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Height information from laser - Altimetry for urban areas
Reconstruction of Buildings
The reconstruction of buildings from high resolution ALS data, seems to become a promising option. The high point density enables the proper determination of the position and orientation of planes, which put together the building, although it may appear that planes with similar orientations are merged (Maas, 1999). For mapping purposes one is usually interested in the outlines of buildings. These have to be determined by intersection of the planes. In ALS data, edges at height discontinuities are difficult to locate, while other structures near the edges of roof faces cause problems in reconstructing the outlines. Therefore, a manual correction step is required. To improve the automation level, a combination of ALS and aerial imagery, from which building outlines can be extracted rather reliable and accurately by (semi-) automatic means, seems to be promising. Also combining ALS and 2D digital map data may be appropriate, although the latter may be often quite out of date. Detailed reconstruction of buildings, like historical monuments, both from above as well as from the faces is becoming a realistic option by combining ALS data with laser mapping data acquired from terrestrial stations. Powerlines High resolution DEMs are suited for inspection surveys of powerlines (Figure 3). From the randomly distributed points hitting the wires the wire-lines can be reconstructed by using 3-dimensional line detection and modelling software. 
Figure 3, Representation of a Powerline from high resolution Laser-Altimetre data, acquired by helicopter(courtesy: Eurosense) However, inspection does not only include the determination of deformations of the wires, but also the identification of obstructions present in the corridors, for example caused by trees or illegal buildings standing too close to the powerline path. Their rapid detection and evaluation allows maintenance crews to react fast and to prevent disasters. Also, the inventory of damage after bad weather circumstances can be done easily from Laser DEMs. The required high resolution mapping necessitates the use of helicopters operating at low altitudes as recording platform. Telecommunication Detailed DEMs are essential to tower planning and micro-wave network design for example to answer the question where antennas for mobile cellular networks should be placed for optimal coverage. As bandwidth increases and higher frequencies are used the task of locating transmitters and receivers and minimizing the number of repeaters becomes critical. ALS data is well-suited for telecommunication purposes. For this application, the city does not need to be reconstructed with great accuracy. Often it suffices to model the buildings by a rectangular block with the base dimensions, orientation and the height of the building at meter level accuracy. An effective building height determination algorithm by combining high resolution ALS data and a 2D digital map has been developed by Lemmens et al. (1997). Additionally, high resolution imagery can be collected to facilitate further the design and planning process. Infrastructure Laser DEMs are beneficial for planning, design, inspection and maintenance purposes of infrastructural works. In the planning process corridor planning and environmental impact simulations can be carried out. The use of detailed DEMs enables to design a solution, which will have the least undesirable effects on the city landscape, vegetation, and hydrology. The DEM provides also accurate terrain information to optimise the construction process, and logistics, like optimal movement of earth works. Major road surface, runway surface, and railroad track deformations can be traced on the fly. Under special conditions, for paved surfaces even potholes deeper than 5 cm can be detected. Large obstructions like fallen trees after storms can be detected and registered. Change Detection of Buildings In many urban areas buildings are often subject to changes because of public construction works or reconstructions carried out by the owners. For tax purposes or subsidy arrangements, where money flows from the central government to local authorities or visa versa, it is often necessary to trace these changes. Automation of the change detection process is highly desirable to reduce the costs of tax collection. Although automated detecting of building changes from aerial and satellite imagery have been investigated intensively, they have never become operational. ALS seems to present new opportunities. Murakami et al. (1999) carried out experiments to trace automatically building changes by subtracting time sequences of ALS DEMs. The areas where changes are traced can be checked by field inspections or inspection of aerial photographs. Another application is the detection and measurement of sub-urban growth, in particular in cities where expansion is a highly uncontrolled process. Pollution and Subsidence In principle, high resolution DEMs are suited for detection of changes of any heights and volumes. In particular, when time series are applied height and volume changes can be traced. So, it could be an appropriate tool to detect the dumping of waste for example in ditches or to monitor (growth of) waste belts. Some cities, like Mexico city are subject of severe subsidence due to groundwater extraction. By regularly carrying out airborne laser-altimeter surveys, the level and rate of subsidence can be estimated and modelled. Concluding Remarks The main property of ALS is that it is able to determine accurately and detailed the 3-dimensional terrain shape. We have identified in this paper some of the abilities of ALS for urban areas. ALS has passed a rapid and successful development. At present, it has reached the stage of being an established, although still rapidly evolving technique. The only restriction for new applications seems to be unfamiliarity and unawareness among users about its full potentials. Although the method is operationally applied, it still is not a settled technique. Much has still to be investigated, including accuracy improvement of the measuring system and accompanying processing software, filtering techniques for automatically processing of the height data set and proper integration with other data sets. The determination of its suitability for a particular application requires a piloting stage. Much recent information on Laser-altimetry can be found on Martin Flood's Airborne Lasser Mapping Website: www.airbornelaserscanning.com References
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