The isochrone step by time horizon
ExpertThe essentials
An isochrone is the line of the points the boat can reach at the same moment. The step is the time between two successive isochrones. It is not the same everywhere: it is short at the start, where an error of a few minutes changes the route (near a coast especially), and longer afterwards, because the weather will change by then and seeking more precision serves no purpose.
Expert mode sets this step for seven time bands, counted from the start of the routing:
| Setting | Options | Default |
|---|---|---|
| “Isochrone < 2 h” | 5 min, 10 min, 15 min, 30 min | 15 min |
| “Isochrone < 6 h” | 5 min, 10 min, 15 min, 30 min, 1 h, 2 h | 30 min |
| “Isochrone < 12 h” | 15 min, 30 min, 1 h, 2 h | 1 h |
| “Isochrone < 24 h” | 30 min, 1 h, 2 h, 3 h | 2 h |
| “Isochrone < 48 h” | 1 h, 2 h, 3 h, 6 h | 3 h |
| “Isochrone < 120 h” | 2 h, 3 h, 6 h | 6 h |
| “Further isochrones” | 3 h, 6 h | 6 h |
The shorter the step, the longer the calculation. The defaults are what we recommend in the vast majority of cases. The “Reset routing parameters to default” button restores them.
The “Isochrone < …” settings at the bottom of Settings > Routing
Going further
When to shorten? Near coasts, lowering the step of the first bands lets the routing hug the coast. Change one band at a time and compare with the route you get by default.
How the band is chosen. At each new isochrone, the calculation looks at the time elapsed since the start of the routing (since the pivot point for a pivot routing) and takes the setting of the matching band. A few exceptions add to this rule:
- boat aground at the start: the routing starts from the nearest sea point and, for the first half hour, the steps are 5 minutes, so that the first leg does not run along a channel before the first turn;
- narrow front near a coast: the “< 6 h” band setting applies;
- close to the finish: the “< 24 h” band setting applies;
- course with marks (when the target is the finish and the course is followed): within 40 nautical miles of a mark (or of the target of the stage), the step is limited to a quarter of the remaining way at the speed of the front, in round values, down to 5 minutes.
Target too close. If the target is within one step of the boat, the routing stops: “The routing target is too close to the boat (less than one isochrone step, N min). Please move the target farther away.”
Old values. 12 h and 24 h steps are no longer offered for the far bands: a setting saved on those values is read as 6 h.