## 3 Crowdsourcing and GIS  
As has already been mentioned in the previous sections, crowdsourcing is often adopted in order for a broad range of problems to be resolved. The present article mainly focuses on applications relating to geographic space, where crowdsourcing plays an important role in collecting or even generating geographic data from scratch. In such cases, it is usually integrated with GIS, facilitating the whole process of data collection and contributing thus to the creation of Integrated Public Participation GIS tools.  
According to [PERSON], the implementation of several methods (in a geographic context) for data collection by distributed people exists, so that the information to be generated and diffused is so-called \"volunteered geographic information\" (VGI) [11]. VGI is increasingly being used in the case of geographic applications, such as map generation or elaboration by several users, as a bottom-up process. In a decentralized system, data are produced \"by the users for the users\". Therefore, one of the challenges of web 2.0 is to introduce intelligence into the way users can create, share and apply data by themselves [12].  
Crowdsourcing has been integrated with several web technologies (e.g., web GIS) and computer tools that offer the opportunity to create and manipulate data and information fast, as well as to diffuse knowledge. The creation and manipulation of spatial data (e.g., maps, geographic coordinates, satellite imagery) has nowadays become very popular among users of the web, who try to find places or routes on a map, analyze geographical data and, in many cases, create spatial data and publish them \"for free\" through the web. Crowdsourcing has essentially contributed to such kinds of applications as the users having the opportunity (in many cases, they are asked) to create geographical data from scratch, edit existing data, _etc._ Consequently, whole infrastructures have been created and are constantly evolving, allowing millions of users to have access to spatial data or to create and elaborate upon geographic content.  
Such types of applications have been developed with the support of GIS, which allow the collection, storage, manipulation, distribution and reproduction of spatial data. Therefore, crowdsourcing has been integrated into other tools and is now a technique that can support spatial and collaborative planning, cartography and many other spatial applications, where the \"wisdom of the crowd\" is needed.  
Interactive maps are usually adopted, as a means of visualization and communication, in cases where spatial problems have to be resolved. Participants in a crowdsourcing process interact with maps in order to add spatial content or represent a spatial problem or a proposed solution on a cartographic background. In some cases, participants are those who create maps and spatial data, so that such kinds of data can then be disposed through the web and be available to an extended number of users.  
It is now clear that crowdsourcing in combination with GIS technology has set the basis for an alternative approach on spatial planning, where the process of spatial data generation (maps, _etc._) takes place through the web, as volunteers contribute to the whole venture. At the same time, maps and spatial data can be exploited, for several purposes, by the participants of a problem solving process, so that the whole process is facilitated and enriched.  
In the fifth part of the article, some cases in which crowdsourcing is combined with GIS are presented. Such cases concern real applications, which have been successfully developed through the integration of the above techniques, and also tools that have been built partially or totally \"by the public for the public\".