On previous posts there have been mentions of "course points" or "waypoints" - is this what the file needs in order to tell the device what to do at that location?
How do I create that?
Rewinding to the title of the thread, and reminding all present of the carefully chosen term "numpty", I don't get how it works, to be perfectly honest.
Well - how it works, GPS is a very complicated system. In some ways, more complicated than a desktop computer, or a car. We don't need to know 'how it works' in that sense, to use these things. But ...
Wikipedia on GPS (a very long page which just gets worse the more you scroll down)
It doesn't help that it's in the nature of GPS, that it's quite difficult to bug-hunt problems that occur at some remote location, when your GPS insists on being 'here'.
So it's difficult to check in advance, that a route is going to work. You don't find out that it doesn't, until you get there.
Since you have an Edge 800 - well that's a very popular model, and there must be several people on here who can give good advice on specific issues. Deadhead's page linked upthread looks like a good quick start guide** to using a Course on an 800, and the RWGPS advice page looks useful too. (Though NB I'm saying this as one who doesn't own a 800 !)
** eg - from that page - open your Course and set Turn Guidance to 'On'
As far as I know, the 800 is only really optimal when using a Course file. That means that a lot of the general advice here about Tracks and Routes doesn't really help much. Most of the online planners can generate a Course file however (TCX format), or convert to that from other formats. Though the 800 will internally convert a Track to a Course (not sure what it does with Routes) so that if you load a Track in the correct way, you'll find it turning up in your Courses menu anyway.
Directions at junctions. Well according to RWGPS these are auto-generated from the map on the GPS. See ** above.
Obviously your Course would need to be following the mapped roads closely, for this to have any chance of working. Some downloaded files are just recorded files which may wander around a bit, and not fit the roads perfectly, or they may have been generated using a different map from the one you have on board which again means the junctions might not precisely coincide and so not generate a prompt.
Although files (Course, Track or Route) can and do often have embedded 'turn' instructions in the data, I think 90% of the time the GPS doesn't use this, but generates its own from the on-board map data, and generally only at a Waypoint (or Coursepoint or some Routepoints). If you can add Coursepoints (as you can in BikeHike for example) these should generate prompts regardless of the mapping. But that is of course a bit of extra work. There is a BikHike deep setting to auto-generate Coursepoints which might be what you need.
I must say that in my fairly long experience with other Garmin models, auto-generated directions prompts have never really come up to my expectations, and I usually turn them off and use other (more planning required) methods. For example, I rarely see the simple "Turn Left at next" sort of message which is what I'd want. More likely, it would be "Turn NW onto Netherbottom Lane in 200m" which may look more informative on paper but in practice is not helpful. Maybe I just haven't found the right settings to make this stuff work properly.
But, I get a bleep and then I look at the line on the screen, and really that works for me.