GPX timestamp goes back in time

IncredibleFax · Feb 10, 2016

I'm developing a MS Access database application for processing and storage of all the data exported by my Gôrmin Nuvey 3490LMT, and by my late Gôrmin iQue 3600 (bought in 2003) and Gôrmin 45XL (1996). It derives all kind of metadata per track, it has graphs, and also an export function to show the processed data in QGIS.
No problem at all, until I tried to process a GPX of a trip in Germany, made 18-10-2015. It turned out that this GPX has a record in which the timestamp is 43 seconds earlier than the record before!

Time Lat Lon Speed (according to GPX)
09:24:25 49,7785250 10,0903030 98,8
09:24:46 49,7760170 10,0832070 98,8
09:25:06 49,7732610 10,0767390 98,8
09:24:23 (!) 49,7698490 10,0700000 98,8
09:24:43 49,7664340 10,0643300 98,8
09:25:04 49,7625770 10,0588040 98,8
09:25:34 49,7569480 10,0511890 98,8

This is the only time this happens in all my GPS data recorded in 20 years up to now (which comprises almost 2 million records).

What's is happening here?

Rgds

IF / FF/ HF

Nuvi-Nebie · Feb 10, 2016

Aren't there other problems with this list?, If the list of 7 times stamps are supposed to be in time order from top to bottom, it seems to me that they are only correct if they are re-arranged as :-

Code:

. 4 . . . . 1 . . . . 5 . . . . 2 . . . . 6 . . . . 3 . . . . 7 . . . top = 1 bottom = 7
24:23 . . 24:25 . . 24:43 . . 24:46 . . 25:04 . . 25:06 . . 25:34 . . Time stamp Mins:Secs

It looks like the measurements are interleaved, if you insert 1,2 and 3 like this they are in order :-

IncredibleFax · Feb 11, 2016

Good try, Nuvi-Nebie! Thanks.
But if we rearrange the records this way the differences in seconds combined with the distances from point to point lead to weird speeds. I was driving there with cruise control at 100 km/h.

Time Secdif Lat Lon Dist Spd
09:23:39 49,7829720 10,1066440
09:24:01 22 49,7810380 10,0986790 610 100
09:24:23 22 49,7698490 10,0700000 2404 393 (!)
09:24:25 2 49,7785250 10,0903030 1747 3144 (!)
09:24:43 22 49,7664340 10,0643300 2297 376 (!)
09:24:46 3 49,7760170 10,0832070 1723 2067 (!)
09:25:04 18 49,7625770 10,0588040 2301 460 (!)
09:25:06 2 49,7732610 10,0767390 1751 3151 (!)
09:25:34 30 49,7569480 10,0511890 2578 309 (!)
09:25:52 18 49,7537990 10,0462470 498 100
09:26:10 18 49,7509260 10,0409170 498 100

The cause of this sudden 43 seconds time leap into the past could be that my Nuvey's internal clock was running ahead by 43 seconds for some reason, and the GPS time corrected it. But I'm in doubt about that. I was using the unit every day and the trip in question started 1,5 hours earlier without any problem.

Maybe a big time leap in the whole GPS system? I doubt, since a short search on the internet gave me no message at all about such a thing on that day.

Nuvi-Nebie · Feb 11, 2016

Yes, I agree that the 1 - 7 order in #1 is correct (See map) and it is the time that that went wrong at #4, I can see now that there is only a single error and that 1,2,3 is one sequence and 4,5,6,7 is another sequence. I made the mistake of starting with #4 as a valid time (which it isn't) and tried to make the other values fit, my guess would be that there is a RAM fault in your nuvi

I also think that #1 measurement 7 has a time error in it, the distance and speed indicate that it should read 09:25:24 so that the time difference is 20 seconds not 30 Seconds

IncredibleFax · Feb 15, 2016

About the 30 seconds timing error: I do not think so. During those 30 seconds I drove 830 m (calculate the distance between two points), that is my speed was a calculated 99,6 km/h (0,83 * 3600 / 30). That corresponds with my recollection that I was driving with cruise control at 100 km/h.

That leaves us with the 43 seconds error. Even Gôrmins have errors sometimes.....

Nuvi-Nebie · Feb 15, 2016

O.K, I can see that the distance between 6 and 7 is bigger than the distance between 5 and 6 in #4, it just didn't look like another 10 seconds worth of travel. I guess you will just have to keep an eye out for other anomalies from your Nuvi

IncredibleFax · Mar 7, 2016

In my data of the last weeks I have discovered two more cases of this phenomenon. In one of them the time jumps almost two minutes back. I'm very surprised. What's going on? I'll try to contact Gôrmin about this.

Nuvi-Nebie · Mar 8, 2016

my guess would (still) be that there is a RAM fault in your nuvi

IncredibleFax · Mar 9, 2016

Could be. However, apart from this rarely occurring phenomenon my Nuvey navigates happily and flawlessly all the time! So the timing error does not seem to affect the rest of the unit.