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Detecting and analysing "urban villages" in the Pearl River delta using multi-source remote sensing data

‘Urban villages’ is the Chinese version of informal settlement. It is a unique phenomenon that comprises mainly low-rise and congested, often illegal buildings surrounded by new constructions and high-rise buildings. Due to a lack of an unambiguous definition allowing for a spatial delineation of such areas, this article investigates a joint use of high-resolution optical and SAR satellite data through building extraction and 3D reconstruction of urban villages in Shenzhen, China. First, potential urban village footprints are extracted through a combined image fusion analysis of multispectral GaoFen-1 (GF-1) and high resolution TerraSAR-X radar (SAR) imagery. Then, building height estimation is performed on the basis of interferometry principles using interferometric X-band SAR (InSAR) from the Tandem-X mission. It can be demonstrated that urban villages and surrounding urban areas are clearly distinguishable through particular combinations of optical data, SAR data and height information. In particular, a rigid analysis identified three types of information as most suitable: 1) Normalized Difference Vegetation Index (NDVI), 2) contextual parameters such as edge and line density from GF-1 multi-spectral imagery, and 3) textural parameters such as Grey-Level Co-occurrence Matrix (GLCM) variables from TerraSAR-X imagery. The additional height information from InSAR clearly improves the detecting of taller buildings surrounding the urban villages. In conclusion, the fusion of SAR and optical imagery can effectively reveal the footprint characteristics of urban villages. It is an effective means to reduce the effects of layover, shadow and dominant scattering at building location. The 3D building reconstruction model based on urban village footprint maps can reduce the continuous alteration of layover and shadow areas from high-rise buildings in the dense urban area.