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Height information from laser - Altimetry for urban areas
Mathias J.P.M. Lemmens Faculty of Civil Engineering and Geosciences Department of Geodesy Delft University of Technology Thijsseweg 11, 2629 JA Delft, The Netherlands Tel: +31 15 2781042; Fax: +31 15 278 2745 m.j.p.m.lemmens@geo.tudelft.nl
Abstract
The need for height information for approaching urban problems has grown rapidly the last decade. The needs include design and inspection of utilities such as water mains, sewer systems, tunnels, bridges, roads, railroads and power lines, and the creation of 3-dimensional digital city models, in which the shape of buildings and other objects have been reconstructed with high spatial detail, for city planning and development purposes. In the past, one could usually suffice with contour lines. At present, the demands with respect to accuracy and level of detail are far beyond this point. In addition, the processing of huge amount of data requires automation. New remote sensing techniques have emerged which are able to respond to these demands. They include Laser-altimetry, matching of stereo pairs of imagery and radar-based systems (INSAR). They sample the terrain blindly by automatic means, while being able to carry out data capturing with high point density. In particular, Airborne Laser-altimetry Systems (ALS) have many beneficial properties. The automation rate is large, while time-efficiency and cost effectiveness are high. This paper examines the suitability and possibilities of ALS for fulfilling parts of the height information needs in urbanised areas. Our considerations are partly based on an inventory study about height needs carried out among local authorities in the Netherlands. The local authorities involved cover both small and large cities. Our overview is interesting for many involved in urban planning, design, inspection and monitoring. Introduction Approaches to urban problems demand geodata, which represent in a highly detailed form the environment in its full spatial and time dimensions. Before being ready for use, geo data need to be acquired. This seems trivial, but isn't. Too many take the existence of geo data for granted. Acquisition of geo data has long been and is still continuing to be a cumbersome, labour-intensive and expensive task. It can be readily envisaged that within the establishment of national or regional geo-information infrastructures (GII), the availability and collection of data will be one of the main limiting factors. A favourable development, the last decade has witnessed, is the rise of new remote sensing techniques, which are able to collect geo-data in an automated way. For example, techniques to collect highly automatically 3-dimensional geo data in the form of Digital Elevation Models (DEMs) have rapidly emerged. In particular, those data acquisition techniques, which are able to deliver accurate and detailed 3-dimensional geo-data in a highly automatic way are forming the leading edge. They include Airborne Laser-altimetry Systems (ALS), matching of stereo pairs of (aerial) imagery and interferometric SAR (INSAR). For being operationally attractive, desirable properties, which these techniques have to fulfil, include:
The present paper elaborates upon the possibilities, which ALS has to offer for approaching space related problems in urban areas. Our considerations are partly based on an inventory study about height needs carried out among local authorities in the Netherlands. The local authorities involved cover both small and large cities. Because a large part of the Netherlands is located below sea level whilst the landscape is relatively flat, the accuracy demands of height information are at the cutting edge of technology. Our experiences are interesting for many involved in urban planning and monitoring processes. To date the possible fields of applications are far from being thoroughly scrutinised, so it is important to have insight into the principles and characteristics of the technique to arrive at a comprehensive understanding of the yet unexplored abilities. Therefore, we reserved also space to treat these features. Principles of Laser-altimetry Laser-altimeters operate usually from an aircraft or a helicopter, although also orbiting satellites are used (laser pulses can bridge long distances.) Airborne Laser-altimeter Systems (ALS) are multisensor systems consisting of a reflectorless laser range system and a positioning system. ALS was first tried out in the 1960s. In the 1970s experimental systems were developed. Accurate positioning remained one of the bottlenecks until a decade ago GPS became operational. A laser ranger determines the distances from the platform to arbitrary points on the earth's surface by measuring the time interval between transmission of a train of pulses (up to 80,000 per second!) and the return of the signals (Figure 1). A rotating or nutating mirror enables scanning perpendicular to the flying direction, resulting in a swath width, which lies in the order of half the flying height of the platform. A flying height of 1000 meter is typically used during operational flights.  Figure 1, Principle of Laser-Altemetry (Courtesy: Survey Department Rijkswaterstaat, Netherlands) The positioning system determines the position and attitude of the laser ranger. This is necessary for geo-referencing purposes, i.e. to determine the coordinates of the sensed points on the terrain surface in a local or national system. Usually one uses DGPS for position determination and an inertial navigation unit (INU) for the determination of the roll, pitch and yaw of the platform. The INU is autonomous and does not require any ground support. The sampling rate of GPS is low compared to that of the laser ranger and the INU. Transformation from the WGS84 coordinates to a local reference system requires a geoid. During flight a (digital) video records the terrain. The quality of the video limits its use to documentation and visual inspection. The final accuracy to be achieved depends on many factors, including the properties of the entire measuring system, flying height, terrain characteristics and applied processing software. During typical operational airborne surveys the accuracy values, to be achieved, are five centimetre systematic error and ±15 centimetres random (root mean square) error, both in the vertical. For a review of accuracy-aspects and issues refer to (Lemmens, 1997, 1999, 2000).
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