Resetting lat/long coordinates

Discussion in 'Technical Support' started by orienteer, Feb 25, 2013.

  1. orienteer

    orienteer

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    My Tom Tom GO910 Lat/Long settings appear to be about 1/2 a mile out. Anyone know how to reset these? :confused:
     
    orienteer, Feb 25, 2013
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  2. orienteer

    Andy_P Moderator

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    TBH, I don't see how they can be.

    Can you give an example and also say where you are comparing the figures to, so we can try to work out what's going on?

    The only possible issue I can think of is that there are several "datums" used for coordinates.
    AFAIK, all satnavs use WGS84, but the Ordinance Survey in the UK do seem to use an alternative on the edges of their maps called OSGB36.
    However, I believe the difference should only be about 100 metres.

    There are more details and converter in several places on the web. Here's a couple:
    http://www.movable-type.co.uk/scripts/latlong-convert-coords.html
    http://www.nearby.org.uk/coord-ll.cgi
     
    Last edited: Feb 25, 2013
    Andy_P, Feb 25, 2013
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  3. orienteer

    canderson Moderator

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    Edit: Nuts .. I see Andy already answered this while I was out of the house for a while. Oh well ... will add my 2 cents anyway since I had it all typed before I left.

    There's more than one way to communicate lat/long data. The native version we all use on an automotive GPS would normally be something called WGS84. HOWEVER, handhelds all offer other datum options, and many others were used previous to the widespread adoption of WGS74. Be certain that the coordinates you are inputting are NOT from some previously used datum! Your device expects WGS84 datum coordinates ONLY.

    Here in the US, we were using NAD27 prior to NAD83/WGS84. While errors between the old and new usually aren't anywhere near half a mile, the difference can be significant depending upon where you are over here.
     
    canderson, Feb 25, 2013
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  4. orienteer

    orienteer

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    Re resetting lat/long coordinates

    Thanks to both for the replies :thumb:. I use my Tomtom mainly in my motorhome when travelling round Europe to orienteering events. I have books detailing coordinates for campsites and the various events normally use coordinates for their venues. I first used lat/long a couple of years ago when I first bought my TomTom to find campsites in Spain but as I said,It took me to within about 1/2 mile of the campsite. I have tried it again recently in Portugal with the same results, but this time I had a friend with me who put the same coordinates into his Garmin and this took us to the correct location :confused:.
     
    orienteer, Feb 26, 2013
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  5. orienteer

    canderson Moderator

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    Not sure where the malfunction is. I use lat/long frequently -- perhaps more than anyone else using a TomTom (special use model) and don't run into this.

    I have Euro maps loaded on all of my units, so can you provide an example of coordinates that you have used in Portugal that got you into trouble? That will give me something to look at, research, and discuss. Perhaps we can figure out what's going on here.
     
    canderson, Feb 26, 2013
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  6. orienteer

    orienteer

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    Many apologies. I hadn't realized tat there were 3 different formats as unfortunately the satnav was set on the one that no-one uses. I have now changed this and it now appears to be spot-on
    Cheers :thumb:
     
    orienteer, Feb 26, 2013
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  7. orienteer

    Andy_P Moderator

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    Ahhh... Yes, forgot to mention that.
    It's easy to get it wrong when there is degrees, minutes ,seconds, or Degrees minutes and decimal minutes, or just Degrees and decimal degrees.

    I just use decimal degrees, because that's what Google maps uses, but it always confuses me when I come across one of the other formats.
     
    Andy_P, Feb 26, 2013
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  8. orienteer

    swiftarf

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    Definately an issue

    I've recently experienced the same issue with incorrect positions on Tom Tom navigators. On a recent Spanish trip we were using the Tom Tom to navigate to the landing positions of paraglider pilots. The pilots were using a mixture of devices (Garmin handhelds, iPhones, android phones and spot satellite communicators) & positions given by all of these devices correlated. The Tom Tom, when given the coordinates would consistently navigate to the incorrect location.

    By simultaneously noting the position on the Tom Tom and another device we calculated a correction factor (+0.00007N & -0.00017W) which could be applied to get to the correct location using the Tom Tom. All the devices mentioned were using WGS84. There seemed to be no way of altering the datum on the Tom Tom. Obviously we were using decimal degrees on all devices.

    It would seem to me that there is a datum issue with the Spanish maps (& Portuguese maps from previous posts). As the Tom Tom belonged to a friend, I don't know what the model is, but I will find out. We've not tried the coordinate entry in the UK so can't comment yet on the UK maps. Interestingly, the positions using Sygic, a navigation app on android phones which apparently uses the same maps as Tom Tom, also correlated with the positions given by the non Tom Tom devices.
    Cheers,
    Simon
     
    swiftarf, Apr 22, 2013
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  9. orienteer

    swiftarf

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    Oops, Too many zeros in correction!
     
    swiftarf, Apr 22, 2013
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  10. orienteer

    canderson Moderator

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    If you're using a map to work the coordinates, odds are quite good that they are not using WGS84. While a Garmin handheld can be adjusted for use with what seem to be 100 different datum selections, the TomTom and other brands of similar automotive devices are strictly WGS84. Creating an offset as you did is the only solution if you have only the map and the TomTom. Just beware that your offset will work only locally, and as you move around the country, you'll start to see different offsets apply.

    This isn't really a 'problem' with the Spanish or Portuguese maps per se. For example, your own British maps based upon OSGB 36 have offsets that are variable against WGS84 all around Britain. It's just a matter of dealing with the differences in the coordinates. Here we still have most paper maps in our old NAD 27 format, which depending upon where you are around the U.S., will have differing amounts of offset against WGS84. All of these variations in mapping are why the WGS84 is now a preferred datum.

