# Green Areas: The Secret of the Happiness of the People of Madrid
## 1 Introduction  
More than 55% of the world's population is currently concentrated in cities, and this is expected to reach two-thirds by 2050 [1]. As a result, sustainable development has become an important issue, and governments and organizations, such as the United Nations (UN), have begun to debate its consequences and actions to reduce its impact. One of the most well-known initiatives is the definition of the UN of 17 Sustainable Development Goals (SDGs) whose aim is to improve human society, ecological sustainability, and quality of life. In particular, SDG 11 deals with sustainable cities and communities. One of the aspects that contributes to this objective is the presence of green spaces [2]. In addition, green spaces contribute to other sustainable goals, such as SDG 3 (health and well-being), as urban forests positive influence cultural aspects such as social cohesion, stress relief, and physical activity [2, 3, 4].  
Research has shown that green areas and public parks contribute to social, psychological, and physical well-being [3]. However, most research studies are based on surveys, limiting the sample size and its applicability in other venues. Since social networks provide a wealth of information on user feelings, this research paper analyzes social networks to understand how green areas affect citizens' well-being.  
In particular, this study analyzes citizens' feelings and emotions in the Madrid region, Spain. In 2014, a study titled \"Happiness in the City\" [5] analyzed perceived happiness in several cities: Argel, Barcelona, Bombay, Chicago, Chongqing, Madrid, Paris, Rio de Janeiro, and Warsaw. To this end, the authors surveyed more than 5000 citizens, and concluded that 92% of the inhabitants of Madrid were happy living in Madrid, only behind Rio de Janeiro and Bombay. The most critical aspect that Madrid's inhabitants highlighted was the presence of green areas (97%). This result is not surprising, as Madrid has a surface area of 15.9 m\({}^{2}\) green areas [6]. Some of the urban parks in Madrid have recently been created for social purposes, while others come from former palaces owned by the Spanish crown.  
Some of the most relevant parks are Casa de Campo (1, 722 ha), Juan Carlos I (1, 600 ha), El Retiro (116.84 ha), Parque del Oeste (98 ha), El Capricho (22 ha), Campo del Moro (20 ha), Real Jardin Botanico (8 ha) and Jardines de Sabatini (2.6 ha) [6].  
Our study aims to better understand the influence of green areas on the happiness of Madrid's inhabitants. Thus, we present the following research questions:  
RQ1. Does sentiment vary across tweets written inside and outside Madrid's green areas? RQ2. Do green areas positively affect people's sentiment? RQ3. Do the tweeting places influence the commented topics?  
To answer these questions, our analysis is based on information collected from the Twitter social network in the Madrid region, Spain, as previously discussed. To this end, geotagged tools were used. The remainder of this article is structured as follows. Section 2 introduces the related work. Then, Section 3 describes the techniques used for data collection and analysis. Next, Section 4 presents the results of this analysis. Lastly, Section 5 discusses the conclusions of this work.