Why does my receiver go haywire?

I can create a route but not follow it

16 messages
01/03/2010 at 18:09
I need some advice on the correct way to create a route on my Garmin Foretrex 401. Note that it is not possible to download maps into this receiver.

I've downloaded several routes from wheresthepath, and also entered waypoints manually to create a route, but when I try to follow the route, the receiver goes haywire and its behaviour makes no sense to me. Here is the sequence of events:

1 Download waypoints or create route manually.
2 Press 'Follow' and select 'To [the final waypoint in the route]'.
3 Pointer swings to correct direction.
4 At the first turn, the pointer bends to the left and indicates correct direction but then, instead of straightening out and pointing to the next waypoint, it bends back on itself until the tail of the arrow merges with the arrow head.
5 After a short distance, the pointer untangles itself but the arrow then points back to the waypoint that I have just passed.
6 If I then select the 'Nearest waypoint' the arrow swings back and points in the right direction.
7 However, at the next turn, the whole process repeats itself.
8 This means that I can't automatically follow a route from beginning to end

I've experimented with moving waypoints to ensure that the next waypoint is further away than the one before the last, but it seems to make no difference.

Any ideas of what I have missed, or am I doing something wrong?

01/03/2010 at 18:35
Don't get mad if I ask this, Hugh but are you entering the correct 100Km square ID letters to set your route? I only ask because I once inadvertently forgot to change the square ID letters when I should have and the behaviour of my GPS was exactly as you have described.

"...the moral test of government is how that government treats those who are in the dawn of life, the children; those who are in the twilight of life, the elderly; those who are in the shadows of life; the sick, the needy the handicapped and submariners. " ~ Last Speech of Hubert H. Humphrey

Edited: 01/03/2010 at 18:55
01/03/2010 at 19:27
Mal wrote:

<Don't get mad if I ask this, Hugh but are you entering the correct 100Km square ID letters to set your route?>

Good point but all my experiments have taken place within the SU 100km grid and every waypoint has the grid square ID.

ed wrote:
<You can still download routes created on mapping software (IIRC you use Memory Map) though Hugh. Does not matter if it is a non-mapping device.>

Thanks, I'm aware of that and I have been downloading routes using wheresthepath.
01/03/2010 at 19:50
<Are you keeping moving as you follow the arrow as stopping sometimes makes it flip about a bit?>

Yes, I keep moving. Here's another odd thing. I can create and save a track and follow it perfectly in either direction. Also, the receiver will take me unerringly to a single waypoint so it seems that it is only in the route function that it is not working properly. I believe that the Foretrex has a few more bells and whistles than the Geko and I suspect that I have omitted to activate, or even de-activate, some function that is interfering with its proper working.

Needless to say, the manual, which I downloaded from Garmin's website, is not very helpful.
01/03/2010 at 22:31

sometimes a gps will get its knickers in a twist if you do not go through a waypoint and will resolutely continue to point to that waypoint until it decides there is a closer one to go to. i have had this happen to one unit i was acrrying as an experiment. i had two gps with the same route. we had to make a detour. one gps resolutely pointed to the waypoint we were missing out whilst the other decided we weren't going to that waypoint and kindly pointed to the next nearest one (in this case the next waypoint).

i've found nor discovered no rational reason for this behaviour. it just seems to happen.

after a while this ceased to be  a problem because i think i subconciously started to place or add extra waypoints to take into account and/or counter behaviours i'd seen the gps display. 

however, there may be a perfectly good very simple reason why this happens.

i put it down to the learning curve of gps/waypoint usage - it's not as simple nor as obvious as it first appears.

sorry hugh, that doesn't help you at all but at least you know you're not alone!

