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Mobile Mapping System Used for Precision Surveying in Danish Rail Project

LE34, a Danish surveying company, used mobile mapping to survey a 70 km section of the Odense-Nyborg rail line to collect sub-centimeter precision data without interrupting traffic.

Source: Trimble

Nyborg, one of Denmark’s oldest and most historic cities, is known for its cross-timbered old houses and the ruins of a medieval castle. The railway line between Odense and Nyborg was originally built in the 19th century and is considered to be an essential transportation corridor, primarily because it is a critical inter and intra-country trade route. The Odense-Nyborg corridor is a major trade artery for Denmark and is also a crucial trade link between Europe and Scandinavia. Owing to its economic significance and the increasing yearly rail traffic, a capacity expansion of the corridor was initiated in 2019. Although a detailed survey for the specified stretch was carried out in 2014, the government did not approve the expansion of the project due to budget constraints.  The project was reinitiated in 2019 but due to changes in some data points ─of the terrain, crossings, bridges, etc. ─ the survey work had to be redone.

The Danish Rail Authority assigned the contract to re-survey the proposed rail section to LE34, Denmark’s largest surveying company.  The re-survey task required the mapping of a 70 km section of the rail line, which also included a busy 35km dual-track rail line section.

Challenges

The biggest hurdles of the project revolved around the schedule, cost, and most importantly achieving the desired precision data. The daunting technical specifications ─ of achieving a desired sub 1cm precision on the survey control points ─ seemed almost unobtainable. Additionally, the client did not want the existing rail traffic to be impacted, further raising concerns over the safety of the survey crew.

Delivering Precision Positioning Using Mobile Mapping System

Breaking 1 cm barrier

A combination of dense survey control, rigorous processing of information, and high precision instruments, made the re-survey exercise of the Odense-Nyborg rail line possible. The integrated scanning solution, along with imaging systems, were used by LE34 to collect rich site data of the project corridor. The team used the Trimble MX9 mobile mapping system mounted on the front of a maintenance lorry to gather the point Cloud data across the desired stretch. This device is a mix of precision and dual high-rate scanner heads, a gyro-GPS positioning module, 5 mega-pixel (Mp) aft and fore cameras, and a 30 Mp spherical camera. The inertial measurement unit (IMU) and GNSS sensors in the mobile mapping system retain highest level of precision between the survey control marks, which can substantially degrade in case of prolonged GNSS outages or if these control marks are at large distance from each other. This led to the finalization of maintenance lorry speed at 45 to 50 km per hour as optimal speed, such that, the precision accuracy was constantly maintained throughout the length. Using the technology, any project manager could remotely operate the system and view the data points on a tablet.  

However, it is important to note that the traditional survey completed in 2014, did not go to waste. The survey points collected in 2014 helped in establishing dense control points of the 70 km stretch. The survey team at LE34 used this dense data of the stretch, post verifying it through the mobile mapping to examine the entire stretch. All the datapoints were then merged and used to create 3D solid models and planimetric drawings for essential attributes like crossings and bridges.

Additionally, LE34 developed its own web portal for the in-house team and for the client to access dense point Cloud data for rapid display. The Graphical User Interface (GUI) on this portal includes the cleaned-up point Cloud data, fore and aft images, and a dynamic map. The processing of inertial and GNSS sensor data is done on Applanix POSPac MMS Software for directly georeferencing the mobile mapping sensor data. The use of the MX9 mapping system allowed several post-processing options of the generated data for the user. Further, the company used Trimble Business Center for data extraction to deliver a robust CAD deliverable.

Outcome

With the data that had already been captured through a traditional survey done six years ago, two surveyors from LE34 verified the existing control points in only a week’s time. The mapping on the entire 70 km route was completed in a record-breaking time of four hours with a precision accuracy of 7mm. On achieving such high precision (sub 1 cm) survey targets, with an interactive 3D solid model as output, the Danish rail authority mandated the use of mobile mapping for survey of rail infrastructure as a standard practice.

Project Facts
LocationDenmark
ClientDanish Rail Authority
SurveyorLE34
Technology partnerTrimble
Digital technologies usedGNSS, Laser Scanning

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