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May 3rd, 2008
Danish StreetMapper

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ImageGeographical information systems (GIS) can offer tangible benefits for those charged with the management of our transport network. However, even with sophisticated spatial analysis tools, advanced display options and compatibility with other systems GIS is just another piece of software if proper consideration is not given to the data held and accessed by the system. Dr Graham Hunter, Managing Director of UK Based 3D Laser Mapping explores how the next generation of mobile mapping systems is being used to capture highly accurate, detailed records of the highway infrastructure and associated assets at speeds of up to 70 kmph.

Laser scanning technology is coming of age. Advances in equipment specifications coupled with a greater understanding of the requirements of specific applications are ensuring that individual system components and complete solutions are being launched that offer unrivalled accuracies, repeatability and ease of use.

For example Riegl has just launched the LMS-620 the worlds first and only laser scanning system to offer 8,000 point measurements per second with a range of up to 2 km and an unrivalled 10 mm accuracy. Specifically designed for the geotechnical, geological and geophysical markets the LMS-620 is expected to revolutionise operational and safety applications within the mining industry. Alternatively Leica recently launched the worlds smallest laser distance meter, the DISTO D2, specifically designed for indoor applications and Trimble has released the SPS730 and SPS930 Universal Total Stations specifically designed for the high traffic scenarios found on constructions sites.

{sidebar id=122} Each of these systems offers a high-end technical specification focused on a specific industry or application, however perhaps more exciting are the opportunities that arise when technologies come together resulting in complete systems that can perform across a number of markets and applications such as the next the next generation of mobile mapping systems that are being used to capture highly accurate, detailed records of the highway infrastructure and associated assets at speeds of up to 70 kmph.

StreetMapper the result of a joint venture between UK based 3D Laser Mapping and German guidance and navigation specialist IGI is a world first that enables highly rapid and accurate 3D laser mapping from a moving vehicle. Combining well-proven laser scanning technology with high accuracy positioning systems the position of features, relative to the vehicle on which the system is mounted, are measured. Each laser transmits a light pulse that is reflected off the road surface or roadside feature and bounced back to the vehicle-mounted receiver. Using the time taken for each individual pulse to be returned and the known value of the speed of light the StreetMapper system can automatically calculate the distance of the feature from the moving vehicle.

StreetMapper is also accurately calibrated so that each laser transmits its optical pulse from a set position and at a known angle, and uses satellite technology to calculate the exact real world position of the system. Using basic trigonometric principles and these measurements and values the on board computer can process over 40,000 points per second to produce highly detailed and accurate three dimensional models of the highway and associated assets and features.

The system is equipped with multiple laser scanners, each with a range of 150m and an 80 degree scanning angle. Each scanner can perform up to 10,000 measurements per second with a scanning rate up to 100 scans per second. Numerous laser scanners can be used, and can be arranged on the scanner platform to suit different requirements and a range of host vehicles. StreetMapper can measure up to 40,000 3D points per second whilst in motion with a typical positional accuracy of 5mm for good GPS conditions and an average point-to-point accuracy of 3cm. The StreetMapper system can also be enhanced with video capture and the addition of other sensors.

During a recent field trial, commissioned by the Danish Road Directorate, StreetMapper was used to survey a stretch of dual carriageway just outside Copenhagen in order to demonstrate the accuracy and coverage of mobile mapping systems compared to traditional survey techniques.

The Danish Road Directorate, part of the Ministry of Transport, is responsible for the management of Denmark’s National Roads that carry almost 30% of the country’s total road based transport volume. The Construction Unit undertakes maintenance of the existing network, including rebuilding, and the construction of major, as well as smaller, new roads and this requires highly accurate and detailed surveys, specifically measurements of elevation. Current survey techniques, for example manual surveying using Total Stations, are slow and disruptive, not to mention dangerous due to the traffic on existing highways.

During the trial StreetMapper surveyed a series of control points. Each point had been previously measured by the Danish Road Directorate using traditional survey techniques in order to obtain exceptionally accurate readings; better than 10mm in both plan and elevation. When the StreetMapper readings were compared against the control measurements the results were directly comparable; 10mm compared to the control points and 12.3mm compared to the surveyed points. When the additional benefits, which include increased speed and coverage of surveying, better passibility on the road during survey and improved safety for survey operatives, offered by the StreetMapper system are considered this is a significant achievement.

