Hey,
to expand on @amandus answer a bit:
Isochrones seem “simple” on the surface but are indeed rather tricky once you get into the nitty-gritty details.
The underlying problem is that you’re trying to generalize a route network (i.e. the network of all streets within 5 minute drive time from your starting point) to an area - the isochrone we’re looking for.
This generalization is necessary, since we can’t transfer the whole reachable network. While this might work for these small (5 minute) isochrones, this would just be huge amounts of data for larger isochrones.
As it turns out, transformation from “this is a huge number of points I’d like included” to “this is the (concave) outline best defining this number of points” is rather complicated.
A few instances of these complications can be seen above:
- The rather shallow “bay” area east of the starting point is included in the isochrone - even though it’s water and you can’t drive on that with a car.
- The tip of the island to the north of said “bay” area is excluded, although it is easily reachable.
- The lake to the west is included as well, as are the railways near hoboken terminal. Neither are driveable by car. Should the isochrone contain a hole there?
- The isochrone endpoint on the highway to the south will be reached by car within 5 minutes easily, but surrounding buildings near it won’t. Should the isochrone be a line here?
These issues can be solved by fine-tuning the isochrone, as can be done with the mentioned smoothing parameter: Setting it to 0 will result in the following isochrone:
As you can see, this more specific isochrone will exclude the parts in Jersey City taht can’t be reached within 5 minutes - but it will also exclude the streets near the bay area west of the starting point.
Isochrone generation can be controlled by a few parameters, but the “optimal” set of parameters is different for each location and every isochrone range - what might work for 222 Jackson Street, Hoboken, 5 min car isochrones might generate horrible results for a 1h hiking isochrone in the rockies or a 30 minute bike isochrone somewhere in the netherlands.
I hope this gives a bit of an insight,
best regards