Isochrone

An isochrone is a line connecting all points that can be reached in the same travel time from a starting point. The word comes from the Greek for equal time. Where a contour line on a topographic map joins points of equal height, an isochrone joins points of equal travel time.

In practice isochrones come in sets: five, ten and fifteen minutes on foot from the same origin, drawn as nested bands. The bands show not just where you can get to but how the reachable area grows, and they make the barriers visible. A band that stretches along one street and stops abruptly at the next is telling you something about the network.

Isochrone and catchment area are closely related and often used interchangeably. The useful distinction: the isochrone is the boundary line, and the catchment area is the surface it encloses. Isochrones are time based by definition, while a catchment area can be defined by distance instead.