TTN5 200yds East of position.

LeighB · Posted: Mon Jul 04, 2005 10:05 am Post subject: TTN5 200yds East of position.

Hi,
I travelled to Brentford from Ipswich yesterday. First longer trip with TTN5 generally very pleased with it. On the waydown it tracked perfectly with no "wobbles"
However when coming home in the evening on the North Circular from Chiswick to Hanger Lane it showed a position some 200 yds or so the East !
It was constantly trying to work out which of the roads we might be on.
Once we got to Hanger Lane it sorted itself out and behaved pretty well after that.
This was the first trip in our Peugeot 306. With the Mio in landscape position the aerial is facing sideways, could this be part of the trouble?
I am not sure if the Peugeot has a reflective windscreen or not. The response etc does seem that much better in our old Volvo.
We are off to Lancaster tomorrow, I think I will try the magnetic roof aerial and see if that makes adifference.
_________________
Leigh

Mio168 Kingston 256SD
TomTomNavigator3 3.07
TomTom Navigator 5
Memory Map 2004 OS 4.4
Checkpoint 2.02

Robin2 · Posted: Mon Jul 04, 2005 10:31 am Post subject:

I had exactly the same thing about a month ago with TT3. It lasted until I got home, a distance of about 40 miles. I also tried CoPilot, with the same result, so it was not specific to TomTom.
It was an evening when I subsequently found that there was a lot of sporadic reflection from the E layer in the ionosphere. I can only assume it was affecting the GPS, because it worked perfectly the following morning
Robin
_________________
TomTom One v3 Europe, Navcore 7.903, Western Europe 835.2420
TomTom Go 720 Europe, Navcore 8.351, Western Europe 855.2884
Satmap Active 10
Plus a lot of other PDA GPS kit, seldom used

LeighB · Posted: Mon Jul 04, 2005 11:04 am Post subject:

Hi Robin,
Thanks for reply, strange that it was only out for a relatively short time, but the error was very consistent, you could just see the North Circular on the left of the screen with our position charging up through the housing to the East!
That area is not especially affected by tall buildings, so it was not that I don't think.
Will wait and see if similar problems happen again.
_________________
Leigh

Mio168 Kingston 256SD
TomTomNavigator3 3.07
TomTom Navigator 5
Memory Map 2004 OS 4.4
Checkpoint 2.02

Robin2 · Posted: Mon Jul 04, 2005 11:11 am Post subject:

Yes, it was similar with me. I could see the road I was actually on at the left of the screen. For most of the time I was in open country, but when I got into the city it kept trying to get me to join the road I was actually driving on.
There has been no recurrence since that evening in late May
Robin
_________________
TomTom One v3 Europe, Navcore 7.903, Western Europe 835.2420
TomTom Go 720 Europe, Navcore 8.351, Western Europe 855.2884
Satmap Active 10
Plus a lot of other PDA GPS kit, seldom used

Diplo · Posted: Mon Jul 04, 2005 11:40 am Post subject:

Remember that TomTom tries to 'snap' your position to the nearest road. If, for some reason it can't get a precise fix then it's possible this would show your position on a nearby or parallel road rather than the one you are actually on.
_________________
Medion 95000 | TT3.07 | Kingston 512MB SD

Robin2 · Posted: Mon Jul 04, 2005 11:57 am Post subject:

No, it wasn't that. For most of the time it was happening it was showing me in open country, well to the right of the road I was actually on, which I could just see to the left of the screen.
When a minor road coincided with the indicated position it did try to snap me onto that, but that was infrequent until I got into a built up area.
As the road changed direction, my track swung round with it, but always staying approx the same distance to the right.
As I said earlier, it was nothing to do with TomTom. I did a soft reset, started CoPilot, and had exactly the same error
_________________
TomTom One v3 Europe, Navcore 7.903, Western Europe 835.2420
TomTom Go 720 Europe, Navcore 8.351, Western Europe 855.2884
Satmap Active 10
Plus a lot of other PDA GPS kit, seldom used

LeighB · Posted: Mon Jul 04, 2005 4:18 pm Post subject:

I was in the same situation. The position kept trying to snap to a road somewhere off to the right, but most of those ran at right angles to the road I was on, and TTN5 got very confused trying to decide which road I was actually on.
Then once I got to Hanger Lane it all went back to normal.
_________________
Leigh

Mio168 Kingston 256SD
TomTomNavigator3 3.07
TomTom Navigator 5
Memory Map 2004 OS 4.4
Checkpoint 2.02

wazzzzupmate · Posted: Mon Jul 04, 2005 8:47 pm Post subject:

This has happened to me today.
A167 near Sheffield, showed me 100 or so yards left of the road, in a field. This carried on for about 1/4 mile.
On numerous occassions it keeps doing this, not a problem on a motorway say, but a real pain when navigating city centres.
This was using the built in GPS receiver on my MIO168, so I tried using the hard wired one on com1 (the one for my Medion pocket pc as I use the same lead) and the same problems keep occuring.
I was assuming this was because the MIO168 is a slow PDA, but I may be very wrong with this assumption.

treborvfr · Occasional Visitor Joined: Aug 25, 2004 Posts: 34

With a modern Peugot you will almost certainly need to use the external aerial due to the heat reflective windscreen.

