Remote Sensing - a tool for environmental observation

Compilation of an error matrix is required for any

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Compilation of an error matrix is required for any serious study of accuracy. The error matrix consists of an n * n array where n represents the numbers of thematic classes. The left hand side (y-axis) of the error matrix is labelled with categories on the reference (correct) classification. The upper edge (x-axis) is labelled with the same categories and refer to the classified image or map to be evaluated. The matrix reveals the results of a comparison of the evaluated and reference image. Together with the matrix, computed by the sum of the diagonal entries, is the overall accuracy given. Inspection of the matrix reveals how the classification represents actual areas in the field. Furthermore, the matrix reveals class-wise how confusion during the classification occurs. Campbell (1996) provides a good description of the error matrix.
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86 Chapter 6 Image Interpretation 6.1 Introduction The objective of most environmental studies is not to determine the usefulness of remote sensing techniques for specific survey. Remote sensing is mostly a tool to collect information on the environment. The technical aspects of remotely sensed image acquisition, pre-processing and processing are often of minor importance. However, a certain level of knowledge of the technical aspects of remote sensing will be very useful for environmentalists to decide whether they should use data from radar systems or optical systems and to separate natural phenomena in their images from system caused patterns. Furthermore, some knowledge is necessary of the spectral behaviour of the objects of interest: soils, vegetation, crops and water. This chapter gives an introduction to the spectral behaviour of these four objects. 6.2 Spectral Behaviour of Vegetation The reflectance of vegetation in the visible wavelengths (0.43 - 0.66 μm) is generally small and reflection in near infrared (0.7 - 1.1 μm) is generally large. Figure 6.1 shows the major influences on spectral properties of a leaf. Three features of leaves have an important effect on the reflectance properties of leaves: - Pigmentation; - Physiological structure; - Water content. Pigments (chlorophyll a and b) absorb radiation of the visible wavelengths. The species-specific structure causes discontinuities in the refractive indices within a leaf, which determine its near infrared reflectance. The reflection properties of a vegetation canopy are affected by the physiological structure: the spatial distribution of vegetated and non-vegetated areas, the number of layers of the vegetation cover, the types of vegetation, the leaf area index, the leaf angle distribution and the vegetation conditions. Figure 6.2 shows an simplified example. Some narrow absorption bands due to lignin, cellulose etc. are present in near and short-wave infrared wavelengths. The presence of water often masks their absorption features and the bandwidth of the TM is in fact too broad to detect these narrow absorption bands.
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