Optical Remote Sensing in Urban Environments

International audience; Cities today face a variety of issues: attractiveness and economic development, living conditions and urban redevelopment, the quality of life of citizens and the environmental conditions of the urban system as a whole. These challenges reflect the situations in urban territories where the economic development and population growth required for stabilizing the urban system come into conflict with the promotion of esthetic urban improvements appropriate for social cohesion and ensuring the safety of users while guaranteeing a reduction of the environmental impact caused by urban spread [WGI 14, WEB 15, MAC 07, WIL 11]. The process of urbanization, which has significantly transformed our countryside over the last hundred years, was the most important factor of the 20th Century and saw the urban population surpass the rural population in the majority of Western countries. The process of urbanization can be defined as “particular forms and structures of space being occupied by the population [RHE 14]” – in other words the transformation of space (natural, agricultural, forest, etc.) through human activity. A recurrent problem is the objective demarcation of the urban system which is partly artificial and difficult to transpose from one case to another (conurbation, administrative boundaries, etc.). The determinants can vary in time and space (from one country to another1). In fact, identifying the place of study can sometimes get in the way of properly understanding the dynamics of urban growth and expansion, because like any other system, it is not exempt from external factors which in one way or another influence its internal dynamics. For instance, environmentally evaluating the quality of the water supply can be carried out at various locations in the urban system: where the water is extracted from, where it leaves the urban system and in the city center. The quality of the supply can only be determined based on sections that have been artificially developed. What renders the urban space so complex is that it only exists insofar as it is “built, worked and used through social ties” [CAS 72]. It can be thought of as a construction where the organization and the structure impact upon urban society but also as one element of a much wider ecosystem on which it depends and/or which it has an influence over. This system incorporates political, economic, social and, increasingly, environmental dimensions and we therefore regard it as a complex and heterogeneous ecosystem whose social, technical and natural components must be studied in order to understand the way in which it functions [MAT 06]. Taking these dimensions into account is necessary for urban studies, whether this is in terms of evaluating environments, modeling dynamics or outlining evolution scenarios. Over the past few years, the focus has been on gaining a better understanding of how the urban ecosystem works. This has taken place under the guise of different investigations: – a study focusing on the social, economic and spatial aspects of the urban system in order to consider the environmental aspect; – another study which focuses more on the interactions between the urban ecosystem as a whole and the natural systems on which it depends. This study examines both the fluctuations (energy resources, food, raw materials, etc.) and the state of these systems (the state of biodiversity, for example).