The large diversity of unmanned aircraft requires a suitable and proper risk assessment. In this paper, we propose the use of risk map to define the risk associated to unmanned aircraft. It is a two-dimensional location-based map that quantifies the risk to the population on ground of flight operations over a specified area. The risk map is generated by a probabilistic approach and it combines several layers, including population density, sheltering factor, no-fly zones, and obstacles. Each element of the risk map has associated a risk value that quantifies the risk of flying over a specific location. The risk values are defined by a risk assessment process using different uncontrolled descent events, the drone parameters, and environmental characteristics, as well as uncertainties on parameters. The risk map is able to quantify the risk of large areas, such as urban environments, and allows for easy identification of which locations the flight has high and low risk. The map is a tool for informed decision making, and the proposed method is validated with different aircraft in a realistic urban environment.