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Remote Sensing - a tool for environmental observation

Graduate and under graduate students are involved in

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Unformatted text preview: Graduate and under graduate students are involved in these projects. Figure 6.8 Suspended matter in the IJssel lake in the central Netherlands. 98 Chapter 7 Remote Sensing and Geographical Information Systems 7.1 Introduction For many environmentalists remote sensing is a tool to collect information on the natural conditions of vegetation or soils in their study area. Consequently, the classified remote sensing image is for them not a final product. The airborne or space born digital images must be com- bined with other sources of information or serve as an input for hydrological models, erosion models for bird or animal migration. Other sources of information comprise spatial (soil maps, vegetation maps, digital elevation models, hydrological networks, roads, administrative boundaries) and non-spatial information or databases (e.g. climate data). 7.2 Data Integration Issues A GIS or Geographical Information System is a computer system to store, process and display spatial and non-spatial information. As remote sensing images are already available in digital format, it is very easy to combine remote sensing data with data stored in a GIS. Figure 7.1 shows the principle of an integrated database containing remote sensing images, GIS maps and non-spatial information). The combination of images or maps from different sources requires an accurate geometric match. Consequently, geometric corrections are often necessary before the data can be integrated. Apart from geometric distortions the two images or maps may have a different pixel size. Resampling procedures can be helpful to overcome these problems. Resampling algorithms may work reasonable well for continuous data e.g. reflectance values, elevation information or groundwater levels. It is much more difficult to compute new pixel values for classified maps. Land use classes cannot be averaged, and other methods such as majority filters must be applied. A majority filter attaches the most occurring class in the input kernel to the output cell. 7.3 Modelling Spatial Processes in a GIS using Remote Sensing Data Models are used to represent complex processes such as erosion or evapotranspiration because the scientific knowledge is lacking to describe that process exactly. A model is a simplified representation of reality, which describes the object or the process in a formal, structured way. Models can help us to increase our understanding of the process: they offer the opportunity to explore scenarios and to assess the importance of each individual input variable. The use of remote sensing information as input to a model requires that: - The scale at which the model predicts the processes should match the scale of the images; - The time-scale of the model should match the acquisition frequency of the images; - It should be feasible to derive some of the model inputs from digital images. 99 Figure 7.1 Integrated image processing and analysis by means of a GIS (Buiten & Clevers, 1993)....
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