An isochrone map visualizes travel-time accessibility: it shows the geographic area reachable from a point within a fixed time threshold (e.g., "everywhere you can drive to in 30 minutes"). The source defines it formally as connecting points where something arrives at the same time. Beyond transport planning, the technique applies to hydrology (runoff travel time in drainage basins) and cardiology (detecting heart abnormalities via electrical timing). Modern implementations compute shortest-path trees on network graphs, then draw boundaries around reachable nodes—a computational approach now fast enough that multiple tools (GraphHopper, OpenRouteService, Mapbox, TravelTime API, etc.) offer it as a service. The historical examples show the idea is older than you'd expect: Galton published "isochronic postal charts" in 1881, showing days-long travel from London. The article lists 20+ production implementations across open-source and proprietary platforms, most using OpenStreetMap or public transit (GTFS) data as their backbone. One sharp question: the source doesn't clearly specify how isochrone boundaries handle network discontinuities—does the algorithm exclude unreachable enclaves, or does it always output convex shapes?