I think I understand what GPSapp is trying to do, but it seems to do the wrong thing often enough that I'm wondering why.

Here's an example of a path I took today. The map data is slightly off-plumb for one of the roads, but otherwise accurate:



The road name has been fictionalized to protect the innocent.

Anyway, the thing is, as I approached the road, it gave me exactly the wrong turn information. The screen looked like this (simulated):



Note that the white "snap" line correctly shows the roads, the turn, the waypoint, everything. The only variable was that my car wasn't exactly on the map data's road line (gray). And it wasn't off far, either, this was zoomed in pretty far. The innacuracy between map data and actual position was probably less than 50-100 feet.

So... my question is... in which universe is that considered a LEFT turn?

I mean, I would understand it telling me to turn left if I'd gone past Foo Rd and turned around and headed back towards it from the other side. But I don't see how, even if GPSapp was assuming an off-course-and-need-to-re-acquire situation, it could call that a left turn.

I suspect there's something fishy about the algorithm it uses to determine left and right turns, and I think it's more than just the fact that it's trying to be helpful when you're off course.