    You'll find this issue on all automotive-specific nav devices.

    One solution is to carry the most inexpensive Garmin handheld you can find. While it won't have the features of the TomTom, what you can do is create a waypoint using one datum (e.g., ED50 for many Spanish maps) coordinates, and then swap to WGS84 datum on the Garmin. Your waypoint's coordinates will automagically be displayed in WGS84, converted by the unit for the area in question. Then you can plug those into your TomTom.

    Just be aware that if your destination for pick-up is too far from any known road, you may get a "Route not possible" message, even with good coordinates.
     
    canderson, Apr 22, 2013
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  11. orienteer

    Andy_P Moderator

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    So what's the "corrected correction" :lol:

    I tried a (very unscientific) experiment on Google maps where I sent it to a set of coordinates and then added 0.00007 to the N(Lat) and subtracted 0.00017 from the W(Long) and I managed to measure it as onlyabout 7metres difference - although it was pretty much at the limit of Google's accuracy.

    Is that the sort of accuracy you are hoping for? I think the fundamental accuracy of the GPS system as used in TomToms is worse than that (hence the quiet "snapping to roads" that goes on).
     
    Andy_P, Apr 22, 2013
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  12. orienteer

    canderson Moderator

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    I was kinda wondering about that, too, Andy.

    +0.00007N & -0.00017W isn't much of a miss for driving purposes. At 40 degrees (we're on about the same latitude here as Madrid), I figure 0.00017ยบ (or about 0.010 minutes) E/W offset to be around 45 feet of error. You won't miss 'glider in that amount of space. But I think the OP said something about slipping a digit or two somewhere.

    No matter .. it's still the fact that nearly all paper maps are using some other datum than that of an automotive GPS.
     
    canderson, Apr 22, 2013
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  13. orienteer

    swiftarf

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    Hi guys,
    The correction had too many zeros should be N 0.0007 and W -0.0017.
    No paper maps were used- literally the garmin (and other device) lat/lon was sent by SMS to the driver who entered the coords into the tom tom. The error wasn't huge but could lead to the driver going down the wrong track (or to the wrong bar) from where the pilot would be waiting. Part of the problem is the Tom Tom snapping to roads when working out the destination, but usually the pilot would make their own way to the nearest road which would tend to be the same one. Obviously not a big issue but an annoyance.

    I wonder whether the Spanish tom tom map originally came from data referenced to something like the ED50 datum and this got lost in translation into the electronic map?
     
    swiftarf, Apr 22, 2013
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  14. orienteer

    canderson Moderator

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    Negative. TomTom's geolocation is based strictly upon current WGS84. They aren't trying to use uncorrected older datum maps for their raw mapping data.

    It sounds as though there may have been some error based upon availability of known roads. When it can't quite get you there, but feels as though it is close, it will place the final destination as close as it can on a known road.

    Is there any chance at all that you could supply us with one example of the coordinates you were using when this problem occurred, and a rough idea of where you were starting? Probably a bit late to be asking for that, but it could prove enlightening.
     
    canderson, Apr 22, 2013
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  15. orienteer

    swiftarf

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    I'm afraid I don't own the Tom Tom and I'm no longer in Spain :-( so I can't give any more info. I can't think of any explanations that would explain the findings other than an error with the datum, but I guess the only thing for it is to go back to Spain and try the GPS again :)
     
    swiftarf, Apr 22, 2013
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  16. orienteer

    Arno Moderator

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    It was my understanding that Android phones use Google maps.

    With the highest enlargement on Google, put the mouse on one spot on the edge of the road. Right click. Click on "What's here?". Enter the resulting coordinated into TomTom and see what difference you find.
    Any time I do that, there is virtually none.

    PS
    For the last 5 years postal addresses were usually out by 30 to 45 metres in longitude. Google and Nokia (Bing) are correct now here but Navteq still moves me 5 houses to the west.
     
    Arno, Apr 22, 2013
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  17. orienteer

    canderson Moderator

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    @Arno
    Depends upon what you've got loaded on your Android. There is a TomTom app!
     
    canderson, Apr 22, 2013
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  18. orienteer

    Arno Moderator

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    Had the composition window open and had to rush away so I didn't see the above post by the time I completed and posted mine.

    Let's go through the exercise anyway.

    You, or your friend know where you had wanted to be. Please post those Google coordinates here and we will check as to where they will get us in the TomTom map.

    As canderson said, if your destination is far off a road TomTom may not allow us to go there. I am game to try to "drive" there in Demo mode if you can remember the place from where you took off or a town you passed on the way, that day.
     
    Arno, Apr 22, 2013
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  19. orienteer

    canderson Moderator

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    I would be too, but posts #14 and #15 seem to preclude reproducing the original problem.
     
    canderson, Apr 22, 2013
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  20. orienteer

    Arno Moderator

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    Right, a question of interpretation.
    I excluded TomTom as the tenet was TomTom being wrong in which case "& positions given by all of these devices correlated" wouldn't make sense.

    I presumed that #15 referred to what had appeared on the TomTom on site.

    What I was asking for was to which place they had been in Spain.
    If swiftarf does not remember, maybe his friend does.
    He also doesn't have to be in Spain to use a Google map and right click on that location and then click on "What's here?".

    If either of that is impossible, so be it.
    I still am very curious and would hope that we can hear from the OP.

    I know your specific interest in this case is to see if you be positive about a difference in coordinate systems.
     
    Arno, Apr 22, 2013
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