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gcb Regular Visitor
Joined: Oct 24, 2005 Posts: 151
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Posted: Fri Nov 18, 2005 1:37 pm Post subject: Tunnels. |
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What does your I3 do when you drive through a tunnel?
Please take tunnel to mean long tunnels over 150 metres,
I would have imagined when I drove though a tunnel that my I3 would complane about losing satalite connectivity. Doesnt seem to do this.
I only have experience of one such tunnel but:
When I drive through the tunnel, the I3 seems to show me driving through almost as if I were still in satalite reception.
When my destination is somewhere above and to the side of the tunnel, the I3 gets confused, moves me impossibly from in the tunnel to one of the roads above the tunnel and Hey presto , Destination on Left, Bing Bong, it thinks I arrived before I even get out the tunnel.
This kind of anomoly could easily be fixed in the software, if all tunnels were maked in the map data. So if unit suddenly lost reception at either end of a tunnel (maked in map data) it could know exactly where it was likely to be when it regained satalite reception. |
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zogman Frequent Visitor
Joined: Sep 05, 2005 Posts: 1417 Location: swindon uk
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Posted: Fri Nov 18, 2005 2:01 pm Post subject: |
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gps's know when theres a tunnel and compensate by estimating the distance and time taken to travel through it.....obviously if you stop in the tunnel then the gps will loose the signal after a set period ... _________________ *************************** |
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gcb Regular Visitor
Joined: Oct 24, 2005 Posts: 151
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Posted: Fri Nov 18, 2005 3:05 pm Post subject: |
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I thought that should be the case , doesnt explain why my I3 was insisting that I turn left to destination half way through one. |
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Mr_Gonzales Regular Visitor
Joined: Oct 05, 2005 Posts: 123 Location: Mmm...medway
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Posted: Fri Nov 18, 2005 3:42 pm Post subject: |
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Mine gets me through the Medway tunnel (100-200m) but loses reception and takes a few seconds to get it back when going through the Dartford tunnel (half a mile?).
Having the display show the car/bike going through the water is always good for a laugh. |
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zogman Frequent Visitor
Joined: Sep 05, 2005 Posts: 1417 Location: swindon uk
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Posted: Fri Nov 18, 2005 3:46 pm Post subject: |
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Mr_Gonzales wrote: | Mine gets me through the Medway tunnel (100-200m) but loses reception and takes a few seconds to get it back when going through the Dartford tunnel (half a mile?).
Having the display show the car/bike going through the water is always good for a laugh. |
looks like theres a distance/time limit before it searchs for the sat _________________ *************************** |
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Skippy Pocket GPS Verifier
Joined: 24/06/2003 00:22:12 Posts: 2946 Location: Escaped to the Antipodies! 36.83°S 174.75°E
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Posted: Fri Nov 18, 2005 5:58 pm Post subject: |
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gcb wrote: | I thought that should be the case , doesnt explain why my I3 was insisting that I turn left to destination half way through one. |
When you enter the tunnel, the i3 loses position lock virtually instantly. It doesn't want to admit this so it keeps on updating your position for 3 or 4 seconds based on the last known direction and speed. Eventually, it has to admit defeat and tell you that it doesn't have a lock anymore.
You can see this in action if you leave an elevated section of motorway and stop under the underpass where there is no GPS signal and the GPS will show you as still travelling at 30 MPH for a few seconds.
Also, the actual position reported by the GPS is typically 10 to 100 feet adrift from reality and the roads programmed into the GPS map database can also differ from reality by the same amount. The StreetPilot masks this by using a "lock to road" technique which will show you as travelling on the nearest road if it is within a certain distance of your reported position.
The reason why you got an odd turn instruction is because the GPS had a very poor quality signal and produced an outlandish position update which put you a hundred yards or more off the road and it snapped to a road then recalculated the route. If you wait a few seconds then it should come to it's senses and correct itself.
I agree about the tunnels though. The GPS should "know" that the road it is on is a tunnel and try to warn the user that satellite lock is going to be lost. Hmmm, if mobile phones can work in tunnels using leaky feeders, then how about putting GPS re-radiating antennas in tunnels to allow people's sat nav systems to work. 8)
Two places where it causes serious problems in London are the Limehouse tunnel (the tunnel splits in two and it can't give the turn instruction) and the Blackwall tunnel Northbound where there is an exit as soon as you emerge from the tunnel (the GPS often doesn't recover quickly enough to guide you). _________________ Gone fishing! |
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Skippy Pocket GPS Verifier
Joined: 24/06/2003 00:22:12 Posts: 2946 Location: Escaped to the Antipodies! 36.83°S 174.75°E
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Posted: Fri Nov 18, 2005 6:01 pm Post subject: |
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Oh yeah, I forgot to add that the expensive ££££ in-car systems (and some top end Garmin models) have "dead reckoning" which monitors the wheel speed of your car and uses a gyroscope to determine your direction so it can compensate for a loss of signal from the GPS receiver... |
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zogman Frequent Visitor
Joined: Sep 05, 2005 Posts: 1417 Location: swindon uk
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Posted: Fri Nov 18, 2005 6:03 pm Post subject: |
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very informative explanation skippy... _________________ *************************** |
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Mr_Gonzales Regular Visitor
Joined: Oct 05, 2005 Posts: 123 Location: Mmm...medway
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Posted: Fri Nov 18, 2005 11:54 pm Post subject: |
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That could explain why sometimes when I'm looking at the GPS it shows me on a nearby parallel road for a second, or why sometimes the i3 shows me taking a motorway turnoff/staying on the motorway when I'm doing the other of the two.
This only happens when I'm not navigating - when I'm just using it as a moving map. I guess when navigating it assumes you'll take the given route and after a few seconds before it realises you're going another way and it recalculates.
It definitely sounds like it's all down to the accuracy of the GPS and it getting confused when two roads are close by. |
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gcb Regular Visitor
Joined: Oct 24, 2005 Posts: 151
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Posted: Thu Nov 24, 2005 12:11 pm Post subject: |
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CF10 4JY.
This postcode is less than 100 ft from a tunnel (above the middle). The Building Future Inns , Cardiff Bay is closer to the tunnel than the road , this Probably confused the I3 which took the Tunnel to be like any other road which could have houses along the side of it. I would think this would catch out any GPS unless the GPS actually distinguished tunnels as tunnels. |
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