Follow way for a given distance

[stanton]

Use cases

When mapping features in tunnels (emergency equipment, different speed limits etc.), coordinates in the form of latitude/longitude pairs are usually not available due to lack of GPS reception. The tunnel itself may have been mapped from other sources (official sources, approximation, overground features).

For locations in the tunnel, the most readily available location is distance from one of its portals: often it is indicated on the walls, or it can be determined with the odometer of a car. Unfortunately, determining a point at “distance X from the tunnel portal” is currently non-trivial. (One option is to draw an auxiliary way that starts at the portal and follows all points, and stop when you have reached the desired distance.)

Desired behavior

Either: Select a point, click a new option which gives you a dialog box to enter the distance. You will then be prompted to select the ways to follow (and the direction, if you are not starting at one of the end nodes). Eventually, either a marker will be shown or an end point will be inserted.

Or: Select a point, choose an option “Measure distance along way”. Move the mouse along the way (and, optionally, others connected to it); as you do, the portion of the way followed is highlighted and the distance is shown in the status bar.

Benefits of the solution

It becomes a lot easier to add features whose location is defined in terms of distance along a path from a given reference point.

Possible drawbacks

Mapping this way is not very precise. Then again, an approximate map is better than none at all.

Additional information

Determining the path along which to measure distance may become difficult if that involves more than one way. For example, you are trying to map something at 1000 m from the tunnl portal, but the first way after the portal is only 500 m long, so we have to follow subsequent ways as well, until we have reached the desired distance. Where ways split and then rejoin, this may become ambiguous.