Remote sensing data do not compete effectively with extant sources of urban data that are in place, efficient and cost effective, and required for legal and other purposes. For example, remote sensing may be a suboptimal tool for obtaining information on the built environment if more detailed information is already available from regulatory, governmental or commercial sources. It is also unlikely that optical and microwave sensors will be able to obtain population counts that are as accurate as those currently provided by census data—although remotely sensed imagery of land cover may facilitate spatial refinement of census-derived population distributions that are provided by conventional censuses. Nor can imagery measure the economic, policy, or social causes of observed spatial phenomena, although it can help quantify their consequences and extent. These factors can often be better understood with a combination of satellite imagery and other types of data, such as household surveys (Entwistle et al., 1998). Here again, there is a difference in the potential use of remote sensing data in developed and developing countries. The municipal administrative infrastructure in most developing countries is generally not capable of supporting the sustained and publicly available record keeping and data collection maintained by cities in industrialized countries. As a consequence there may be more new urban applications of remote sensing data in the developing world than in the industrialized countries. Implementing these applications will, however, require a sustained investment by local governments in technical staff and equipment (Steering Committee on Space Applications and Commercialization, 2001).