The use of landsat imagery in mapping the west rand-portion of the grassland biome in South Africa

Abstract

The purpose of this reconnaissance survey was to redefine and confirm more accurately the main classes of vegetation as established by Acocks (1975). Visual analyses of Landsat MSS images, based on standard aerial photo interpretation techniques, were carried out and uniform physiographicphys iognomic units were delineated. These annotated units were verified by field checks. Quantitative analysis of floristic data , sampled according to the Braun-Blanquet technique was used to define the mapping categories . Larger scale aerial photographs in conjunction with data from ground surveys were used to refine the Landsat derived map. The mapping scale of 1 :250 000 was reduced and the final map was produced at a scale of 1:1 000 000. A mapping accuracy assessment was done. The overall mean mapping accuracy was 79,8 per cent, with mapping accuracies for individual mapping categories ranging from 62,5 to 95 per cent . The final mapping units may be regarded as landscape types, due to the fact that they a re based on vegetation features as well as land attributes such as terrain form, climate, soil patterns and sometimes even the main land use practices. A catenary relationship between the vegetation and the landscape-soil complex is evident from the vegetation map . The geological structure and therefore geomorphology and topography, seems to be the overriding factor in determining the development of the vegetation in a semi-arid region such as the West Rand. Another important factor is the decline in rainfall from east to west . The local distribution of individual plant communities , within a landscape type is strongly influenced by the relief, slope direction, soil type ·and local moisture regime. During this study it was possible to identify and map vegetation variations occuring within the Bankenveld. Unfortunately, extensive areas of the Cymbopogon-Themeda Veld, the Dry Cymbpogon-Themeda Veld and Themeda to Cymbopogon-Themeda Transition Veld are presently under cultivation and it was therefore not possible to confirm or redefine these Veld Types. Although it can be concluded that Landsat MSS imagery could be a useful aid in the in the preparation of a vegetation map of the remaining natural vegetation of an area, a great amount of ground surveys would still be necessary for developing meaningful maps. It is hoped that this study will contribute towards the creation of a sound basis for the extrapolation of research results.