TomTom 720 Occasional Inaccuracies with Current GPS Position

Discussion in 'General TomTom Discussion' started by jdholman, Feb 17, 2008.

  1. jdholman

    jdholman

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    Hello All:

    I have had my 720 for about 3 weeks now and have been enjoying leaning how best to use it. I have had 2 occurrences in the past 2 weeks that have caused me problems in navigation.

    When I am not on a road that my TomTom knows about (a new subdivision for example) and from this position select to navigate home, the 720 asks if I want to avoid unpaved roads. I choose ?no? and have it plot a navigation for me.

    The 720 tries to get me back to the main road I am closest to, but I am not on a road as the road I am on is too new. When I get to this main road, I expect the 720 to find my position on the road and recalculate based upon this new position. The problem is that my current GPS position for this trip was 40 yards or so SW of my actual position, so the 720 had no idea how to direct me. It thought I was SW of my actual position, and as I drove I could see the road I was actually on the left of what the 720 thought I was.

    I pulled over, removed the 720 from my windscreen mount, and reset the device. When it powered back on, I selected a new trip to navigate home. I accepted the default route and again it mad my current position 40 to 50 yards off my actual position.

    This has happened once before, and also when I was on a new road and tried to navigate from this ?unknown? road home. The 720 again had my current position displaced by 30 ? 50 yards or so.

    Any idea what might cause this? Has anyone else ever had their TT displace their current position like this?

    Thanks,

    Jim
     
    jdholman, Feb 17, 2008
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  2. jdholman

    baggss

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    This has happened to me twice in the last week, both on the same day. Multiple on/off actions did not work, nor did the hard reset via the bottom pin-hole. What did seem to work was docking it and downloading the latest QuickfixGPS update. After that, no problems. All of mine were on roads that are on the map yet it was often 60 to 100 yards off.
     
    baggss, Feb 17, 2008
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  3. jdholman

    jdholman

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    That is wild...

    Each time mine has messed up, I was "off the map" and each time I remember docking and downloading the "quick fix" and it worked again.

    I wonder why this happens? I don't have TomTom Home on my laptop for when I travel - only at home. If this happens when I am on the road, I could be in trouble!

    I wish there was another way to reset it without docking & downloading. Is this a known issue perhaps with a technical explanation?

    Regards,

    Jim
     
    jdholman, Feb 17, 2008
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  4. jdholman

    baggss

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    Not that I know of. I have TTH on my personal laptop that I carry when I go out of town on business. I've had my TT for 2 months now and it's the first time it's happened so I'm not overly worried about. I had considered doing a firmware reset via the TT Prefs but figured I would try docking it first.
     
    baggss, Feb 18, 2008
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  5. jdholman

    dawidgatti

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    First of all you don't have to reset the device for clearing the rout, there is a special icon for that in the menu.

    second - the maps aren't perfect the software compensate that by dragging your icon on the actual road. when you are somewhere off, tomtom will display the actual position until he will snap to a road near you.

    when that happens just drive forward until the maps get better form that small part and tomtom will snap back on the right road.
     
    dawidgatti, Feb 18, 2008
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  6. jdholman

    boren

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    No, it does not fix itself easily. I've had exactly the same thing happen and it's a real bear to get it right again.

    The parking lot for my work is about a 1/4 mile from the mapped road. When I leave at lunch if I turned on the GPS and started driving around the big loop driveward towards the road, which takes a minute or more. The GPS initially gets a lock at where I stopped and gets a good location, but about half way around the drive way it jumps and tries to place me on the road, jumps back, and seems to give up. From then on I've been as much as a 1/4 mile offset.

    I've driven into town, and back, about 30 minutes round trip with my truck flying along a 1/4 mile south of the road I'm on. It's interesting in a way. Resets, power cycles, nothing reliably puts you back on the road.

    I think it has to do with how quickly it tries to aquire signal lock. I've owned a lot of GPS systems, from an old 7 satelite lowrance, through many garmins, a newer nuvi, etc. None aquire gps lock as quickly as this 720. It can have a tentative position by the time it finishes booting.

    I think the way they do it is assume your location. It maybe where you stopped, if you're moving they try and put you on a road and guess your current location. Once you have your current location getting a better lock I bet is much much easier.

    But if you're not on a road, and it has your current location wrong then you get the offset. Unfortunatly it seems to store that bad information and then you get screwed.

    Downloading a new quickfix might solve the problem since the correct ephemeris data overrides what ever it came up with.

    Just speculation, I need more testing before I file a bug report. Seems to work will if you don't fire up the gps till you're on a road it knows about. Or maybe sit in your last location till his has a good lock? It does not happen at home when I'm only 50-100' from the road, so there seems to be a distance threshold involved.
     
