Home: October - November 2007 › ABA Surveying Ltd - Getting the ERTMS project off to a flying start

ABA Surveying Ltd - Getting the ERTMS project off to a flying start

ABA Surveying Ltd - Getting the ERTMS project off to a flying start

04/12/2007 | Channel: Infrastructure

ALAN BARROW describes the survey process in support of the ERTMS project

The European Rail Traffic Management System (ERTMS) is a cab-based signalling and train control system being introduced for the first time in the UK on the Cambrian Line. Network Rail intends to implement the system to level 2 of the ERTMS on this route. All trackside signals will be removed and new axle counters, interlocking and balises will be fitted along the route together with digital radio links. A new control centre to manage the system will be built in Machynlleth.

Using data relayed from these systems each train’s onboard computer will allow the train to reach its maximum permitted speed while maintaining a safe braking distance. Because the time and distance needed to maintain safe train separation
will in future be managed dynamically the work of the surveyors in producing the
survey database has become hugely safety critical to ensure the continuing safe
operation of the system. One essential asset that the design team cannot do without is an up-to-date asset database of everything along the route together with the track alignment, both horizontal and vertical, related to the spatial position of all assets to the highest possible accuracy.

Compiling a 21st century dataset
ABA Surveying had been discussing with Ansaldo for several months before the
contract award about the various options available for the survey dataset. When Network Rail announced the award of the design contract to Ansaldo just before Christmas 2006 it signalled the start of the survey design phase for ABA. With such a multi-discipline project the designers were consulted early on and the dataset designed to be robust enough to satisfy all the questions that might be asked of it without any need to revisit the site. All this data had to be collected and packaged within a timescale of fourteen weeks to meet the demanding programme.

Well known for their world-leading expertise using 3D scan datasets for surveying the railway infrastructure, ABA have added airborne 3D scanning to this project in order to make possible the rapid data capture required to cover the 250km Cambrian route. BKS Surveys were contracted by ABA to carry out the airborne data capture using the helicopter-based Fugro Fli-map system. The airborne dataset will comprise a 3D pointcloud model of the route corridor with a point density of more than 50 points per square metre on the ground together with scale correct video imagery. In areas of complexity where greater accuracy may be required ABA will supplement the airborne dataset with ground-based scanning.

Designing the dataset
For conceptualising the design and developing the feasibility stage the 3D model and the video imagery will play an important part in bringing the site to the design office. For detail design, however, a full CAD model is needed and this will be extracted from the scan data and compiled in Microstation digital format. In parallel with the office-based data extraction, ABA field teams will be walking the whole route
logging all trackside assets. This walkthrough has dual purpose – it will be used to validate and add value to the ground detail extracted in CAD from the aerial survey and it will also be used in its own right to compile an asset database in spreadsheet format of assets against line chainage.

A survey planned for the future
Anticipating that, at the very least, the new assets to be installed will require to be added to both the database and the CAD drawings, ABA have installed a system of
permanent ground markers in pairs at nominal ten kilometre spacing along the route. Designed to remain stable for many years these markers consist of mush-room shaped rivets set into concrete bases. These survey points will be used as the datum points for any future survey works and are of such a high accuracy that they may be used also for future permanent way or gauging surveys. The grid chosen as the reference grid is true to scale (TTS) and is at track elevation level. This means that 1km measured on site is also 1km when calculated from the coordinates in the CAD
model. This removes the possibility of slippage or error in chainage as would be the case with scale factors and distances related to sea level. The grid has been commissioned from University College London who have designed similar grids for both the ECML and the WCML.

How is it being done?
The first thing for ABA to do was to establish a primary network of reference markers at 30km intervals along the route. These points provide the backbone on which all survey data will be based.

A total of nine such points were established and simultaneous GPS observations recorded at each point over a period of eight hours. This gives us a network of GPS vectors with expected accuracies in the region of 1-2mm per km. The coordinates of these points are calculated initially in the ETRS89 coordinate reference system. A mathematical conversion then converts to the project grid which, as previously mentioned, is TTS and at track level. A mathematical conversion exists that enables the ETRS89 coordinates to be converted to National Grid coordinates, thus allowing the survey to be registered to the National Grid mapping. GPS receivers are set at these control points for the duration of the airborne data capture.

A twin-engine helicopter flying at 500 feet is used for the airborne scanning. The helicopter is mounted with a rig carrying forward and vertical 3D scanners and interlinked forward and vertical highresolution digital cameras.

On the rig there are also two outriggers that carry the GPS antennas that continually record the attitude and orientation of the aircraft. An onboard Inertial Navigation System with sensors monitoring pitch and roll completes the airborne equipment. As the helicopter flies along the site the scanners scan a corridor continuously from left to right. Ground terrain points in this corridor are collected at a density of 50 points per square metre – approximately one point every 100-150mm.

The simultaneous operation of the ground-based and airborne GPS enables us to precisely relate the position of the helicopter to the survey control grid. The sensors on the helicopter monitor the pitch and roll and heading very precisely so that we can calculate the grid position of every scanned point. Putting it all together the accuracy of the scanned ground points will be better than 50mm in absolute
positioning. The final component of the data capture is from the downward and forward-facing video cameras. This imagery is geo-referenced to the survey grid for plan position and corrected for height distortion to provide an orthorectified image.

Although the imagery is good there will inevitably be information that cannot be
recorded from the air – either because it is too small or simply because it is hidden in
bushes or under trees. ABA field teams will now walk the whole route to validate the survey and record all asset numbers (S&C or mloc boxes for example) together with the permanent way infrastructure details that are needed to complete the asset database for the designers. This information will be added to the CAD drawings and scheduled into spreadsheet format against feature code and chainage. Yet another task for the field teams will be to survey the tracks through the tunnel sections together with any assets inside the tunnels.

Did it work?
It certainly did. The data capture over the entire 250km route was completed in just two days during the week following Easter. The rest of the data collection and the
processing is in hand now and scheduled for completion for the beginning of July – well in time for the finalising of the design for the new installation works.

Minimum risk
Because the dataset will be so complete there is minimal risk that it will not be able to answer any question that may be posed by the designers. The dataset will last
through the project and can be used for 3D visualisation, fly-throughs, detailed mapping including contouring and a host of other mrailway-specific uses. It can be updated with changes and will provide the asset database for future maintenance planning. It will provide the lowest project-life survey cost and beyond if it is kept up to date.

ABA Surveying Ltd
Tel: 01483 797111
Email: alan@abasurveying.co.uk
Web: www.abasurveying.co.uk