The UK rail network houses over 30,000 bridges, tunnels and viaducts, of which a large percentage are from the Victorian era and are in challenging to reach locations…

This leads to vast investment in ongoing inspection and maintenance, carried out both by Network Rail internally and also by a network of trained and experienced suppliers. With challenges such as this to face, Network Rail is continually looking to innovative methods of survey and inspection to create efficiency, free up time, enhance safety and deliver enhanced datasets which can be used to assess the condition of its assets while minimising the requirement to put people in high risk environments.

Traditional survey and inspection has often had to take place outside of operational hours and away from running trains, and as such the quality of the data gathered and reports produced can suffer due to time and weather constraints, and also due to collecting survey and inspection data during darkness. The use of UAVs (Unmanned Aerial Vehicles) can add another aspect to this process, and by using UAVs as a ‘first pass’, much useful data can be gathered quickly and with minimum intrusion, which can then be used to identify where more detailed survey and inspection is required.

With the formation of Bridgeway Aerial in 2017 came an opportunity to merge aviation, rail, geomatics and inspection services into one overarching body. As a Network Rail approved supplier of UAV services, and holding enhanced permission to fly at night, with spotters, and in closer proximity to buildings and infrastructure than usually allowed, Bridgeway Aerial are able to maximise the links between internal teams and are in a prime position to analyse the requirements of survey and inspection projects, and to advise where UAVs may be employed as an alternative to more traditional means.

More and more, Bridgeway are finding themselves engaged by Network Rail and its Tier 1 suppliers at an increasing frequency to adopt UAV technology to capture initial survey and inspection data. When defining a survey methodology, multiple factors need to be taken into consideration including accuracy requirements, the most appropriate sensor technology, and deliverable formats. It is here that Bridgeway are able to offer advice and guidance to their clients to ensure that the deliverable that they ultimately receive is actionable and appropriate to the scheme.

The project detailed below is just one of a number of UAV surveys and inspections that have recently been carried out by the Bridgeway team, and gives a good overview of their approach to planning and undertaking complex works on the rail infrastructure. This particular project relates to a survey and inspection of the ‘Great Wall of Todmorden’, a 400-metre retaining wall on the MVN2 lines.

Todmorden is a town on the borders of Lancashire and Yorkshire in the Pennine hills of northern England, and this specific structure, said to be built from four million bricks, separates the 32-mile-long Rochdale Canal, and the Calder Valley rail line running at higher level. The structure is of local significance, was built in the 1840s and remains one of the town’s most iconic landmarks. In terms of surveying a structure of this type, several challenges present themselves as the wall is almost entirely uniform with few distinguishing features, is curved on two planes, is inaccessible from the canal side, and is also in an area of heavy vegetation. Careful thought and planning was required in order to create useful deliverables.

Deliverables for the project included:

• nadir and oblique (top down and angled) high resolution photography

• HD video

• georeferenced photographs to enable spatial contextualisation

• a 3D colour pointcloud

• digital Elevation models (DEM)

• a structural condition and defect report

• a high level overview orthorectified photograph

• detailed orthophotos and orthorectified elevations

• 3D revit model with embedded defect data.

In order to carry out the works successfully, a team was chosen including members of Bridgeway’s Aerial, Geomatics, and Examination teams, and a high level of desktop study and walkouts was undertaken to assess launch and recovery positions, methods of geospatially orientating the survey, areas of specific interest, airspace considerations, flights in proximity to the railway and other infrastructure, and other considerations directly relating to the UAV equipment.

Once planning was completed, survey control was installed and coordinated via Total Station and GPS observations in order to fully orientate the survey, and additional ground-based laser scanning was undertaken to be used as verification data later on in the process. The UAV survey was then undertaken over a period of two days, with Network Rail representatives in attendance alongside the Bridgeway Aerial and Examinations team. In total, just under one thousand individual high resolution photographs were taken, referencing 29 survey points and ultimately leading onto the creation of a 3D pointcloud containing over 178 million individual measurable points. When compared against the ground-based scanning, a comparative accuracy of 14mm was achieved along the retaining wall.

Added value deliverables included the creation of orthorectified photographs in widely useable formats, which are essentially merged overarching photographs including all of the individual images. The key benefit of this deliverable is to be able to geolocate individual defects or characteristics on a structure, within the context of the entire site.

The full suite of deliverables allowed Bridgeway Examiners to produce a Visual Report in line with NR standards, and also to identify areas where further tactile inspection would be advisable. A 3D BIM model was also created of the structure, and defects embedded within the model enabling the long term tracking and identification of maintenance or repair regimes.

While the collection of large and complex datasets can provide real benefits, it can also be a challenge to turn the data into a deliverable that the client can use. Different datasets can require significant investment in both IT hardware and software, can mean a change in internal workflows, and can also lead to changes in how established technical standards are applied and interpreted.

It is here that Bridgeway’s far reaching engineering expertise comes into its own, as we are able to advise our clients from a multi-faceted perspective and highlight the true benefits of complimentary datasets while having a knowledge of established practices.

Bridgeway’s approach to data collection is relatively simple. We ask that our clients come to us with problems that they need to solve, and we can find the most efficient way of gathering and processing the data required for their projects.

For more information or guidance on UAV derived survey, mapping and inspection techniques, please contact Richard Cooper, Geomatics and Examinations Director, at [email protected]