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Height information from laser - Altimetry for urban areas
Properties Depending on the system, the flying height, the speed of the platform and the merging of data stemming from multiple flyovers, densities up to various dozens of height points per square meter can be acquired. Helicopters are better suited for high resolution coverage, because they can easily limit their speed. Since ALS is an active system, data acquisition is independent of sun illumination, while no shadows are generated. Weather and sight conditions do only slightly affect flight surveys, making the technique fairly well independent of season and daytime. For many countries, these are beneficial properties. Contrary to photogrammetry, height values in areas with low texture such as beaches and dunes are obtained effortless. The used wavelengths (visible and near infrared) do not penetrate material. Consequently, when transmitted in the direction of a forest stand or bushes, pulses will reflect on the foliage cover. Nevertheless, depending on the density of the vegetation and the footprint size, parts of the signal will reach the ground; the footprint size depends on the characteristics of the ALS and the altitude of the carrier above ground surface, but is typically between 20 and 100 cm. By recording the last part of the return signal the distance to the ground is obtained. By recording in addition the first part of the return signal, canopy heights can be derived. Some systems are able to record the entire signal characteristic of the reflected pulse. Using advanced signal detection and processing techniques, information can be derived about the vertical structure of the surface, such as roughness, height and shape of objects, canopy density and height of trees, and the reflectivity of the surface. The amount of accompanying ground surveys is usually modest. Problematic accessible areas can be mapped relatively easy. When waterbodies are hit, parts of the pulses may penetrate water and reflect on the bottom of the waterbody, enabling the measurement of water depths. However, when waters are troubled, depth measurement fails. The pulse may also reflect on suspended layers causing spurious values. The absorption of pulses hitting water-bodies is rather high. Many pulses may be also subject of specular reflection, especially when the surface of the water body is smooth, yielding reflection of the signal in the direction away from the recording platform. Nevertheless, when choosing small scan angles sufficient signals may return to map water surfaces, in particular when the surface has a certain roughness. Experiments in the Netherlands have shown that laser-altimetry is able to map water levels of rivers. In urban areas, spurious height values may occur when a pulse is specularly reflected on a ground point, e.g. on the paved road surface. The reflected pulse moves away from the recording platform, but may hit next the face of a building. When this multipath signal reflects in the direction of the carrier, it may be picked up by the receiver of the ALS. Because the measured travel path will be longer than the actual distance, a false height value, up to several meters below the real surface, will show up in the data. Height Demands in Urban Areas Since ALS provides high resolution height data with an accuracy level slightly above the decimetre level, the technique is particularly suited for planning, monitoring and control purposes. Other applications like construction of e.g. traffic tunnels and bridges, and deformation control purposes, which require a dense and well-distributed network of bench marks with centimetre precision, remain to rely on trigonometric levelling. In the remaining part of this paper a number of applications are treated, which are needed by City Administrations. The treated applications are already operational, have passed successfully the piloting stage or are in the phase of experimental investigation. Flood Risk Management Floods cause nearly one third of the economic losses resulting from natural hazards. The role of floods as cause of natural catastrophes is increasing due to amongst others rising sea levels and increased urbanisation. The last issue results in a substantive growth of the earth's surface, even river-beds- covered with asphalt and concrete. Flood prevention and river management tasks are therefore important activities in urbanised areas of lowlands, such as the Ganges Delta, the Yellow River Delta, the Rhine Delta and the Mississippi Delta. For these purposes, detailed and accurate knowledge is required concerning the variations in elevation of the river-bed and surrounding area, height of dikes, flood waves, along-and across-track slope of the river and the water resistance of different vegetation areas. Studies in the Netherlands have shown that high resolution ALS is able to provide detailed information about the water level and the along- and across-track slope of the river Rhine (Bollweg, 1999). In the UK the applicability of ALS data has been studied for quantifying flood risk for insurance purposes (Murtagh & Cheesman, 1999). By incorporating the DEM in a GIS as input for a flood model, insight into which areas are susceptible to inundation is obtained. The height data provided by ALS is so detailed and accurate that they enable verification and even refinement of the input parameters of hydraulic models used in these simulations. Rain Water The heights of areas in many cities are subject to natural or man-induced changes. Man-induced changes may for example result from the development of a new living area. What will be the effect on the flow of rain water, once having reached the paved surface? Is the sewerage system still able to deal with the drainage of the all water, even in periods with heavy rain falls? Is the rain water still streaming in the direction of the river or reservoirs? Should the drainage layout be improved? These are all question a City Administration may be confronted with and which can be answered with the help of height information derived from airborne laser-altimetry. Urban Planning and Development The need for easy evocation of the environment is as old as is the human capacity of constructing buildings, bridges and roads. For example, integration of an architectural design with its surrounding, represented by a 3-dimensional landscape model that includes existing vegetation, facilitates highly the design process and gives engineers and planners an accurate impression of how their design interacts with its surrounding (Figure 2). Maps, scale models and later on, also aerial and satellite imagery, have always represented an essential tool for serving these purposes. However, these analogue media often give a rather poor backdrop. In addition, this type of representation is often hard to understand for the general public, especially when complex spatial structures are involved. Furthermore, urban planning and development requires increasingly 3-dimensional urban topography models. 
Figure 2, Part of a 3-dimensional City Model of Mannheim created by using Laser-altimetry (Courtesy: Toposys Germany) Computer technology has been for some time capable of creating 3-dimensional virtual fantasy worlds for entertainment purposes. This technique can also be used to support the urban planning and development process. When creating a 3-dimensional virtual world of existing or proposed reality, real data is needed. This data should not only be 3-dimensional, but also very accurate and highly detailed. The direct acquisition of 3-dimensional information is very time-consuming and expensive. This is one of the main problems to be overcome in the advancement of virtual reality for monitoring and design purposes. ALS, providing highly automatically spatially highly detailed geo-data, is a promising technique to tackle this data problem. |