477 Samuel Nájera Bailón Spacial analysis of video surveillance... impact on society and on various state institutions. Consequently, these must be considered as an active focal point. Subsequent to defining the scope of analysis, a further step was to determine whether or not a census of installed systems existed. This would exclusively consider cameras of interest to the current study as those directed at public highways, or those that serve as inter- or intraurban connection points, irrespective of whether these were of a public or private nature. After having taken this into consideration, three spatial levels were defined: a) Subterranean domain: this is understood as any public form of infrastructure below ground allowing for the transit of people or vehicles. This essentially boils down to Metro services as well as the tunnels pertaining to the urban road network. It is worth mentioning the existence of car parks of a magnitude large enough to generate small-scale suburban distribution networks. Since it is clearly impossible to access Metro network control systems, modelling this environment from a systemic perspective calls for this to be deemed a closed element due to its potential capacity for complete shutdown. In fact, this would make it impossible to access or escape via this means. b) Public domain at ground level: understood as conventional public areas, i.e. streets. It is at this level that practically all public and private CCTV systems are to be found. c) Subaerial domain: this is defined as the upper part of any structure or installation existing above ground level in the public domain for which the access route is either public or private property. Systems installed in this domain tend to serve two basic purposes: peripheral control, of a strategic nature, or control of access to the location in question. Given the inability to access these sites, as well as the infeasibility of registering them by means of any other medium (high resolution imagery, photographic flights, etc.), this has been classed as a domain not to be significantly evaluated. After defining the spatial scope, a classification system was selected. Crime Prevention Through Environmental Design,21 based on a logic of functionality, aims to redesign those areas where there is a high sense of insecurity. As part of a classification system that is easily extrapolated to the use of CCTV systems, it defines four categories: a) natural surveillance b) natural access control c) territorial reinforcement d) maintenance and space management. Müller and Boss,22 underscoring this functional logic, consider a 21 CPTED. Crime Prevention Through Environmental Design Guidebook. National Crime Prevention Council. Singapore. 2003. 22 MÜLLER, Christoph.; BOOS, Daniel. Zurich Main Railway Station: A typology of Public CCTV Systems. Surveillance & Society. Ed. Norris. 2004. Pp.161-176. http://revista.ieee.es/index.php/ieee
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