{sidebar id=123} StreetMapper has also been used in the UK to facilitate accident investigations on one of the UK’s major freight routes. The A14 is 125 miles long and runs between Europe’s largest container port, Felixstowe, in the East and the M1 in the Midlands. Cambridgeshire Police commissioned a trial survey of a notorious stretch of the road located between Cambridge and Huntingdon in an effort to reduce traffic congestion and delays that occur after an accident. Using StreetMapper a highly accurate and detailed 3D map of the highway, surrounding features and environment was produced enabling accident investigators to concentrate on measuring vehicle debris, tyre marks and other information directly related to a specific incident rather than the road itself, decreasing the amount of time spent on the highway and therefore reducing the impact on other road users.

Engineers at Halcrow are also using a 3d model generated from survey measurements captured using the StreetMapper system to assist with the redesign of a Coventry roundabout as part of a project to create a high-class business park on land adjacent to the intersection. Halcrow are working for St Modwen Properties Plc, one of the country’s leading regeneration specialists, who purchased the 53 acres of surplus land at the Jaguar Engineering, Whitley, Coventry for the creation of a high-class business park. Whitley Business Park, as the site will be known, is located at the A45/A46 interchange, two miles south east of city centre and close to Coventry Airport. The junction carries 86,000 vehicles every day, according to Highways Agency figures, a level of traffic significantly greater than the existing junctions design capacity already resulting in congestion, queuing onto the Tollbar roundabout and long delays. By mapping the highway with precision from a moving vehicle, StreetMapper eliminated any disruption to normal traffic flows, improved the safety of the workforce and achieved significant project savings.

StreetMapper is also proving useful for ensuring the safe transport of abnormal loads. In one example National Grid used the system to survey from Ellesmere Port to Cellarhead Substation in Staffordshire to ensure route was suitable for transporting a large transformer. The 19km survey was completed in just one hour by driving the length of the route at normal road speed. The 93 million points of survey data were then analysed to highlight problem areas on route called ‘pinch points’.

Further development of individual components of the StreetMapper system are opening up potential applications outside of the already proven highway related projects. An upgrade to the navigation system used by StreetMapper is delivering higher and more consistent positional accuracies in the built up environment. IGI have upgraded their TERRAControl navigation system especially for city modelling applications where there is reduced GPS visibility due to tall buildings. Using a technique called ‘Direct Inertial Aiding’, an inertial navigation system assists the GPS receiver in areas of poor GPS signals. After losing the GPS signal when passing a tall building, the receiver can rapidly lock on to GPS signals again and maintain positional accuracy.

Initial tests have confirmed that typically the GPS receiver will get back to a high positional accuracy up 5 seconds faster when using the Direct Inertial Aiding after losing the signal. The overall result is higher positional accuracy across the whole survey, which has been proven in recent tests in Germany. The resulting computer generated 3D models will provide city authorities, architects and developers with an invaluable tool to assess the impact of proposed changes to our urban environments. 3D cityscapes allow existing and proposed developments to be viewed from any angle and orientation. This will allow anyone to undertake an on-screen walk around or fly through of an area and project planners can add and change developments easily to illustrate the impact on the environment.

Mobile mapping using laser scanning technology is not however limited by the transport networks that span the countryside and urban centres. Flexible system configurations enable mapping projects to be undertaken from a range of host vehicles and even portable systems carried around in a rucksack. For example, a laser scanning system was recently used to map part of the South Coast of England in order to monitor changes in coastal defence system over time. The two part survey involved mounting the system first on an off road vehicle and then a boat in order to get two very different perspectives of the coastline. When compared against known control points, surveyed using traditional techniques and static GPS, the mobile laser scanning solution achieved comparable accuracies with standard deviations in elevation of 13mm, 20mm and 22mm at three sites.

The above examples demonstrate just a few of the potential applications of mobile laser scanning and when the additional benefits of mobile data collection, such as speed and flexibility of data collection, reduced post processing requirement, increased coverage and resolution of collected data not to mention the reduced costs and potential risks to the
survey team, are considered it can be concluded that mobile laser scanning is definitely coming of age. This maturity brings in turn an immense potential for the application of GIS for improved management and development of our transport infrastructure.

Dr Graham Hunter is Managing Director of UK based laser scanning specialist 3D Laser Mapping and can be contacted via graham@3dlasermapping.com.

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