Oregon 400T - glitch with odometer?

[catalyst81]

I was hiking San Jacinto Peak in Southern California and I took my new Oregon 400T along. It was performing well when I noticed the unit had powered off. I turned it on and the trip odometer had jumped from 8 miles total to 333 miles. The hike round trip was 15 miles. Any ideas what may have caused this? I recorded the hike with tracks and it appears to have recorded the hike correctly. I can't figure out what happened to the trip odometer though. After looking, it appears the max speed and moving average speed changed as well.

Edited May 18, 2009 by catalyst81

[+Jeep4two]

I was hiking San Jacinto Peak in Southern California and I took my new Oregon 400T along. It was performing well when I noticed the unit had powered off. I turned it on and the trip odometer had jumped from 8 miles total to 333 miles. The hike round trip was 15 miles. Any ideas what may have caused this? I recorded the hike with tracks and it appears to have recorded the hike correctly. I can't figure out what happened to the trip odometer though. After looking, it appears the max speed and moving average speed changed as well.

 

Any luck figuring this out?

 

I had this happen to me once shortly after getting my Oregon. Unfortunately I don't remember what software revision I was on - it shipped with 2.80 I think, then I upgraded to 2.97b.

 

In my case I noted a max speed that was really high (in the hundreds of MPH) and my mileage jumped to over 3000 miles on the odometer (I didn't note specifics - but realizing later the uniqueness wish I had used the screenshot feature). I just attributed it to a glitch in reception.

 

I haven't seen it again as of yet.

[+g-o-cashers]

Typically this is caused by a large (but temporary) location error when you first acquire lock. I've seen it happen at some point on just about every GPS I've owned.

[+DavidMac]

Here's my speculative understanding of what happens... I've seen it occur numerous times on every GPS unit I've ever used. It's easier to explain with diagrams, but I'll try with words.

 

Your GPS unit calculates it's position from each satellite based on the delay in time that it takes for the signal to reach you. If we know our distance from only one single fixed point above us (a satellite), we can do a little math to calculate a large circle on the earth's surface. We expect that we are somewhere along this circle, but we do not know where.

 

If we add a second fixed point (another satellite), we can now draw two circles on the earth, and conclude that we are at one of the two points where these circles intersect each other. However, we cannot determine which of the two intersections we are really at. To do that, we need to know our distance from a third satellite. This is why your receiver needs at east 3 satellites to acquire a signal, a process known as "triangulation".

 

The problem is that the GPS signal can be reflected, bounced, or blocked by nearby objects. When this happens your GPS can miscalculate it's distance from one of the satellites, and in some cases can mistakenly conclude that it is hundreds of miles from it's true location. Sometimes these wild readings will only last for a second or two, then your GPS will again pinpoint the proper location. Normally your GPS has algorithms which will correct for these outliers so that you don't even see them, but once in a while one will slip through. When this happens, your GPS mistakenly concludes that you have moved several hundred miles in the last second.

 

Take a look at your track log when you either first acquire reception or when you loose it, and you may see that one or more tracklog points (often the first or last point) are wildly inaccurate. Since speed is simply the distance traveled over a unit of time, the GPS mistakenly reports your maximum speed as if you traversed the distance from the erroneous point to the correct one in a very short time (my personal record was something like 8,000 MPH ). Same thing goes for the odometer. If the erroneous point was 160 miles away, your GPS could report you having traveled there and back in a few seconds, adding 320 miles to your odometer.

Edited May 18, 2009 by DavidMac

[+DavidMac]

double post

Edited May 18, 2009 by DavidMac

[catalyst81]

Well I checked my track log and everything was spot on. Tracked the hike perfectly. No rogue points anywhere. I'll have to just keep my eye out and see if it happens again. Its really strange that the total distance would be so large when the track log correctly recorded my movements.

[+Jeep4two]

(my personal record was something like 8,000 MPH ohmy.gif )

 

Heheh - Nice! I've often thought of pulling my GPSr out if I got pulled over and showing my 'max speed' to prove that "I wasn't going that fast officer" however this little glitch would seem to indicate that to be a bad idea.