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СЪЕМКА ТРАМВАЙНЫХ ПУТЕЙ В ХЕЛЬСИНКИ С ПОМОЩЬЮ МОБИЛЬНОЙ ЛИДАРНОЙ СИСТЕМЫ
Ханну Корпела
Фирма Terrasolid Ltd, Kanavaranta 7 B 28, 00160 Хельсинки, Финляндия, директор по маркетингу и продажам, тел: +358 400 648 391, факс: +358 14 645 002, e-mail: hannu.korpela@terrasolid.fi
Точное определение положения рельсов является сегодня самой актуальной задачей, которую практически невозможно решить традиционной съемкой. С этой целью фирма Terrasolid с 2011г. Стала использовать мобильную лидарную систему для съемки трамвайных путей в Хельсинки. Главная цель проекта заключалась в распределении легко распознаваемых цветных лидарных точек, с которых определяется точное положение рельсов и всех прилегающих к ним конструкций.
На стадии проекта разрабатывались программные модули и среди них модуль для автоматического согласования переездов для контроля дорожных знаков и выделения путей из лазерных точек в виде 3D векторов.
Ключевые слова: мобильная лидарная система, трамвайные рельсы, съемка, сканнер, TerraScan, TerraMatch.
HELSINKI TRAM LINE SURVEY BY MOBILE LIDAR SYSTEM
Hannu Korpela
Terrasolid Ltd, Kanavaranta 7 B 28, 00160 Helsinki, Finland, Marketing and Sales Manager, tel : +358 400 648 391, fax : +358 14 645 002, email: hannu.korpela@terrasolid.fi
To survey the accurate xyz-location of rails is a very demanding task and practically impossible in traditional field surveying. Therefore Terrasolid applied in 2011 a mobile LiDAR system to survey the tram license in Helsinki city. The targets of the survey project were to produce well geo-referenced, colored LiDAR points and from those points detect the accurate location of rails and all structures close to the tram lines.
During the project the software modules were developed among others to match automatic the drive passes to control signs and to extract rails as 3D vectors from laser points.
Key words: mobile LiDAR system, tram lines, survey, scanner, TerraScan, TerraMatch.
The Traffic Administration of Helsinki city - the capital of Finland - takes care about the public transportation in Helsinki city. Among bus and metro networks in downtown there is a relative dense tram line network, of which length is total 97 kilometers. About 200.000 people use trams daily Today trams with modern wagons are living a renaissance due to their good transportation capacity and the pollution problems of busses.
The Helsinki tram network system was built at the beginning of 1900. Due to narrow streets at that time curves are very tight in many places. This causes turning problems to modern wagons. Today the bottom of wagons should be on a much lower level than earlier, but on the other hand a low bottom can hit ground, if the vertical
curves are too tight. Street curbs, platforms and other structures may cause problems as well.
Mobile LiDAR mapping offers an advantageous and fast method to survey the location of the lines and close environment without causing essential harm to the other traffic. The targets of the survey project were as follows:
- To survey an exact location of the rails and the electric wires, their holder and conductors above the lines
- To survey in 3D objects close to the tram lines such as street curbs, stops and other objects
- To search places, where the space between rails and street pavement is too narrow. This information is needed to estimate the magnitude and places, where man must cut street payment to replace current wheels with wider and less noise wheels.
- To detect the 3D geometry of rails and deduce then their horizontal and vertical components. These components can then be used in redesigning the alignments for renovation.
Terrasolid Ltd from Finland was selected as contractor of the total project. To subcontractor Terrasolid selected 3D Laser Mapping (U.K.), which used their Street Mapper mobile LiDAR system. The system consisted of 2 kHZRiegl scanners and one forward looking oblique camera. The system was installed on the top of a tram wagon to get a higher position compared to a car roof as used normally as a platform. The survey took place from 29th to 31st May.
Due to high buildings and trees satellite visibility and the GPS access is often very poor in urban areas and forest. This results as a poor absolute positioning of the total system. In mobile LiDAR mapping the major errors are the xyz positioning of drive passes.
To correct the xy-errors of drive passes one must have control points on the ground features, which can be identified also from LiDAR points. Therefore small, white signs were painted to the asphalt just before scanning. Then later on they were surveyed by high-precision total-stations. The distance between the locations of signs was in open areas 250 meters. If the visibility was limited, then a 50 meters distance was applied.
For elevation correction airborne LiDAR data was applied. This data was collected in previous year by a helicopter LiDAR system. The point density was around 20... 25 points/m and the accuracy of height around 3,5 cm. The mobile data was fixed to match airborne data.
All drive passes we categorized according to their accuracy of quality. In the final product the worst points were removed including as follows:
- Points due to stops
- Points, of which the survey length was longer than 100 m. In mobile mapping we are mainly interested about points of the street lanes, where the distance from scanners are only some meters. Thinking about their accuracy, the position of the system is critical. The longer is the distance from the scanner to the target, the more error is caused by the misalignment angles.
- All overlapping points were rejected after their drive pass quality. Only points from the best quality drive passes at each location was kept.
- In the delivery product the mobile and airborne data was merged together. Points from streets and building facades were mobile. Building roofs and other targets further away from the scanner than 100 m were airborne LiDAR.
The LiDAR data was fixed and classified by TerraScan and TerraMatch from Terrasolid. The oblique images were processed by TerraPhoto. Among others during the project until today a new function was developed to detect the control points automatic from LiDAR, compare their location to the surveyed points and then solve thexy errors automatic.
The project is still under way. The final deliveries are going to be available in late spring 2012.
About Terrasolid
Terrasolid Ltd. is a Finnish private software company. Today Terrasolid is the worldwide market leader, which develops and sells software for LiDARdata and image processing. Terrasolid packages - TerraScan, TerraModeler, TerraMatch and TerraPhoto - suit for airborne as well as mobile LiDAR data processing. Experiences about the Helsinki tram line mapping projects in 2011 and the latest news about city modelling texturing will be shown at Terrasolid stand in “Interexpo GEO-Siberia-2012”.
CONTACT
Hannu Korpela
Marketing and Sales Manager
Terrasolid Ltd
Kanavaranta 7 B 28
00160 Helsinki
Finland
Tel: +358 400 648 391
Fax: +358 14 645 002
Email: hannu.korpela@terrasolid.fi
www.terrasolid.fi
© H. Korpela, 2012
