Isochrones are usefull to show which area can be reached within the next minutes. Espacially if some time is left in a city before you have to go way home and you want to explore more of a city isochrones enables you to plan your day. The following video shows 30 minutes traveltime for cologne, starting in front of the cathedral for one day in 5 min timeintervals. The color changes by the reachable are. For calculation public transport without ICE/IC and EC is used. Furthermore only one kilometer by foot are allowed.