THE rail corridor is a difficult and potentially dangerous location for engineers and surveyors. Track possessions are usually short, at night, and can take considerable time and effort to arrange. All these factors can lead to delays in upgrade works and increased costs.

Using high-resolution imagery to map the rail network can help mitigate both delays and costs and, thanks to the reduced track access demands, significantly lower the risks to staff from working in these locations.

AtkinsAs a result, the use of high-resolution stereo aerial imagery to map London Underground's (LU) network is now an established process. Having accurate topographical mapping and high-resolution imagery accessible on every desktop via the intranet gives LU's designers access to information which would normally need numerous site visits.

Since summer 2012 Transport for London (TfL) has been working with design, engineering and project management consultancy Atkins on a programme of aerial surveys to provide digital mapping and ortho-rectified imagery to inform upgrade works to the above-ground rail network.

The initial works included sections of the Piccadilly, Bakerloo and Central lines followed by the Docklands Light Railway network in 2013. The latest project, which started in early 2014, includes sections of the Jubilee and Northern lines and further sections of the Piccadilly Line. Approximately 169km of lines as well as 21 depots and sidings, have been flown over since August 2012.

With continuing improvements in large-format digital metric aerial cameras, along with the right aircraft, Atkins has developed a robust methodology for achieving very high accuracies.

"The key benefit to this approach is that the mapping is produced without the need to enter the rail or highway corridor to undertake the works," says Mr Cory Hope, the geomatics capability lead at Atkins. "All projects require far less intervention on the ground and, apart from the health and safety advantages, the method also allows accurate remote mapping of other sensitive areas where access may be difficult or inappropriate."

In general the highest resolution imagery available from a fixed-wing aircraft has given a ground sampled distance (GSD) of 3cm. However, since 2012 Atkins has been able to employ an even higher resolution of 2cm GSD from the UltraCAM Eagle camera sensor. This camera creates a huge 262 mega pixel image with a footprint on the ground of 400m x 261m at 2cm resolution. The imagery is taken from a height of approximately 365m with the aircraft flown on a pre-agreed flight plan, with each image taking a massive 700Mb of disk space.

To allow stereo use, and therefore viewing in 3D, each forward image has an approximate 60% overlap with the previous one, with lateral overlap of parallel runs reducing to 30%. Furthermore, in order to prevent trains in operation from obscuring certain sections of track, the images are examined following the flight to identify the relevant sections which need to be re-flown to achieve comprehensive coverage.

This improved high-resolution imagery, combined with calibrated GPSINS exterior orientation data (accurate photo centre and aircraft orientation information) and high-quality Global Navigation Satellite System (GNSS) ground control, has enabled the production of aerial topographic mapping with an absolute 3D accuracy of +/-2 cm root mean square error (RMSE).

"A key benefit to the data users is the considerable enhancement of the quality of the ortho-rectified imagery which can be created," Hope says. "In fact, that quality is so good that it's arguable that staff working on site during a night possession would be able to gather less visual information than they could while sitting at their desk viewing the image mosaics and digitised topographic mapping."

Very rigorous procedures are in place to achieve these accuracies. To produce a good photogrammetric model for extracting the data, ground control, which are points measured on the ground visible on the flown images, needs to be comprehensive and accurate.

This can be achieved by installing a network of primary control stations - a surveying term used for the most accurate reference points - at regular intervals along the routes with the photo control (secondary control points) installed at approximately 500m intervals on both sides of the rail corridor and measured via dual GNSS baselines from two primary points.

All control during this project was measured using static GNSS baselines in closed and adjusted networks and linked to the London Survey Grid and height datum using the Ordnance Survey Active GNSS Network.

The final stage before data extraction can begin is the aerial triangulation. This, in effect, combines all the flight and ground data to create the working photogrammetric model.

At this stage the original images are first compressed from 700Mb down to 70Mb each to allow faster visualisation within the photogrammetric software. The ground control is then combined with the calibrated GNSS/Inertial Navigation System (INS) external orientation data from the flight to undertake a 3D 'triangulation' of the imagery and produce a set of working stereo models for extracting the 3D topographical data photogrammetrically.


Comprehensive quality checks of these results are carried out and, before any mapping is started, the 3D position of the ground control points is reverse checked using the final model and the differences examined to determine whether the observations meet the required tolerances.

The 3D digitisation of the topographical data is made in accordance with the specification set out by LU for both content and structure. Atkins' mapping team uses advanced photogrammetric workstations to digitise the data. This is by no means a simple task as it requires considerable concentration to ensure the detail meets the accuracy constraints and that the content is correctly identified and attributed, assisted by a comprehensive pictorial set of different LU asset types. The mapped information then goes through a rigorous three-stage quality assurance process before it is cleared for issue or further processing.

The mapping can be issued in a variety of formats depending on the users' requirements. For the projects undertaken for TfL the data is issued in Microstation DGN format and in LU's ICS format. Atkins worked closely with the LU survey team to refine and improve their ICS survey data format providing seamless data input into the track department's design software, using the in-house software which LU developed to manage the conversion.

Image resolution

Arguably the most important benefit from the improved image resolution is the quality and clarity of the orthophotos (ortho-rectified imagery) which are produced to complement the digital mapping data.

"With the advent of online services like Google or Bing Maps in the last decade, most people now use orthophotos in their everyday lives, but for those unfamiliar with the terminology, an orthophoto is an image which has been 'draped' over the ground terrain," Hope says. "This terrain can be a digital terrain model (DTM) which includes only ground features or a digital surface model (DSM) which includes buildings, and foliage canopy."

Using specialised software along with the DTM, a montage of the images is constructed with the scale distortions removed to create a photo map which can be viewed alone or as an underlay within programmes such as AutoCAD, Microstation, ArcGIS, Geomedia or other GIS software.

The orthophotos for these projects are developed to extend 100m either side of the routes to give additional context to the specific area of interest and they provide an invaluable tool for the LU upgrades. The DTM is generated photogrammetrically during the mapping phase and involves a careful mapping of the heights and break lines (change of surface shape) of the ground.

LU initially commissioned the photography and mapping as a cost-effective way to meet the requirements for a single project to avoid physical access to the railway. However, engineers and planners across TfL are now using the technology for a variety of projects.

"Having the data available at a 2cm resolution gives greater confidence, which in turn allows for the feasibility stage of projects to be accelerated," says Mr Adrian Lintott LU's land survey manager. "This enables LU to deliver upgrade works, such as new rolling stock design and track configuration changes, in shorter timeframes and with less risk when moving from feasibility to design. The data is available to all staff in TfL as both mapping and ortho-rectified images and is used on a daily basis."

While there are several methods to capture the data required to deliver the new TfL works, this high-resolution imagery provides a level of confidence in the identification and digitisation of features and assets not previously possible with lower-resolution imagery or aerial LiDAR.

There have been significant changes in the way topographical and asset information is captured in recent years. As remote sensing technology continues to advance, increasing amounts of data can be collected to the exacting standards required without the inherent risks of design and maintenance staff accessing dangerous locations directly.

This method has brought welcome time and cost savings to LU's network upgrade programme, while the technique has been adopted for another seven projects across Britain totalling nearly 800km of lines. And with further improvements in camera sensor technology expected given the rate of advancement already witnessed, it could play a major role in future project planning.