Field Group has successfully scanned all non-electrified railways in Norway using its state-of-the-art mobile mapping system (MMS). The system combines Lidar, GNSS, IMU and a 360º camera to ensure that the horizontal accuracy of the tracks is within 10mm. Field is one of Europe’s leading companies in aerial surveying, mapping and land surveying.
Temperature changes or the braking of heavy trains can cause movement in railway tracks. Too much movement can lead to derailments, which is why it is so important to regularly monitor such movement.
Rail authorities in Norway need to measure the tracks with a horizontal accuracy of 10mm every year to know whether the tracks are moving – and if so, by how much. The Field Group was approached to survey a total of about 1,400km of non-electrified railway track, up and back. This large-scale activity is not feasible with regular land survey methods. Therefore, Field designed and developed a new, state-of-the-art MMS combining Lidar, GNSS, IMU and a 360º camera, and a highly advanced post-processing workflow.
Combining sensors for smooth scanning
The tracks are scanned with a combination of several sensors. This includes a very high-quality Lidar unit from Zoller+Fröhlich (Z+F) which grants sub-millimetre accuracy. The generated point cloud is then georeferenced using a combination of GNSS and IMU devices and post-processed with Field’s self-built Terrapos software.
Terrapos takes advantage of the numerous permanent GNSS base stations set up by the Norwegian authorities. The surveyors scan all of the tracks twice, after which the point clouds are matched using a very advanced algorithm called ‘Tpmatch’, which has also been developed in-house. Tpmatch utilizes all the available information from the point clouds and navigation sensors to achieve the optimal solution.
From the georeferenced point cloud, it is possible to measure the tracks with an absolute accuracy of approximately 8-10mm, which is within the specifications of the rail authorities. “We have done several land survey campaigns to verify the result, and we see that in most cases, we are at about 4-6mm accuracy,” said Kim Willem van Woensel Kooy, product manager at Field.
The challenges of mapping in winter conditions
The MMS also includes the Mosaic X camera, a 360º mobile mapping camera. Some objects, such as signs, are more easily identified using images rather than the point cloud. Typically, the images could be used for colourizing the point cloud as well. However, as the survey was conducted when the ground was covered in snow, this was not the case for this project. Another advantage of capturing image data is that the images also can be published in Field’s 360 web viewer, called Mapspace, enabling the customer to use the images to view or inspect their infrastructure.
This newly devised mobile mapping rig and the method of data capture reduced the overall time frame of the project from six months to only 10 days of data capture and about four to five weeks of data processing and quality control. Additionally, using traditional methods, the labour requirement of the project would most likely require hiring all available land surveyors in Norway. “Without this method, the rail authorities would actually not be able to do these kinds of measurements,“ added Van Woensel Kooy.
Despite the project being carried out under severe weather conditions, there was no cause for concern regarding the functionality of the equipment in use. Arkadiusz Szadkowski, vice president software products, IT & cyber security at Field Group, reported: “The Mosaic X operates very well in harsh Nordic conditions; it operated well in North Norway in the middle of winter. In extreme conditions, it has proven how robust it is and it can cope with tough conditions.”
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