01/03/2010 at 23:00

There is a setting which controls how close to one waypoint the GPS deems is sufficient before it will switch to directing you to the next one. On page 14 of the manual it mentions a 'ROUTES' setting which allows you to 'select auto or distance for route leg transition'. It may be that the setting you have for this value generates the wierd behaviour you experience. I sometimes get a similar behaviour when skirting a waypoint - my GPS will direct me back to the waypoint I bypassed for a while, until it figures out that I'm not going there. Sometimes it can point back for a while - it might depend on how close the next waypoint is. Parky has obviously experienced similar behaviour - but it happens in the context of bypassing a waypoint, not hitting one. I would check the ROUTES setting first and see how it has been set. Since you are probably navigating quite accurately, I'd try selecting DISTANCE for the route leg transition, and putting in a modest value if it lets you - say 50metres.

P.S. On the odd instance that I do get this problem I do a GOTO to the next waypoint.

P.P.S I notice that your Foretrex has an electronic compass, to which it will switch when your speed drops below some value that you set up. Have you calibrated the compass properly? Are you slowing down at waypoints when you are checking your GPS and so possibly getting issues with the compass? Can you set the display to show the name of the waypoint it is directing you to? Is the waypoint the right one,  just the direction being indicated wrong, or is the waypoint the name of the one you have passed?

P.P.P.S I've found on Page 17 of the manual that you can set one of the data fields on the COMPASS screen to show your next Waypoint. I would suggest you do this, so that you can see whether your problem is an issue with the compass needle, or the transition between waypoints.

Edited: 01/03/2010 at 23:41
02/03/2010 at 13:10
ALS has solved the problem for me, yet again!

I set the Route Leg Transition to 50 metres, followed the route that had given me all the problems yesterday, and the Foretrex behaved perfectly showing me exactly where to turn. Marvellous! And many thanks, too.

I suspect that in most instances I shall use my receiver more in the manner of Parky than ALS. I lead walks in the Home Counties where navigation in woodland and in popular areas such as Box Hill, Leith Hill and the Devil's Punch Bowl is often more difficult than in upland areas (assuming fine weather). Joe public has created a maze of paths, many of which are not recorded on Explorer maps, so it's helpful when arriving at a junction of paths to know which path you want, and even to be able to identify whether it is the right junction.

So I have two further questions. According to 'GPS for Walkers" by Clive Thomas, the best location for waypoints at a path junction is

one 50m before the junction to alert you to the fact that you are approaching it
one in the middle of the junction
and one 50 metres along the path that you intend to follow

Does this accord with your experience?

What exactly does 'Route Leg Transition' mean? When set to 50m does it mean that the receiver picks up the next waypoint 50m after passing the current waypoint? Or something else? I had it set to 'Auto' when it was behaving so peculiarly and in order to complete each of the three nine-mile routes on which I conducted the experiments, I had to switch to a GoTo every time I passed a waypoint.

Now that I have made the correct settings to the Foretrex, I don't need to follow all the helpful suggestions made by ALS. However, I'm under the impression that an electronic compass, properly calibrated every time I use the receiver, works when standing still. is this not the case?

02/03/2010 at 13:41

my use of waypoints (WP). a WP just before you need to turn - set so it's just before i.e. you have to go past it and then turn. with the inaccuracy of the map and the gps - only metres but wandering about trying to find exactly where the WP is isn't fun only to discover it isn't where the turn is..

the gps in one of its data boxes will tell you how far the next waypoint is so you don't need a WP before the turn to warn you it's coming up unless you want one. your next waypoint after a turn should be in a position so you know you are going in the correct direction. it doesn't matter how far it is.

as you are fully aware of trying to navigate in places like box hill and leith hill even WP won't always help as you can easily veer along a parallel path without realising it. however. enough of this gloom.

you should have WP where they will be useful to you. there is no rule for this other than the fun of finding out for yourslef and experience depending upon what you would like to see and just how much you want to look at the gps. i appreciate that is all very vague hugh but it really is the best way.

to summarise. a WP just before a turn. you then pass it and the gps will point in the direction you need to turn - this requires some patience and practice to get right.

the electronic compass will work when you are stationary. as i always carry a compass anyway i've never used the electronic one.

