## 4 Crowdsourcing Applications: From a General Framework to Spatial Planning Applications  
The focus of this part is on the general framework, featuring several crowdsourcing applications, and also on the way the whole process \"works\". Emphasis is given to the role that crowdsourcing can play in applications concerning spatial planning. Crowdsourcing basically involves the definition of a problem, the publicizing of the respective problem or request as an \"open call\", the recruitment of possible participants and the proposal of alternative solutions to the problem under study. The best solutions are awarded, and the company or the institution, which firstly published the problem, keeps and exploits the proposed alternatives. Generally, crowdsourcing can be adopted by companies, institutions and universities in several ways, so that fast, efficient and innovative solutions are discovered. Commercial companies that sale retail, such as clothes, shoes _etc._, request from the crowd to design products for them, and the most interesting designs receive a certain award. Several websites ask the crowd to upload products, such as photographs, and select the best among them in order to be uploaded on the site. Every download by the users of the site implies an award to the designer of the respective product. In the case of scientific institutions and universities, scientific problems are discussed and solutions are proposed by the users of the respective sites. As a result, innovative ideas and solutions arise, and organizations are offered the opportunity to discover skillful scientists from all over the world [9]. It is now clear that the contribution of crowdsourcing to the problem solving process is undoubtedly essential, and this is the main reason for its wide acceptance, either as an autonomous module during the problem solving process or as part of a general methodology, in various scientific or professional fields. Crowdsourcing enables the introduction of public participation, especially e-participation, into the planning process. In this context, a huge number of applications have been designed to serve several planning purposes and are implemented at both the urban and regional level. Some applications consider the issue of reaching consensus among stakeholders who are affected by the planning process, and thus, the purpose of such applications is stakeholders being involved in the process and expressing their opinions and preferences. Stakeholders can also discuss with each other, share content and knowledge, depict the opinions expressed on a map by the support of the respective web applications and propose and evaluate alternative solutions for the problem under study. In other words, they can \"actively participate\" and not simply get passively informed by the planners [13]. In some other cases, citizens may participate even during the design stage of a platform through which a web community can be created. Such actions can take place at the level of a neighborhood,where citizens may act collectively towards a common end. In this way, citizens have the chance to meet each other, exchange the experiences of their daily life and discuss several issues relating to the problems of their neighborhood or city [14]. As mentioned above, crowdsourcing is usually combined with GIS technologies, especially in cases where spatial information needs to be manipulated or generated. Recent advances referring to public participation GIS (PPGIS) applications include published maps relating to several environmental and spatial planning issues at the homepages of organizations, the implementation of real and dynamic GIS software that enables users to get in touch and interact with spatial information, Internet mapping for planning purposes, Internet-based public participation systems, information distribution and the citizens' chance to build scenarios and suggest alternatives. Some applications of this type relate to the shortest path problem.