tl;dr: Flatten SF is a browser-based tool that computes walking and biking routes between any two points in San Francisco, letting users trade off distance against elevation gain via a slider. It uses USGS 1-meter lidar elevation data and the Overture/OpenStreetMap street graph across ~160,000 segments, with stairways permitted for pedestrians but excluded for cyclists. Place search and all routing run offline in the browser.