Author Topic: eTrex 20 baffling routing  (Read 1717 times)

eTrex 20 baffling routing
« on: March 11, 2017, 11:17:46 am »
Hi everyone,

First post here. Nice to be here.

I bought a Garmin eTrex 20x recently to act as a SatNav for long cycle routes. I carefully prepared a route with 40-odd routepoints, designed to pin the Garmin down to a particular road route. I have it set to "car / motorbike" because the first time I tried this in bicycle mode, despite being set to "on road" it tried to send me off onto a non-existent trail in the very first mile, and a glance at the "distance to go" field told me it had somehow planned a 120 mile route when I'd actually intended 71.

Setting it to "car / motorbike" has mostly been successful over the route I planned. I scrutinised the map screen carefully and it stuck nicely to the roads I'd pinned it to with the routepoints, with one baffling exception.

 

The above image shows part of the route that passes east to west over the M42 in Leicestershire. But for some reason my eTrex thinks that the best route between the two routepoints shown to the right and left of the roundabout is to take the first exit down the A444, perform a bit of a triangle to the south, then come up the B493 and do a u-turn at the routepoint on the left. It's as though there were a brick wall between the first and second exits on the roundabout.

Anyone seen anything similar, or have a suggestion to get round it? I've tried pinning the route directly onto the roundabout, but it just does essentially the same thing. I could, I know, just ignore the directions at that roundabout and force it to recalculate betore the two routepoints, but I'm concerned that it will trip me up one day at some roundabout I'm less familiar with.


And a supplementary question if I may: if I plan a route with (say) 40 routepoints, but decide to take a detour that avoids routepoints 18-22, will it start to guide me toward routepoint 23 when I close in on it, or will it insist that I go back to 18?

Thanks for any help.

Samuel D

Re: eTrex 20 baffling routing
« Reply #1 on: March 11, 2017, 11:32:55 am »
Although others claim success in other ways, the only way I’ve found to get any Garmin to auto-route with useful reliability is to purchase Garmin’s City Navigator maps.

Problems like the one you describe have otherwise been so common for me as to render auto-routing useless.

Feanor

  • It's mostly downhill from here.
Re: eTrex 20 baffling routing
« Reply #2 on: March 11, 2017, 11:54:27 am »
What maps do you have on the Garmin?
Bad routing is usually down to a problem with the maps, not the device.

It sounds like the maps have a broken link in them at that point.
That sometimes happens, particularly on OSM maps where someone has made a bad edit and not 'joined' the roads up properly.

( I've just tried routing between those points on the OSM site, and it seems OK right now. )

I'd advise planning the routes using the exact same maps that you have on the device to minimise unexpected routing issues like this.
Also, I usually have a plain ( non-routing ) Track plotted in the background, to sanity-check the routing on-the-road.

Another thing that can cause odd routing and u-turns ( but not in this case ) is where you have placed a waypoint on the wrong side of a dual carriageway ( usually because you were not zoomed in enough to notice ).  You will get routed to the next roundabout, u-turned till you touch the waypoint, and then u-turned again to continue in the original direction.



Kim

  • Timelord
Re: eTrex 20 baffling routing
« Reply #3 on: March 11, 2017, 01:37:46 pm »
Although others claim success in other ways, the only way I’ve found to get any Garmin to auto-route with useful reliability is to purchase Garmin’s City Navigator maps.

Problems like the one you describe have otherwise been so common for me as to render auto-routing useless.

Agreed.  Although OSM is much better than it used to be, you can still get errors that send you miles out of your way outside urban areas.

In the real world, you're going to come up against this problem even with City Navigator, due to the low weighting of some minor roads, and because as a cyclist you're going to break the rules of the map from time to time.  A typical situation is where you follow a residential dead-end street up to a main road, become a pedestrian for a minute, cross the road, and then resume on some other minor road on the other side.

While I like to use auto-routing to make the display more easy to read on my recumbent, my preferred approach is to supplement the route with a track which is set to be displayed, so I have something to refer to when there's reason not to trust the auto-routing.  If you're deliberately going to pull a manoeuvre like the one described above, I find a waypoint reminding you what's going on is prudent.

The auto-routing will sort itself out when you re-join the plotted route further on.  Which also answers this question:

And a supplementary question if I may: if I plan a route with (say) 40 routepoints, but decide to take a detour that avoids routepoints 18-22, will it start to guide me toward routepoint 23 when I close in on it, or will it insist that I go back to 18?

Yes; it will start guiding you towards 22 as you approach 22.  For this reason, circular (and figure-of-8) routes are considered harmful - it will likely take a shortcut right to the end.  The work-around for this is to split your ride into more than one route.  When you get to the end of the first route, you select the next one.  Split the route at a cafe or control and it's no big deal.


Other useful tips:

When crafting a route for autorouting, it works best if you put the routepoints in the middle of the roads you want to use, rather than near junctions.  A particular issue with dual carriageways and slip-roads is that they're composed of logical one-way roads in the map, and a routepoint on the wrong 'side' will cause it to perform a detour and u-turn to join up.

Use Basecamp (or Mapsource) to plot your route against the actual map that's on the eTrex, rather than some online tool.  This is the only way to ensure consistency in the maps. 