"What exactly does 'Route Leg Transition' mean? When set to 50m does it mean that the receiver picks up the next waypoint 50m after passing the current waypoint?" means that when you get to within 50m of the WP the unit will assume you have passed it/found it and move onto the next WP. this amount of leeway doesn't help with turns though and should be set lower.

Edited: 02/03/2010 at 13:44
02/03/2010 at 13:51

Cheers Hugh. I am determined to get you and your toys working.

I'm not exactly sure of Garmins algorithm for deciding you have visited a waypoint - I am not sure Garmin know it either - but I think the logic is something like "If the GPS has got within 50m (or whatever the setting is) of a waypoint and is now travelling away from it, switch to the next waypoint".  You can experiment by looking at your GPS as you walk towards a waypoint - if you pass it to one side for example the needle should swing to that side then flip to indicate the next one just after you pass it.

As far as waypoints around a junction .. not sure I'd bother with one 50m before - after all, the GPS will be saying "79 m to Waypoint xyz" anyway. (If you have set the compass page to indicate that, which I suggest you do). And as long as the next waypoint is in the correct direction, it doesn't need to be as close as 50m - anything useful will do that gives you the right direction.  I always have a section of map with the waypoint names printed on it which helps greatly.  And give waypoints useful names if it helps: BRIDGE, FORK, etc.

Yes the compass works when you are standing still, but I believe it can be very sensitive to how level you hold it. There is a threshold speed above which it switches to the GPS compass. Notice you can set magnetic variation which can save mental arithmetic.

Edited: 02/03/2010 at 13:56
02/03/2010 at 17:57
<Cheers Hugh. I am determined to get you and your toys working.>

I love that!!!

Thanks to ALS and Parky for their patience and sage advice. I've been out this afternoon conducting more experiments and the Foretrex is operating to my complete satisfaction.

I take Parky's point that there are no firm rules and one has to learn from experience; just like navigating with map and compass, really!

Thanks again, everyone.
02/03/2010 at 18:25
Hugh Westacott wrote (see)
4 At the first turn, the pointer bends to the left and indicates correct direction but then, instead of straightening out and pointing to the next waypoint, it bends back on itself until the tail of the arrow merges with the arrow head.
5 After a short distance, the pointer untangles itself but the arrow then points back to the waypoint that I have just passed.

I suspect the 'bends to the left' will depend on the approach you take to the waypoint, and that it's likely to 'bend to the right' just as often.

Determining when a waypoint has been passed isn't a simple exercise for a GPS receiver.

For a car satnav, it's easier, because the unit can do what's called 'map matching', which assumes (not unreasonably) that the car is on the road, and we can therefore make the GPS position more accurate (effectively) by forcing the determined position to be somewhere on the current road (i.e. we force a 2D error circle onto a 1D error line).

For an outdoor GPS unit, the calculated position will have an error circle (it's actually quite a complex and dynamic probability density function, but 'circle' is a good enough description).  As we approach the waypoint, there are two sources of error: in the GPS position fix and in the waypoint position entry (i.e. the error between the mapped path and reality, which includes the error of selecting the waypoint with the mapping tool).

If we approach a waypoint such that our position as determined by the GPS unit doesn't match the entered waypoint position, the GPS receiver will helpfully try to direct us back to the waypoint, on the assumption that we actually want to go to that waypoint.

So, what we need to do if we're not trying to go to a genuine waypoint (as we might if we were geocaching, or intending to meet someone or something), is to specify a 'this will do' distance so that the GPS receiver will mark the waypoint as reached when we get 'close enough' for the purposes of following a route.

I'd suggest that you set the 'this will do' distance to a value equal to the possible size of the combined error between GPS position and waypoint entry position.  This will depend on the accuracy of your GPS, the availability of WAAS/EGNOS corrections, the scale of the map you used to create the route waypoints, and your ability to place waypoint positions at the appropriate point on the mapping tool.

