Eyes in the sky Feet on the ground

September 1st 2009

Eyes in the sky Feet on the ground

Rollo Rigby of Severn Partnership reports on how Irish Rail has benefitted from 3D survey data capture.

Iarnród Éireann - Irish Rail - has embarked on an extensive track renewal project between Dublin and Cork with the aim of increasing linespeeds, network capacity and improving customer safety, service and reliability on the route. Fugro-BKS led a team that included Severn Partnership to provide a comprehensive 3D dataset of the double-track line for use in the new permanent way design. While Fugro-BKS undertook the aerial survey and produced the final photogrammetric digital mapping deliverables, Severn Partnership's role in the project was to provide a permanent primary and secondary control network.

Although familiar with such challenges, Severn Partnership was up against the Irish weather in the months from October to January. Having been told it can rain every 15 minutes in Ireland, it soon became clear that it rained in-between as well!

Custom site grid

Given the overall project objective of providing continuous mapping and a DTM of the Dublin-Cork route, a high-accuracy geodetic control network - with an engineering grid scale factor of 1 - was vital for the vast amount of survey data which had to be combined accurately on this project.

Severn Partnership has past experience of long, linear rail projects, most notably on the East and West Coast main lines. Using this, it commissioned University College London and Dr J Iliffe from the Department of Geomatic Engineering to produce a custom site grid using the SnakeGrid software.

The grid maintains a scale as close to unity as possible along the route of the project, accounting for horizontal and vertical scale distortions. This allows a single grid to cover the whole project without the need to compromise accuracy with local scale variations. The custom grid - DC08 - allowed Severn Partnership and Fugro-BKS to achieve their goal of ±40mm accuracy from the aerial imagery, a figure that is close to the limit of what is possible from traditional aerial techniques.

Photo Control Points

Coordination began with the 12 primary networks located at major stations along the route at intervals of approximately 30 kilometres. To create a homogenous network, minimising any daily differences in GPS conditions, all 12 control points were occupied simultaneously with 12 geodetic grade receivers for a full 12-hour period. The observation time was more than required. The 12 hours of data allowed a baseline to be computed, leaving plenty of redundancy to perform QA checks within the network. These primary control points were then used to coordinate 53 secondary control points - one every 4 kilometres - and some 400 Photo Control Points (PCPs) along the route.

One major benefit of remotely placing primary, secondary and photo control on Irish Rail operational land was the huge cost savings in terms of track access planning and critical safety staff resource costs.

All the control points were placed within a 120-metre corridor of the Dublin-Cork line. With PCPs in remote areas of farmland with no hard detail or vehicle access, it was a logistic problem to actually find the locations and then a survey problem to locate the point to within specification. A combination of handheld GPS units, maps and common sense ensured a team of 12 installed all 400 control points in just six days, come rain or more rain! Across 267 kilometres, these were GPS coordinated in two weeks, keeping ahead of the planned flight routes for aerial photography.

Problem with ‘black box' systems

The recently increasing use of GPS technology has led to the national provision of level data being switched from fixed benchmarks to GPS active networks. Whilst GPS provides an efficient survey tool with many benefits, there is a risk of poor data if it is not used and checked carefully. This is a common problem with many ‘black box' systems.

One such check on computed GPS primary and secondary level data was a comparison with the double spirit-levelled data. Two Zeiss DINI high-accuracy digital levels were used across the whole 267km route. This was a check for gross errors in the GPS and substantiated Severn Partnership's rigorous GPS site and processing work. A further check on the GPS processing and the custom DC08 SnakeGrid was the on-site measurement between some of the GPS fixed primary and secondary controls. Total stations measured the straight line horizontal distances and compared them with final adjusted values. The differences were all less than 10mm.

Aerial survey

The objective of the aerial survey was to provide continuous mapping and terrain data of the route to assist in the future vertical and horizontal alignment design. In order to meet the survey accuracy requirements, Fugro-BKS's survey aircraft flew along the line of the railway at a height of 1,250 feet above mean ground level, capturing imagery at a scale of 1:2,500 using Fugro-BKS's Leica RC30 large format camera system.

Despite winter weather conditions and civil aviation restrictions, the project was flown in a few days and the imagery processed in Fugro-BKS's facilities in Coleraine, ready for digital photogrammetric mapping and orthophoto production.

The 3D topographic mapping was produced with a horizontal accuracy of ±50mm and a vertical accuracy of ±40mm with all features >1mm in size when represented at 1:500 scale, captured in their true dimensions. The survey was also designed to allow Irish Rail to generate cross-sections along the full length of the route.

A further deliverable of the aerial survey was orthophoto mapping with a ground resolution of 10cm suitable for interpretation of the main ground features required for design, such as drain outfalls, access and egress. Around 2,500 orthophoto tiles covered the route with each tile in uncompressed format containing 12MB of data.

The invisible parts

Not all the main line was visible from the aerial photography. Severn Partnership's final but crucial remit for Fugro-BKS was to survey the track detail in these areas, providing ground-truth survey detail at every station - both active and decommissioned - every overbridge obscuring more than 20 metres of track and at intervals of no more than 10km down the track. This data would provide permanent way rail accuracy at the pinchpoints to the main line and act as a further check when combined with the aerial survey data.

The collection of so much data requires the best from a company's quality assurance system.  Both Severn Partnership and Fugro-BKS are accredited to BS EN ISO 9001:2000 and view quality as an intrinsic element to all business systems.

Some of the specific railway checks - with specialist written software to quality-assure large datasets - are track gauge and SC0 file checks. Having completed a lot of work on the UK infrastructure, one of the first QA parameters Severn Partnership had to reconfigure for working in Ireland was the track gauge change to 1600mm!

Communications

With Fugro-BKS based in Coleraine, Northern Ireland, Severn Partnership based in Shrewsbury and the client, Irish Rail, based at Inchicore in Dublin, an efficient means of communication was needed. An online schedule for the project - which was password protected and accessible by all members of the project - was put together that allowed all parties to keep up-to-date with 24-hour working. This gave good planning, flexibility and effective communications at all levels, allowing the many parts of the project to be combined and ensuring its completion on time.

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