Using sequential aerial photographs to detect land-use changes in the Austro Ecuatoriano / Utilisation de photographies aériennes séquentielles pour détecter les changements d'utilisation du sol dans l'Austro Equateur

During the last decades, there is a boom of the use of aerial photographs and remote sensing images to collect quickly accurate digital information in remote areas. Due to an easier accessibility of aerial photographs and a significant decrease of their price as well as that of the image analysis and GIS software, these techniques are becoming increasingly popular. This article presents a methodology to use Geographic Information Systems for quantifying past land use changes from aerial photographs. An application of this technique to the Austro Ecuatoriano has demonstrated that such a methodology requires that the data are both spatially and temporally compatible. In order to obtain spatially compatible data, the air photos are geo-referenced using a set of ground control points (GCP's). Our analysis shows that, if certain conditions are satisfied, GCP's obtained from topographical maps are of sufficient quality to carry out the geo-referencing procedure with a very good precision. On the other hand, temporal compatibility appears more difficult to achieve : the scale (and probably the quality) of the air photos has a significant effect on the resulting land use classification. Consequently, a calibration of the various datasources is necessary in order to avoid bias in the final results.