As the adoption and usage of Unmanned Aircraft Systems (UAS) is growing over time, the safety of the surrounding environment when they are used autonomously over urban areas has been questioned. Urban areas are characterized by high population density therefore a potential UAS crash may involve people injuries or even people death, as a consequence, it is essential to preserve human safety in mission planning. This thesis is based on a previous work done by the research group, in which a path planning algorithm for UAS has been developed to determine a safe flight mission. The proposed methodology uses a risk-based map that quantifies the risk of fly over specific areas.