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Methode de determination des invariants radiometriques adaptee au paysage semi-aride de l`Afrique de l`Ouest

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Abstract. The follow-up of changes by multi-temporal spatial remote sensing studies can be carried out by successive classifications or by dynamic study of the radiometric signal. In this last case, a prior stage of radiometric rectification is required. In developing countries, determinist methods of radiometric rectification are not operational due to lack of atmospheric optical properties automatic recorders. Ideally, empirical methods use only data included in the image. They are founded on research of invariant radiometric points. However they are unusable in rural areas of these countries as the usual invariant radiometric points (asphalt road, roof, etc.) are missing. A new approach, an automatic selection of invariant pixels, is proposed in this paper. This is carried out in two stages: firstly, the bare soils are selected by use of a vegetation index, secondly, an index of brightness is used to distinguish the areas of extreme brightness and darkness in the selected areas. On a site of central Senegal, the quality of the algorithm is assessed by an identification of the sets of invariant pixels using high resolution aerial photography and in-situ observations. Dark pixels arise mainly on lateritics and uncultivated lands. Bright pixels are divided between the sandy trails and the crusted surfaces of some fields. Potential applications of multi-temporal spatial data with high resolution are suggested in terms of multi-annual vegetation studies. Resume. Le suivi des changements par etude multi-date d`images de teledetection peut se faire par classification successive ou par etude dynamique du signal radiometrique. Dans ce dernier cas, une etape prealable de correction radiometrique s`impose. Dans les pays en voie de developpement, les methodes deterministes de correction radiometrique ne sont pas operationnelles faute de station d`acquisition des proprietes optiques de l`atmosphere. Ne necessitant pas de donnees exogenes aux images, les methodes empiriques fondees sur la recherche d`invariants radiometriques sont egalement difficiles d`emploi dans les zones rurales de ces pays car les invariants classiques (route goudronnee, toit, etc.) sont absents. Nous presentons ici une methode de selection automatique des invariants. Elle consiste en un double seuillage: le premier, au moyen d`un indice de vegetation, selectionne les zones de sol nu, le second, a l`aide d`un indice de brillance, distingue les extrema sombres et clairs de la droite des sols nus. Dans notre application, sur un site du centre du Senegal, l`identification physique des pixels determines invariants par selection automatique est verifiee par interpretation de photographies aeriennes a haute resolution et par verite terrain. Les invariants sombres se situent principalement sur les surfaces lateritiques et incultes, les invariants clairs se partagent entre les chemins sableux ou les surfaces encroutees de certains champs. L`utilisation de donnees satellitaires multi-dates a haute resolution spatiale, ouvre le champ du suivi interannuel de la vegetation.
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Document Type: Research Article

Publication date: January 20, 1997

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