Case study: Tracking soil erosion with mobile mapping, GNSS and LiDAR

Hexagon | NovAtel invited Dr. Antero Kukko of the Finnish Geospatial Research Institute (FGI) to share his work on tracking soil erosion, flooding events, and a changing topography through mobile mapping and GNSS, IMU, and LiDAR technologies.  

Dr. Kukko leads the mobile mapping group at FGI and holds an adjunct professor position at Aalto University in Finland. In this case study on fluvial geomorphology, he applies his knowledge of laser scanning systems and data processing to understanding the changing topography of the Pulmanki River in Finland.  

Read how Dr. Kukko’s work helps fluvial researchers to better understand and mitigate future soil erosion in rural and urban areas.  

Precise GNSS+IMU for Mobile Mapping Laser Scanning

On a breezy autumn day in September, North of the Arctic Circle in Finland, the silence of the wilderness surrounds. The sun rises slowly above the horizon between the rounded peaks of glacier-ground fells. Crystal clear water in Pulmanki River, a tributary to much larger Tana River streaming between Finland and Norway, flows calmly towards the Arctic OceanHowever, in the springtime, the stream exhibits a much more violent sight with roaring waves and turbulent waters of an ice-cold spring flood powered by the melting snow. 

This example of mobile mapping combines NovAtel SPAN technology with GNSS+INS, IMU and LiDAR sensors

The Akhka-R4DW backpack mobile laser scanning unit captures the river’s channel topography for an efficient data collection with the precise accuracy required for geomorphological analysis. (Photo: Antero Kukko, FGI, 2019)

A gentle humming sound disturbs the peaceful sight. A man carrying a backpack with optical instruments wanders across the sandy point bar. This instrument, called Akhka-R4DW, is a mobile mapping tool based on GNSS+INS positioning through IMUs and laser scanning. It generates accurate 3D maps of the river channel for fluvial geomorphology studies. 

The Akhka-R4DW backpack system used in this study is a result of proprietary research and development at FGI. We’ve relied on NovAtel products since 2005 in multiple research instruments and projects, including the SPAN CPT7 on drones, the PwrPak7-E1 on a mobile mapping autonomous research vehicle, and the FlexPak6 on airborne mapping systems. To develop the Akhka-R4DW for mobile mapping, we turned to NovAtel’s tightly integrated GNSS and IMU PwrPak7 enclosure to deliver highly precise positioning and inertial measurements with low power consumption.

With a NovAtel PwrPak7 GNSS receiver tightly coupled with their VEXXIS GNSS-850 antenna and ISA-100C Inertial Measurement Unit (IMU), the Akhka-R4DW uses GNSS signals from GPS and GLONASS constellations and inertial measurements to record position and attitude changes as the instrument travels along the riverbank. This raw observation data is then processed for accurate and precise trajectory information using Waypoint Inertial Explorer post-processing software.

Waypoint Post Processing allows mobile mapping analysis to be fully comprehensive

Waypoint Inertial Explorer’s differential, multi-pass solution to determine our trajectory. Top left: The number of satellites in view. Top right: The estimated attitude accuracy (arcmin) of our trajectory. Bottom left: Our trajectory along the river with the base station indicated as a green triangle. Bottom right: Our estimated position accuracy in meters.

Waypoint incorporates differential GNSS corrections between the base station and the Akhka-R4DW. The trajectory Waypoint produces and refines is then used for direct georeferencing with our LiDAR laser scanner that uses 3D point positions to develop environmental modeling. Through this whole process, we begin to understand how the streambed topography has changed over time.

From Mobile Mapping to Point Clouds

The team reconstructs river channel topography and vegetation through mobile mapping and LiDAR.

The team reconstructs river channel topography and vegetation through our mobile mapping and laser scanning instruments. The high bank seen in the middle-left reveals patterns of glacial deposits. Researchers can anticipate future topographical changes as well; for example, it is likely that the river will breach the top of the channel’s loop, creating a small lake at the bottom of the point cloud. (Image: Antero Kukko, FGI, 2020)

To generate our point cloud, we mount high-speed profiling scanners on top of the Akhka-R4DW for advanced modeling of the hydrodynamic system. These GNSS, IMU, and highresolution laser scanning data sets are combined for a holistic understanding of the Pulmanki River’s flow dynamics, current turbulence, and sediment changes. As you can see in the above point cloud, our Akhka-R4DW instrument’s combination of laser scanners, GNSS positioning and IMU motion measurements culminates in a very accurate representation of the Pulmanki River’s topography. 

Through additional techniques, like data fusion with sonar and acoustic Doppler bathymetry and drone imagery, our fluvial researchers can better understand the channel bed topography for hydraulic modelling and more. These combinations of different methods in our mobile laser scanning instrument facilitates the crucial time and temporal resolution on topographic data for researchers to examine rates of erosion and accumulation of deposits over time. 

Mobile Mapping with GNSS+INS and LiDAR creates this point cloud map that describes the changing landscape

Shown here is a survey trajectory on the sandy point bars of Pulmanki River, Finland. Features of the terrain, fell and tundra vegetation, and man-made structures are captured in the point cloud. Using LiDAR in our solution provides data on the changing topography and surface properties, allowing for different modeling approaches during analysis. (Image: Antero Kukko, FGI, 2020)

Tracking Erosion, Year Over Year

The latest paper of the research team published in Geomorphology (Salmela et al., 2020) focused on the maintenance of riffle-pool sequences and the morphological changes of the river over a five-year period. The results indicated that riffle-pool sequences in a meandering river are maintained by high discharge events like Spring flooding. Using our point cloud data sets, the team was able to document how, during these high discharges, the riverbed eroded on its concave sides and developed riffle-pool sequences. During low discharges of water, the team documented riffle erosion and pool filling along the river.  

The research team observed that while pools and riffles were not stable in size and shape, their longitudinal location remained the same. This observation would not have been possible without the combination of GNSS positioning, IMU measurements, and LiDAR point clouds.  

The changing erosion levels of the Pulmanki river year over year, captured through mobile mapping

Seasonal morphological changes at meander bends during the study period based on the team’s GNSS, IMU and LiDAR data sets (From: Salmela et al., 2020).

You can see the areas of sediment erosion in red and of sediment deposits in blue along the riverbank in the image to the right. By including representations of vegetation, researchers can understand the evolution of the erosion and deposition over years. Long term, this research provides insight in how to mitigate soil erosion in populated areas. For example, we can better understand how man-made erosion protection structures around suspension bridges have a strong influence on the river’s sediment changes through the year. 

About FGI

The Finnish Geospatial Research Institute FGI is a non-profit governmental research institute operating under the National Land survey of Finland. FGI conducts innovative research and expert work within the field of spatial data. The esteemed international research institute offers reliable information for the benefit of society. FGI has multiple NovAtel SPAN technologies through Navdata Oy in Finland. 

The Centre of Excellence in Laser Scanning Research aims to create new knowledge, science, openings and breakthroughs in the emerging field of Laser Scanning (LS). Our research covers the full complementary technology chain of Laser Scanning: hardware electronics, system integration, positioning technologies, information extraction and data processing, applications and visualization. Latest news and more information on laser scanning and the Centre of Excellence can be found on their website. 

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