TomTom Weirdness on the approach to the Severn Bridge (M4)

jweaver · Regular Visitor Joined: Apr 11, 2005 Posts: 135

I am using TomTom 4.40 on my Palm T3 and have observed something strange.

The first time I approached the severn bridge (heading East) just before the tolls (which are on the other side), I happened to glance at the Palm and noticed that my position was shown in the field to the right a few 100 yards.

This continued for a while and then my position 'snapped' back onto the M4 but on the wrong carrageway. Then after a few 100 yards it put me back in the right place.

I thought nothing more off it, thinking that it was a map error.

But a week later, I did the same trip and again, at the same point, the same thing occured.

Then at the weekend, I went across for the 3rd time and saw exactly the same thing again.. This time I was ready for it and saw that my position doesn't "drift" off the road, but its 'snaps' there, in a single jump..

I have 2 friends with the same setup and asked them to do the same test.. On theirs, it followed the M4 correctly.

This is the only point that I have ever seen TomTom go off course and wondered if anyone knew why this occured at the same point!

Jon

hornist · Posted: Wed Apr 27, 2005 1:18 pm Post subject:

I'm afraid I don't know the answer, but I can report that I used to have a similar problem on the A46/A4 Batheaston bypass near Bath.

Every time I drove the road Southbound/Eastbound I would find myself jumping off into a field, then driving down the Bath-London railway line for 1/2 mile or so, then jumping back onto the road. It didn't happen if I drove in the opposite direction.

That was with TTN 2. Once I upgraded to TTN 3 it stopped happening.

Paul
PS As a safety professional I strongly advise readers against attempting to drive down the railway line.

Skippy · Posted: Wed Apr 27, 2005 1:35 pm Post subject:

Sometimes the GPS receiver doesn't have a very good fix and it will put your position a few hundred feet adrift from reality. The sat nav software tries to compensate for this by locking your position to the nearest road but sometimes you get strange results, as you saw.
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oddsock · Posted: Wed Apr 27, 2005 1:57 pm Post subject:

Was this the old or new crossing Question

If it was the new one.. (M4) there are a mass of support/wind protection cables on either side which may block Gps signals of certain receivers ,if it was the old bridge.. M48 then Pass
dave

oddsock · Posted: Wed Apr 27, 2005 2:13 pm Post subject:

Ok,I'm as blind as a bat Exclamation

I can now see it's the M4 ..Must be all those flashing ads :D Wink

jweaver · Regular Visitor Joined: Apr 11, 2005 Posts: 135

Interesting ideas.. Yes, it was the new Severn Bridge (M4) that I see this problem on.

I have only been over the bridge 4 or 5 times since I got my TT system.. 3 times, I have seen the phenomenon in exactly the same place, but the other 2 times, I am really not sure as I can't specifically say that I DIDN'T see it!! I am certainly going to keep an eye out for this next time I go over!

The only common denominator is the time.. The 3 times that I have seen it has been on Sunday afternoon.. The times that my friends went over were on different times/days.

Its strange though.. They have the same version, BlueTooth GPS receiver and PDA and theirs is fine..

I will keep my eye open and see if I can work this out.

Maxi · Regular Visitor Joined: Jan 06, 2005 Posts: 147

Hmm... this all sounds like an episode of the X-files that I saw once about Alien abduction. You get abducted by aliens and dont know anything about it apart from a small and very brief blip in time.......only on this occasion its a small and very brief change of location!!! Maybe those little green fellas only come out on a Sunday afternoon.

Be afraid...be VERY afraid!!! Laughing

Skippy · Posted: Thu Apr 28, 2005 2:58 pm Post subject:

@ Maxi. I think you have the best theory yet.

Alternative theories are that it's either a mapping error or the GPS reception is degraded at that point (multipath error from all the cables and stuff or blocked signal).

As for the snapping, generally the sat nav will show you as being on the nearest road if you are within (say) 200 meters of one but if you are (say) 210 meters away then it will show the actual position that the GPS is reporting (which might be in an adjacent field).

It's strange that it would show you as being on the wrong carriage way because it knows the direction of travel on each carriageway. I guess that it corrected it pretty quickly once it got a grip on your direction of travel. (Even if I am in a contraflow, my sat nav shows me as being on the correct side of the road).

Get someone else to drive the route for you and watch the satelite status screen on your GPS. Do you have a good reception? You should probably have 7 to 12 sats in a wide open area like that.
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geraint · Posted: Thu Apr 28, 2005 4:27 pm Post subject:

I once had a similar experience heading west on the A31 as it crosses the New Forest. It happened consistently along the same stretch for a couple of days, each time showing me driving cross-country on a course parallel to the road. It eventually returned to normal, and has never happened again. Each time it happened, I had at leat 6 satellites, and there were never any "cables and stuff" around which could be blamed.

I blame the extraterrestials. There's no other explanation Wink

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Geraint

TomTom for iPhone v1.9
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oddsock · Posted: Thu Apr 28, 2005 6:50 pm Post subject:

If you click on some of pics on This page two top right photos you can see the cables ( not as many as I thought though) I dont use this bridge my self but on the M48 crossing I get 9 or 10 sats.
Dave

SimonCatlin · Posted: Thu Apr 28, 2005 10:23 pm Post subject:

Lets not forget the rifle ranges underneath the bridge, powerful transmitters either side of the crossing, GPS jamming from the Black Mountains and of course the English bending it... :P
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jweaver · Regular Visitor Joined: Apr 11, 2005 Posts: 135

I went across the Severn Bridge again on Sunday, this time watching TomTom closely.. It behaved perfectly.

However, after the Severn Bridge, I went onto the M49 to cut out the Almondsbury Interchange and it went NUTS!

Most of the journey, I was off the motorway and when ever I passed under/over a road, TT would think that I was on that road and would recalculate a route accordingly.

There are 100s of power cables across the M49, is it likely that this was the cause?

Does this indicate a problem with my GPS receiver (i.e that its easily affected by external factors) or is this normal for GPS??

jweaver · Regular Visitor Joined: Apr 11, 2005 Posts: 135

There is definitely something weird about the maps as you go along the stretch of the M4 listed in my original post.

I went over again last night and this time I was in the outside lane... As you come around the bend (right next to the tolls which are on the other side) my position jumped over to the West Bound carrageway and back twice.

The fact that it thinks that I am able to travel East on the other carrageway is strange.

I have a feeling that the times i have seen the original problem, I have been overtaking (from the inside to the outside) exactly as I went around the bend.. I wonder if this fact, combined with a map error and perhaps an inaccurate signal, could 'force' me of the road as I have seen a few times.

I will see what happens once TT5 comes out!

jweaver · Regular Visitor Joined: Apr 11, 2005 Posts: 135

Sorry to bring this up again, but I am still convinced that there is something 'funny' about the maps at this point.

I have found that the key to this is being (or moving) into the outside lane as you go around the left hand bend.

There is nothing to explain interference.. The only anomaly is that you go over the railway line just before it goes 'weird'.

Its like you go around the bend, but TomTom thinks that you have gone straight on.

I have now seen this too many times for it to be a co-incidence, but I can't work out why it only happens at this point!

Jon

oddsock · Posted: Mon May 23, 2005 7:03 pm Post subject:

Since You raised the topic I have had a chance to go back and forth over the bridge .I was running ttn3 on my ipaq 2210 with the tt wired receiver I also had an etrex yellow on as well with the following results: My wife watched The etrex in advanced "sky view "and it had a fix of between six and eight sats..I watched tt and it showed a constant nine sats and perfect routing..as steady as a rock so your experience is a bit of a puzzel Exclamation

Dave

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