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GIS-basierte Quantifizierung der Denudation im Zentralen Kenia Rift / S. Rössner (1997)

Public Titre : GIS-basierte Quantifizierung der Denudation im Zentralen Kenia Rift Titre original : [Quantification basée sur le SIG de la dénudation du rift central au Kenya] Type de document : Thèse/HDR Auteurs : S. Rössner, Auteur Editeur : Munich : Bayerische Akademie der Wissenschaften Année de publication : 1997 Collection : DGK - C Sous-collection : Dissertationen num. 475 Importance : 111 p. Format : 21 x 30 cm ISBN/ISSN/EAN : 978-3-7696-9515-1 Note générale : Bibliographie Langues : Allemand (ger) Descripteur : [Vedettes matières IGN] Applications SIG [Termes IGN] données géologiques [Termes IGN] érosion [Termes IGN] géologie locale [Termes IGN] Kenya [Termes IGN] modèle numérique de terrain [Termes IGN] point de vérification [Termes IGN] positionnement par GPS [Termes IGN] rift [Termes IGN] système d'information géographique [Termes IGN] variabilité Résumé : (Auteur) In this work a GIS-based approach for quantification of average surface lowering (denudation) was developed. The goal in using a GIS (Geographic Information System) was to explore the methodological opportunities of a GIS in order to obtain spatially high-resolution denudation rates which form the basis for investigations of the regional variability of denudation and its relationship to other landscape-forming factors. Such a spatially differentiated view of denudation requires a landscape frame of reference which allows direct quantification of the amount of eroded material in the source area of denudation. Within the Bahati-Kinangop Plateau (Central Kenya Rift) this condition is fulfilled for a 700 sq. km. area, where the highest topography is formed by relict elements of a dated erosional surface. These relict parts of the present relief are used to reconstruct the pre-erosional surface at the time when the plateau came into existence (about 3 Ma ago). The elevation difference between the pre-erosional and the present relief represents the amount of denudation since plateau formation. Thus, present topography is a direct measure of the long-term avarage intensity of denudation during the last 3 Ma in this area. Calculation of denudation rates using the elevation difference method consisted of several steps. It included the derivation of modelling assumptions which were needed for reconstruction of the initial plateau surface based on the geological-tectonic evolution of the area. Additionally, the analogous geo-scientific and topographic data had to be integrated into a digital database in order to perform a quantification of denudation rates within a GIS. Due to the close morphological relationship between inital and present surfaces, the generation of a Digital Elevation Model (DEM) of the present surface formed the starting point of the GIS-based data analysis. This DEM was obtained from contours of the 1:50.000 topographic maps by creating a triangulated irregular network (TIN) forming the basis for a regular raster interpolation. In order to achieve an average height accuracy in the range of half of an equidistance, a control point based approach was developed. It assess the height accuracy of the DEM in relation to map control points as well as to the real surface as determined by relative GPS measurements in the field. The reconstruction of the initial plateau surface is based on a combined analysis of topographic and geological information within the GIS which resulted in 96 relict surface elements representing the input information for digital modelling of the pre-erosional relief. Plausibility checks showed that the generated initial surface is in overall accordance to the modelling assumptions. The denudation rates which were derived from the elevation differences between the two DEMs vary between 1 and 20 m/Ma in about 75 % of the study area. The most deeply eroded areas are characterized by rates between 45 and 100 m/Ma and are limited to 5 % of the area. The relict parts of the relief with denudation rates between 0 and 1 m/Ma occupy about 5 % of the area. These results compare well with rates typical for tectonically stable highland areas of Africa. This GIS-based approach allows assessment of denuation rates in a well understood and reliable way. The results are characterized by a high spatial resolution which has not been obtained so far by other methods used for quantification of denudation. These high resolution rates offer the opportunity of a differentiated regional analysis. As long-term avarage rates they reflect the natural intensity of denudation. Such information are of special value in this area of present intense farming, because they can be used for estimating the influence of human landuse on the intensity of denudation. (Auteur)In this work a GIS-based approach for quantification of average surface lowering (denudation) was developed. The goal in using a GIS (Geographic Information System) was to explore the methodological opportunities of a GIS in order to obtain spatially high-resolution denudation rates which form the basis for investigations of the regional variability of denudation and its relationship to other landscape-forming factors. Such a spatially differentiated view of denudation requires a landscape frame of reference which allows direct quantification of the amount of eroded material in the source area of denudation. Within the Bahati-Kinangop Plateau (Central Kenya Rift) this condition is fulfilled for a 700 sq. km. area, where the highest topography is formed by relict elements of a dated erosional surface. These relict parts of the present relief are used to reconstruct the pre-erosional surface at the time when the plateau came into existence (about 3 Ma ago). The elevation difference between the pre-erosional and the present relief represents the amount of denudation since plateau formation. Thus, present topography is a direct measure of the long-term avarage intensity of denudation during the last 3 Ma in this area. Calculation of denudation rates using the elevation difference method consisted of several steps. It included the derivation of modelling assumptions which were needed for reconstruction of the initial plateau surface based on the geological-tectonic evolution of the area. Additionally, the analogous geo-scientific and topographic data had to be integrated into a digital database in order to perform a quantification of denudation rates within a GIS. Due to the close morphological relationship between inital and present surfaces, the generation of a Digital Elevation Model (DEM) of the present surface formed the starting point of the GIS-based data analysis. This DEM was obtained from contours of the 1:50.000 topographic maps by creating a triangulated irregular network (TIN) forming the basis for a regular raster interpolation. In order to achieve an average height accuracy in the range of half of an equidistance, a control point based approach was developed. It assess the height accuracy of the DEM in relation to map control points as well as to the real surface as determined by relative GPS measurements in the field. The reconstruction of the initial plateau surface is based on a combined analysis of topographic and geological information within the GIS which resulted in 96 relict surface elements representing the input information for digital modelling of the pre-erosional relief. Plausibility checks showed that the generated initial surface is in overall accordance to the modelling assumptions. The denudation rates which were derived from the elevation differences between the two DEMs vary between 1 and 20 m/Ma in about 75 % of the study area. The most deeply eroded areas are characterized by rates between 45 and 100 m/Ma and are limited to 5 % of the area. The relict parts of the relief with denudation rates between 0 and 1 m/Ma occupy about 5 % of the area. These results compare well with rates typical for tectonically stable highland areas of Africa. This GIS-based approach allows assessment of denuation rates in a well understood and reliable way. The results are characterized by a high spatial resolution which has not been obtained so far by other methods used for quantification of denudation. These high resolution rates offer the opportunity of a differentiated regional analysis. As long-term avarage rates they reflect the natural intensity of denudation. Such information are of special value in this area of present intense farming, because they can be used for estimating the influence of human landuse on the intensity of denudation. Numéro de notice : 28015 Affiliation des auteurs : non IGN Thématique : GEOMATIQUE Nature : Thèse étrangère Permalink : https://documentation.ensg.eu/index.php?lvl=notice_display&id=63362

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