Time-Varying Segmentation for Mapping of Land Cover Changes
Among all satellite imaging systems, Synthetic Aperture Radar (SAR) is useful for monitoring purposes, as they provide data at any time under all weather conditions. Today, three spaceborne SAR systems are operational, while by the end of 2007 three further SAR instruments will be launched, thereby allowing to obtain an almost continuous monitoring of the Earth coverage. In order to translate these multi-temporal data into information in an unsupervised (automated) and reliable way, sophisticated algorithms must be available. In the past years several approaches - primarily based on texture analysis and statistical scene estimation - have been proposed for multi-temporal SAR data analysis. Such methods, based on probability density functions, perform well under strictly controlled conditions, but they are often limited with respect to sensor synergy where complex joint probability density functions must be considered - and to the temporal aspect. To address these limitations, an original set of algorithms for the unsupervised multi-temporal SAR data analysis is proposed. To this end, several issues have been tackled: filtering, edge detection, and image segmentation. Moreover, condition sine qua non for the system design, was that i) it is sensor independent, and ii) data from different SAR systems can be ingested without any a-priori knowledge about the probability density function. Basically, the proposed time varying segmentation involves four independent steps. In a first step, a multi-temporal anisotropic non-linear diffusion filter is applied to filter the images which ultimately allow feature extraction. Subsequently, an extension of Canny edge detection algorithm for multi-temporal edge detection is applied, hence obtaining an edge map consistent across the whole sequence of temporal images. In a third step, closed regions are obtained using a two-part coding scheme with the edge map (as side information) and region growing technique (using the multi-temporal stack). Finally, the number of underlying spectral class composing a segment histogram at every frame is estimated, thus detecting changes due to temporal land cover fluctuations. Results are presented based on a set of 16 ENVISAT ASAR Alternating Polarization and Radarsat-1 Fine Beam images acquired between November 2004 and July 2005 for an agricultural (maize and sun flower) area in South Africa.