Expansion urbaine et recul des surfaces végétalisées à Bouaké et Korhogo (Côte d’Ivoire) : analyse diachronique par télédétection
Abstract
Cette étude analyse l’évolution des surfaces bâties et végétalisées dans les départements de Bouaké et de Korhogo entre 2000 et 2025. Elle repose sur une classification supervisée par Random Forest d’images Landsat 7 ETM+ pour 2000 et 2010, et Sentinel-2 L2A pour 2025. Les images ont été harmonisées à une résolution de 30 m. Quatre classes ont été retenues : végétation, zones bâties, sols nus et surfaces en eau. La validation interne, réalisée à partir de pixels extraits des polygones utilisés pour l’apprentissage, a donné des précisions globales comprises entre 99,75 % et 99,95 %. En l’absence d’un échantillon indépendant, ces valeurs mesurent uniquement la cohérence interne des classifications. Entre 2000 et 2025, les surfaces bâties ont augmenté de 56,2 % à Bouaké et de 131,1 % à Korhogo. Dans le même temps, les surfaces végétalisées ont diminué respectivement de 12,6 % et de 39,2 %. À Bouaké, le rythme d’expansion du bâti a nettement ralenti après 2010. À Korhogo, il est resté élevé et le bâti est devenu la classe dominante en 2025. Ces résultats montrent des trajectoires d’expansion différentes. Leur interprétation doit toutefois tenir compte de l’agrégation de la classe « végétation » et des limites du dispositif de validation.
This study examines changes in built-up and vegetated areas in the departments of Bouaké and Korhogo between 2000 and 2025. It uses Landsat 7 ETM+ images for 2000 and 2010 and Sentinel-2 L2A images for 2025. The images were harmonized at a spatial resolution of 30 m and classified using the Random Forest algorithm in R. Four land-cover classes were considered: vegetation, built-up areas, bare land, and water bodies. Internal validation, based on pixels extracted from the training polygons, produced overall accuracy values ranging from 99.75% to 99.95%. These values measure the internal consistency of the classifications and do not represent independent validation. Between 2000 and 2025, built-up areas increased by 56.2% in Bouaké and 131.1% in Korhogo. Vegetated areas decreased by 12.6% and 39.2%, respectively. Urban expansion slowed markedly in Bouaké after 2010 but remained high in Korhogo, where built-up areas became the dominant class in 2025. These results show different urban expansion patterns in the two departments. However, their interpretation must consider the broad definition of the vegetation class and the limits of the validation method.
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