What Garmin call the 'Route Leg Transition' (which, helpfully, they explain nowhere in the manual), is my 'this will do' setting.

Of course, you need to be aware that there are hazards associated with such 'waypoint skipping' if you're following a very precise route that might pass just metres from death if you don't stick to the path.  Then again, I don't know anyone who would blindly follow a GPS rather than look at the ground in front of them...

You are to be forgiven, Hugh, if you cannot understand the manual.  It's not a User Manual, it's simply a list of menu options, none of which are explained to the User.  Clearly written by the software authors who know everything about how the unit works, but forget that the User doesn't.

I also notice that there's no mention of WAAS in the manual, so I suspect the unit isn't equipped for WAAS/EGNOS, so the accuracy of the fixes will be the basic 10m (95%).

Edited: 02/03/2010 at 18:52
02/03/2010 at 18:26

As for setting junction waypoints, Clive's three points per junction are sensible (assuming your receiver doesn't have waypoint alerts).

The first waypoint before the junction will guide you to the junction, and warn you that it is approaching (possibly).  The second is intended to tell you that you have arrived at the junction, and you need to decide which way to go.  The third waypoint should then show you which if the paths you should take at the junction.

The distances for the first and third junction waypoints will depend on the complexity of the junction, and the proximity of paths leading from it; sometimes, 50m may not be enough to discriminate between two paths that diverge slowly from a junction.  And, if you've set your Route Leg Transition to 50m, your unit may well skip all three waypoints at once if they're only 50m apart...

Edited: 02/03/2010 at 18:30
02/03/2010 at 19:49
Thank you, CP, for a very lucid explanation which supplements the information from ALS and Parky.

Just a couple of points. The Foretrex, which was purchased in America, is WAAS enabled which I believe means that it is also EGNOS enabled. Also, my receiver does alert me to the distance from the next waypoint.
02/03/2010 at 19:55

> The Foretrex, which was purchased in America, is WAAS enabled which I believe means that it is also EGNOS enabled

Another failing of the User Manual, then; it makes no mention of WAAS or EGNOS anywhere in the text, or in the instructions.  This is in marked contrast to the Geko manual, which does both (I checked before I posted).

I noted that the Fortrex can have waypoint alerts enabled, but my comment was in view of the fact that Clive's book may be a little dated (no idea; don't have a copy), or that simpler receivers might not have features like alerts.  In which case, the pre-junction waypoint can be made to serve that purpose.

02/03/2010 at 20:37

as a further observation about WP. if you are on paths and choosing a direction e.g. straught approaching a Y fork. i would set an "aiming off" WP well over to the right for the right fork i.e. off to the right of the path so that the gps would make an exaggerated "go this way" arrow point. i found this very useful especially for many paths originating in one place - a not uncommon feature in the south east. by always setting the WP "off route" in this way i was nearly always confident that i would pick the correct path to go along.

another example is where there is a path on either side of a field boundary - set an "aiming off" WP to enusre you go the down the correct side of the hedge/fence. set another WP soon after to get back on route.

by investigating, in my case playing, with differing WP plans you can make the gps do exactly what you want it to do.

02/03/2010 at 20:51

I just  can't see the need for a pre-junction waypoint 50m before a junction(and I've used a GPS for a decade). The Compass page allows you to choose two data fields to display; if you make one of these the distance to the next waypoint (ie the junction in question), and the second the waypoint name, then the compass page will always be telling you how close you are to the next waypoint, and it's direction.  By all means have waypoints at useful intervals along the way. To ensure you take the right turn at a junction, just make sure the next waypoint is at the correct bearing - this might need a waypoint 30m away, it might need several, or one 1km away might be fine, it all depends on the point in question. I find it particularly useful to have plenty of waypoints coming off the top of mountains for example.

Edited: 02/03/2010 at 20:55
Your say
email image
16 messages
Forum Jump  
Sign up to our weekly newsletter


Sign up to our twitter feed