https://www.bigdataspain.org/2016/program/fri-assessing-spatial-accessibility-primary-health-care-services-metropolitan-area-madrid-using-big-data-gis.html https://www.youtube.com/watch?v=DYhQy9oKV6k&list=PL6O3g23-p8Tr5eqnIIPdBD_8eE5JBDBik&index=52&t=3s