In their infinite wisdom(!) Garmin have used a different routing algorithm in Mapsource/Basecamp than they have in the unit, so while the maps will be consistent, the routing decisions won't be.  You'll have to sanity check the routing decisions made by the device itself.  This is tedious to do in advance due to the slow scrolling/zooming, and a source of delay on the road.  Your choice.

Most audaxers will tell you that auto-routing is a mug's game and you're better off following tracks by eye.  They certainly have a point.  Personally, I find it more worthwhile in urban areas, particularly if I'm just wanting to get somewhere and I'm not really fussed about what route it actually takes.
To ride the Windcheetah, first, you must embrace the cantilever...

Re: eTrex 20 baffling routing
« Reply #4 on: March 11, 2017, 02:33:55 pm »
Thanks a lot for the replies, very interesting and helpful.

I'm just using the built-in maps as supplied with the device, which are the "Preloaded Western Europe Garmin TopoActive with shaded relief". Not sure if these are available to use online somewhere?

I'm a Linux user so Basecamp and other Windows tools aren't an option I'm afraid.

Any thoughts on the last question? Can I purposely avoid a set of intermediate route points and continue from a later route point, or will I be directed to the first one I missed?

Kim

  • Timelord
Re: eTrex 20 baffling routing
« Reply #5 on: March 11, 2017, 02:36:44 pm »
Any thoughts on the last question? Can I purposely avoid a set of intermediate route points and continue from a later route point, or will I be directed to the first one I missed?

As I said above:

The auto-routing will sort itself out when you re-join the plotted route further on.  Which also answers this question:

And a supplementary question if I may: if I plan a route with (say) 40 routepoints, but decide to take a detour that avoids routepoints 18-22, will it start to guide me toward routepoint 23 when I close in on it, or will it insist that I go back to 18?

Yes; it will start guiding you towards 22 as you approach 22.  For this reason, circular (and figure-of-8) routes are considered harmful - it will likely take a shortcut right to the end.  The work-around for this is to split your ride into more than one route.  When you get to the end of the first route, you select the next one.  Split the route at a cafe or control and it's no big deal.
To ride the Windcheetah, first, you must embrace the cantilever...

Re: eTrex 20 baffling routing
« Reply #6 on: March 11, 2017, 04:23:51 pm »
Many thanks, sorry I missed that. Useful to know.

frankly frankie

  • I kid you not
    • Fuchsiaphile
Re: eTrex 20 baffling routing
« Reply #7 on: March 12, 2017, 10:03:54 am »
I'm just using the built-in maps as supplied with the device, which are the "Preloaded Western Europe Garmin TopoActive with shaded relief". Not sure if these are available to use online somewhere?

AIUI, Garmin Topoactive maps are 98% OSM, and 2% Garmin (the Garmin bit being a few additional POIs and presumably some customised styling).  Of course they may not be as up-to-date as a new OSM download would be.  When I bought a 30x about a year ago the included map was one year out of date (I could tell by looking at some local footpaths, some of which were added to OSM by me - some are present some are not).  The up-to-dateness affects not only what roads are present/absent, but also how well the routing will work in some random locations.  Even if you update your Topoactive map using Garmin Express, that is still likely to lag some way behind the core OSM data.  As indeed will any OSM download - but some sources are closer to the core than others.

Regarding routing - if you think of your routepoints as an ordered list (which is, actually, what they are) the Garmin rule seems to be, simply, "head (by mapped road) for the nearest point that is further down the list".  This is true even in 'off-road' mode, in that case without the bracketed bit.  This is not the same as "head for the next point in the list" which is what what we'd really like it to do.   But if you're travelling from A to B, most times it works just fine.  Where cyclists catch themselves out is that more often than not they want to travel from A to A.
"This is a complex subject, with a need for more than one highlighter pen."

Re: eTrex 20 baffling routing
« Reply #8 on: March 12, 2017, 10:47:19 am »
Many thanks for all the replies, lots of useful information. Sorry I replied in a hurry yesterday without reading through the thread properly.



I've repeated the problem using OSM maps at http://www.yournavigation.org/, as shown above.

In this particular case the solution will be to start navigating west of the roundabout; I can remember the route up to that part easily enough. Fortunately the problem isn't present when using the roundabout from west to east, so it shouldn't mess up my "distance to dest" on the return trip. But I'll test the return journey on the same facility anyway (granted the routing algorithm might be a bit different, and the maps aren't exactly the same).

Feanor

  • It's mostly downhill from here.
Re: eTrex 20 baffling routing
« Reply #9 on: March 12, 2017, 01:42:40 pm »
It looks like the site you are using has got an outdated version of the OSM maps, with a broken link in the map.
If you do the same thing on the actual OSM site, it's clearly been fixed.

http://www.openstreetmap.org/directions?engine=osrm_car&route=52.68972%2C-1.54443%3B52.68867%2C-1.55096#map=17/52.68856/-1.54595

I expect yournavigation.com will scrape an updated copy of the OSM database at some point.

Phil W

Re: eTrex 20 baffling routing
« Reply #10 on: March 16, 2017, 08:44:32 am »
Yep and here it is on my Etrex 20, which has OSM mapping from May 2016. Works just fine.