a safer place to live. When geotechnical engineers have access to a strong data foundation for assessing stability and implementing the right measures, it contributes to safer and more resilient communities.”