Add message if no elevation data is available

[bagage]

Fix #365

[nrenner]

I would find it nice if there were also a message for partly missing elevation data, e.g. if a route is only half in the no-data-zone, and for cases like in abrensch/brouter#137, i.e. the check would be for one or more undefined alts. What do you think?

[bagage]

Yes I initially wanted to handle it like this, but did not investigate the difficulty:

But maybe for now, a simple label that warns about partial missing elevation data could be enough?

[nrenner]

My personal opinion is, that any kind of interpolation is just an assumption and might be completely wrong, so I implemented gaps on purpose, see also GIScience/Leaflet.Heightgraph#108.

For me, a simple message to inform about partly (or all) missing data, so gaps are not interpreted as mistake, is completely fine.

[bagage]

As far as I know, it is BRouter engine that gives use waypoints altitude.

If you do a route from A (no altitude known) to B (altitude known), BRouter will send a list of points without altitude, then some with altitude.
However, if you route from B to A, all points will have an altitude value (the last one known). And there is no property telling you that some elevation data is actually missing/replicated from previous points.

While I could detect the first situation easily, the second one cannot be detected easily (except by patching BRouter to add a property). Any thoughts @nrenner? What would you suggest?

PS: I cannot check for points above the 60°N 10°E-30°E limit, because there are other places on Earth where elevation data is missing (for instance here)

[nrenner]

When BRouter interpolates we can't do anything.

I was more thinking about examples where BRouter doesn't, because data is missing at the start of a segment, e.g. your via on bridge example. My idea was simply to check if any alt is undefined instead of all.

But thinking about it, it doesn't look as broken as when there is no data at all, and it shows Elevation: - m when hovering as an indication of missing data, so I would wait on feedback for such cases and am merging this now as is.