Last year I picked up a Peugot as a hire car in Barcelona, my satnav didn't get a fix whilst trying to navigate from the centre to the outskirts of the city. At first I put it down to the fact that the streets had quiet a lot of tall buildings and trees either side. I pulled over at the first suitable stopping point, opened the car door and, hey presto, got a fix immediately! Fortunately I had taken my external aerial with me (first time ever on a foreign trip), fitted that and never had another glitch all trip.

Bob

Gustman · Occasional Visitor Joined: Jul 06, 2005 Posts: 4

Sounds like a map datum difference. A map datum is the mathematical projection used to convert the curved surface of the planet to a flat map. There are many different versions and they're all a bit different.

I had this problem in the past with non-tomtom software and a Garmin GPS. I was always 200m out. But I fixed it by changing the Garmin's map datum settings.

I don't have TomTom5 yet (still waiting), so i don't know if you can use the software to change such settings on your GPS or in software on your PDA.

Basically, if your GPS can use different datums then you need to match the GPS datum to the datum of the TomTom map. If TomTom don't specify the datum then it's almost certainly the universally popular WGS84 datum you need.

If map datum can't be configured out of TomTom or on your GPS, then it may be a different problem.

Hope this helps.

Robin2 · Posted: Wed Jul 06, 2005 2:47 pm Post subject:

But in my case this happened during the last 40 miles of a 250 mile journey, for most of which TomTom 3 had been behaving perfectly. I switched to CoPilot and got the same result. Next day it was fine, and has been ever since. Doesn't sound like a mapdatum setting to me.
Robin
_________________
TomTom One v3 Europe, Navcore 7.903, Western Europe 835.2420
TomTom Go 720 Europe, Navcore 8.351, Western Europe 855.2884
Satmap Active 10
Plus a lot of other PDA GPS kit, seldom used

Gustman · Occasional Visitor Joined: Jul 06, 2005 Posts: 4

True, if it fixed itself without any reconfiguration or hardware reset then that probably wasn't a datum issue.

But in any case where the indicated gps positition on a map is the same distance and direction in error all the time (eg 200 yards east), the first thing I would check would be the map datum setting.

I would guess (not an educated guess, just a common sense one) that atmospheric interference or an obstacle to reception such as glass or trees or clouds would be more likely to randomly scatter the indicated positions by 10 or 20 metres in all directions around the true position, than to continually offest the indicated position on exactly the same vector from the true position by as much as 200m. 6 to 12 satellites would need to triangulate your position wrongly, but consistently and precisely wrongly.

A datum error however is indeed likely to show exactly such a large fixed inaccuracy, and only requires one little GPS setting to be wrong. I'd run it through the "what's more likely?" test.

Depends on the symptoms I guess. TomTom's algorithm which snaps the position to a road on the map may be clever enough to hide small map datum matching errors for a lot of the time (e.g. if you follow one cross country road for a long time) and only come unstuck when it has too many road options in town to correctly guess which one you're on. That way you might experieince an intermittent fault caused by a continuous datum error.

Like I say, I don't have TomTom yet, so I don't really know. It's just supposition based on one of my own infuriating gps experiences!

BTW, in Spain, WGS72 is offset from WGS84 by about 120m. Doesn't matter much for general map reading but matters a whole lot for gliding competiton turnpoint verification. It's the difference between winning and wasting a day. Not sure what the offset between these 2 datums is at UK latitudes.

Cheers

Robin2 · Posted: Wed Jul 06, 2005 5:33 pm Post subject:

Yes, in my own case the error wasn't a fixed distance east, but a fixed distance to the right of my true position. As I drove round bends, the error moved with me and so the road I was actually on was always on the extreme left of my screen.
I don't have any rationale for the effect I noticed, but I did find out subsequently that there was a lot of E layer activity in the ionosphere that evening, leading to exceptional sporadic E activity on the 6 metre amateur radio band. It's pure speculation on my part that there is any connection, but it was certainly nothing to do with my software (same effect with TomTom 3 and CoPilot) and the GPS receiver was in the same position for the whole time
Robin
_________________
TomTom One v3 Europe, Navcore 7.903, Western Europe 835.2420
TomTom Go 720 Europe, Navcore 8.351, Western Europe 855.2884
Satmap Active 10
Plus a lot of other PDA GPS kit, seldom used

gg2000 · Posted: Sat Aug 27, 2005 6:23 pm Post subject:

Yesterday while travelling clockwise along the M25, my position jumped to a few hundred yards to the left of the M25. This continued for about twenty miles with only the occasional jump to my correct position. At one stage I pulled off the motorway and did a soft reset but the problem reoccurred shortly after returning to the motorway. For the majority of this time the GPS signal strength was 5 or more satellites.

Each time the false GPS position contacted a road off the motorway it started to recalculate a route

I have also noted several times the GPS seems to suddenly loose all satellites but moments later has a full signal again. I never had these problems with TT3. Now wondering if I should reinstall TT3 Does anybody know if TT5 maps work with TT3
_________________
TTGO 700 v6.15 External Aerial

treborvfr · Occasional Visitor Joined: Aug 25, 2004 Posts: 34

	PocketGPS
	Web