3d mapping landslides

3D Mapping of Landslides at Mam Tor with ROBIN

In by Harriet Brewitt

Subject: The Shivering Mountain

Mam Tor in High Peak, Derbyshire is an imposing 517m hill thought to be one of the first hill forts in Britain, however Mam Tor remains more famous for the active landslip and the effects this has had both on the local transportation links and the surrounding landscape.

“Mother Hill” was so called because of the recurring landslides on its eastern face which have resulted in an array of smaller, infant hills created out of the collapsed shale. The eastern face is continually in motion and appears to shiver as the surface falls away in heavy rain and winds, giving the hill its alternative name – the Shivering Mountain.

The Challenge: Effective Scanning of Mam Tor to Monitor Landslide Stability

The landslide advances at a rate of up to 500mm per year and in 1979, the continual battle to maintain the A625 was lost and the road finally closed. Two metre thick collapsed layers of gravel and concrete can be seen in places where the road had previously been repaired, creating an unusual topographical landscape which is now unpassable by vehicles. As the spread of Mam Tor continues, local farming land and outbuildings are now under threat of being buried under the dark grey mudstone.

One of the biggest challenges when assessing landslide stability and damage to local infrastructure is capturing suitable 3D data for detailed analysis and interpretation. Using terrestrial laser scanners has become common place to capture 3D data at landslide locations, though the often complex terrain and difficult conditions means that scanning even a small area can take days, with repeated scanner set ups not always being enough to ensure delivery of a seamless dataset. The additional challenge was to scan the designated area with only one operative.

3DLM’s Research & Development team have been working on a solution to overcome the limitations of traditional terrestrial LiDAR scanning and took to the hills with ROBIN, the world’s first all-in-one Mobile Mapping System (MMS).
Mobile Mapping allows for the fast capture of accurate data on any terrain, regardless of accessibility limitations. The ROBIN’s unique mounting system enables the scanner to be fixed to a backpack, vehicle and Helicopter/UAV, meaning that the system can be mobilised quickly to scan areas which were previously unreachable. Mam Tor presented the perfect opportunity to test the system’s off-road capabilities.

The Process:

The ROBIN was mounted in backpack mode and the project areas were captured by either walking or cycling, or a combination of both. As ROBIN’s standard battery pack lasts up to 3 hours, a comprehensive scan of the most affected areas was completed, with only a three minute initialization period necessary.

One of the key advantages of using the backpack system is the ability to get full access across a whole site, allowing for the examination of the most important features in this type of infrastructure stability assessment; in the case of Mam Tor, the fissures and fractures of the road created by the landslip.

A backpack mounted system allows for the capture of thousands of individual measurements per square metre, enabling comprehensive 3D maps to be created which capture the full geometry of the study area, accurately mapping the size and depth of the individual blocks. This type of very detailed 3d data is extremely difficult to capture using alternative surveying technology and represents a step-change in the way that walk-over surveys are performed.

The raw GNSS data captured by ROBIN was processed against the local OS GNSS station to improve accuracy down to centimetre level. The processed GNSS was then combined with the ROBIN IMU data using a Kalman filter to produce an accurate trajectory. This was in turn combined with the ROBIN raw scanner laser data to produce a geo-referenced pointcloud in UK National Grid coordinate system.

The Results:

From the first scan of Mam Tor, it was shown that a mobile mapping system was not only capable of capturing a large quantity of highly accurate data but that it was also possible for a lone-operator to cover a large area with only one initialization session. The highly portable nature of the ROBIN system allowed the operator to access all areas across the site and a 500m linear section of the road, enabling all data to be captured within a 30 minute period.

The landslip has caused extensive fracturing of the road surface, creating large detached blocks of compressed tarmac and hardcore. The main advantage to using MMS here is the ability to capture the width and depth of these fractures when in walk mode, producing precise and clear datasets which will become part of a detailed monitoring project over the coming months.

Mobile mapping has been shown to be 80% faster than terrestrial scanning, allowing for faster working in difficult conditions, such as damage mapping after natural disasters or structural collapses. Mobile Mapping Systems allow operators to assess stability of structures and dangerous areas and help to accelerate the decision making process to deal with ongoing crises and remediation such as the allocation of resources and response teams.

Given its flexibility and portability, an MMS can be suitable for a whole range of different geohazard applications as well as for urban planning, asset mapping and environmental studies. Regular repeat surveys are made easier for continuous monitoring projects as areas can be mapped quickly and using less resources.

The unique ability to mount an MMS onto a vehicle or transform into an airborne unit widens the number of applications which can be attempted using the same MMS unit, meaning that Mobile Mapping Systems are now accessible, flexible and deliver a rapid return on investment.