    Last edited: Feb 18, 2008
    boren, Feb 18, 2008
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  7. jdholman

    kichu1979

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    The suspected reason for this problem is faulty QuickGPSfix file from TomTom, as many of the folks got it at the same time and after GPSfix upgrade problem is gone.
     
    kichu1979, Feb 18, 2008
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  8. jdholman

    baggss

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    That is what I was thinking. Once I updated the Quickfix, the problem went away.
     
    baggss, Feb 18, 2008
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  9. jdholman

    yur_maker

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    This is the solution

    You have to download the GPS FIX weekly.
    The GPS FIX is not a software patch, its the changing co-ordinance of Your relative position to the GPS satellite's.
    The satellite's relative position changes slightly every day so enable for Your GPS to remain accurate You need to download the new co-ordinance aka "GPS FIX" weekly.


    I hope this helps You all..
     
    yur_maker, Oct 11, 2009
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  10. jdholman

    gatorguy

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    AFAIK, Quickfix data has nothing to do with correcting your location. It's simply predictive software to assist with locating where in the sky the currently available satellites should be. It speeds up the satellite fix by avoiding your gps chipset doing a full sky search. In the past that could sometimes take 2-5 minutes minimum, and much longer if the device had been off for several days. But it doesn't then take that location data and move your position relative to the map. Your location is determined by decoding the satellite signals themselves. The map display is independent of that. More satellites discovered and complete broadcast data received, the more reliable your reported position. And FWIW, only one satellite fix is required to get the expected, or almanac, position of all the satellites. Each individual satellite found then broadcasts it's own ephemeris data stream, which is extremely precise orbital corrective data for that one specifically. That ephemeris data is what Tomtom supplies in it's quickfix update files. Some other devices from other manufacturers can build their own "quickfix" files without the need for any downloads from a website. That can go by different names such as HotFix, or InstantFix or Quickfix2 for instance. And of course there are some that don't offer the feature at all. There is no manufacturer other than TomTom that requires downloading an update file that I am aware of . They could easily make that a function of the device if they chose to like other manufacturers. IMO, the only reason TomTom requires you to do so thru TT Home is to gather your tracklogs/travel data as often as possible. If quickfix didn't have to be updated from there, many more users would seldom if ever connect to Home. TomTom wants that data on a weekly basis. Waiting much longer and some data from some of the heavy travelers would already be overwritten with new.
     
    Last edited: Oct 11, 2009
    gatorguy, Oct 11, 2009
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  11. jdholman

    mvl Moderator

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    I've had a bad quickgpsfix put me in the wrong location. If the almanac was offset by a similar amount for multiple satellites, the Tomtom could assume the wrong location. When the Tomtom put me in the wrong place, it eventually corrected it after 2-3 minutes. I assume that this was when it picked up the correct almanac from the satellites.
     
    mvl, Oct 15, 2009
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  12. jdholman

    yur_maker

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    OK but if thats the case please explain why when I called TomTom support, this is what they suggested and as long as I update the fix often , My TomTom is accurate?
    It takes four satellites to get an accurate position and if a satellite is not the same distance from Me next week then My tom will read incorrectly.
    I think You should talk to TomTom Tech support as this is how it was explained to Me..
    I agree that the fix should be streamed by the satellites, and it is on the units with Live.
    Your statement is correct that the satellite fix also tells the unit where in the sky the satellites are located to get a faster position lock but this is only good for around seven days also.
    All I can say is update and check the results or call TomTom support and ask them.
     
    yur_maker, Oct 15, 2009
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  13. jdholman

    gatorguy

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    I seriously doubt that very many TomTom support personnel really understand how the gps system works. They may somewhat understand how the TomTom devices are intended to work with the satellite data, but probably have no idea that where the TomTom understands you to be, and where on a map it places you are two independent events. "Snap to road" is used in part because of road geometry errors. Alot of roads are probably mapped anywhere from a few feet to a quarter mile from where they truly are. But your TomTom when operating properly is accurate to around 40 ft with a 90% probability. TomTom's appear to me to have a very generous leeway for this. But I still occasionally find my TomTom "driving off road" on on a parallel highway, altho it's much more rare than with some of the older 7.x maps.

    With that said, TomTom is correct in the sense that a fresh Quickfix file will get you to that 40' accuracy much faster if you don't use your device daily (assuming it's a good file). But even without that, your TomTom would attain that accuracy within 30 minutes or so all on it's own once the available satellite's ephemeris data had been received.
     
    Last edited: Oct 15, 2009
    gatorguy, Oct 15, 2009
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