Changing Horizons in Geography Education - HERODOT Network ...

images visualise the “tree vegetation”: value “1”was assigned to the vegetation classes “palmerais”,“forêt galerie” and “forêt claire”. For the multi-spectralimages these categories correspond with theNDVI-classes NDVI4 up to and including NDVI7(all parts with NDVI-values over 0.4). On theCorona image all digitised parts were visualised.Combining these binary images results inmulti-temporal colour composites. To illustratethe procedure, three images taken in the dryseason were compared (Figure 5). This wayseasonal variation should be eliminated.The binary ‘vegetation’ categories, as definedabove, on the Corona image of 1965, the SPOTscene of 1987 and the ASTER image taken in2001, are combined in Figure 5. Corona is visualisedin blue, SPOT in green and ASTER in red.Figure 5. Multi-temporal colour compositeInterpretation of the multi-temporal colour compositeA multi-temporal colour composite is a very useful tool to compare different images.This way seasonal as well as temporal variation can be visualised. The exampleshows how this method can be used for different studies.The colour composite in Figure 5 allows a quick visual interpretation of the vegetationdynamics in the study area over a period of 36 years. Together with the statistics (table2), which are derived from the composite, following conclusions can be drawn.The considerable amount of blue on the image, which stands for vegetation thathas disappeared since 1965, is the result of the high geometric inaccuracy of theCorona and of the fact that digitised vegetation categories were compared withNDVI-classified images.The cyan coloured part in the central area of the image stands for vegetation thatwas present on the image of 1965 and on the image of 1987, but not on the mostrecent ASTER image. This apparent vegetation degradation is caused by a haze onthe central part of the ASTER image (Figure 3) and is no indication of the disappearanceof valuable vegetation.There is a considerable white part visible on the composite. This means thatvegetation has been present on all three images, since white is the combination ofblue, green and red.These three points lead to a remarkable conclusion. The vegetation in the studyarea has not degraded in a significant way since 1965, in spite of the enormouspopulation growth.Classroom applicationThis study illustrates how a useful application of using remote sensing for interpretationand analysis can be done in class. With experiments like this, students can93