Trip Odometer Discrepancy Etrex (or all Garmin?)

[seldom_sn]

This was originally placed as an off topic post in the Magellan Explorist thread about the same topic.

As I posted there, my Etrex30 tracklog for a trip showed 19.5 miles. My trip odometer showed 20.3 for the same trip. Can anybody explain why this would occur? I would have thought the trip odometer would simply display the totals from the tracklog, but obviously it wasn't. I'm posting this here, because, if answered, it might be of general interest. Also, because Garmin doesn't have forums that deal with the operation of their handhelds or automotive devices, and Garmin's FAQ server crashes whenever I try to put trip odometer, tracklog, and discrepancy in the same query.

[tr_s]

Well, consider this.

 

Tracklogs are drawn between points. How often points are taken depends on the GPS. A very coarse track log setting will result in much shorter tracks.

 

The trip odometer more like "sums up continual movement in every instant". Hence if you walk a street in a zig zag pattern it will show a much longer distance than the length of the recorded track, which the gps may have created track points for at the start and the end of the street only.

[seldom_sn]

The trip odometer more like "sums up continual movement in every instant".

But it's still got to have some sampling rate. Otherwise there would be an infinite number of points of zero length in the track.

The track/trip odometer sample was an in and out hike with maybe 1/4 mile of side trips on the way in, but as short as possible on the way out. (Water was running low and it was getting hot.)

But the odometer to the turn around was 10.1 mi.(agrees with tracklog) and the odometer at the end was 20.3 mi.(tracklog says 19.5).

Edited June 2, 2012 by seldom_sn

[tr_s]

The trip odometer more like "sums up continual movement in every instant".

But it's still got to have some sampling rate.

 

Of course, and my guess is that this would be the GPS refresh rate (1 second on most devices).

[+user13371]

Every GPS I've ever had (Magellan, Lowrance, DeLorme, and Garmin) has done this, to some degree. More noticeable to me on tracks that were recorded at slower places -- that is, worse for walking than cycling, and worse for cycling than for driving. Every GPS I've ever owned has also under-reported speed at low speeds and gets better at higher speeds.

 

I take it for granted now and always just figured it to be a collection of rounding errors. If I really need a more accurate measure of the track afterwards I'll load the track in Google Earth and measure it that way.

Edited June 4, 2012 by user13371

[xyzee]

From an outside wandering around in the yard test:

 

Garmin GPS 60, the yellow entry level receiver.

Units: Distance set to yards.

Tracks: Setup set to Distance, 1.00 yd

 

Results:

Tracks showing Distance as 413.68 yd.

Trip Odom reads 414 yd.

 

And then there is the multi testing under Stature, Metric, etc, and Tracks set to Auto and Time. Some day...

[seldom_sn]

Results:

Tracks showing Distance as 413.68 yd.

Trip Odom reads 414 yd.

That looks like a simple rounding error, but it's a pretty small sample. Of course you're setting to Record By Distance could help as well. My 19/20 mile sample was set to automatic.

[John E Cache]

Put aside errors caused by operator errors like zig zag walking and picture the GPS on fixed spot on a moving train. My view is that there will be points on both sides of the tracks because of random GPS position errors that happen on all GPSs that publish position errors.

[rsaxvc]

The trip odometer more like "sums up continual movement in every instant". Hence if you walk a street in a zig zag pattern it will show a much longer distance than the length of the recorded track, which the gps may have created track points for at the start and the end of the street only.

 

Have you tried setting the track log recording rate to 1/second and seeing how it matches up?

[Grasscatcher]

Was the odometer reset at exact place as the tracklog started?

 

Were both meas. ended at the same location?

 

Was the GPS turned off and left off until reading the differences? At the same location?

 

What about points logged while at any stationary rest stops?

 

Set the GPS outside in a static location and leave it for an hour or so. Then read the tracklog length and the odometer reading.......do the same thing with the GPS indoors and you will learn something surprising about "random points".

Edited June 5, 2012 by Grasscatcher

[seldom_sn]

Was the odometer reset at exact place as the tracklog started?

Yes

Were both meas. ended at the same location?

Yes

Was the GPS turned off and left off until reading the differences? At the same location?

Tracklog was saved and cleared at the end of the trip. Trip odometer was read when tracklog was saved.

What about points logged while at any stationary rest stops?

Since both measurements were continuous any spiderwebs should have been incorporated into both measurements. If anything, I'd have expected the trip odometer to exclude the spiderwebs.

Set the GPS outside in a static location and leave it for an hour or so. Then read the tracklog length and the odometer reading.......do the same thing with the GPS indoors and you will learn something surprising about "random points".

I'll give it a try. I've done spiderwebs before, but never compared them with the trip odometer.

[Grasscatcher]

After 18+ years GPSing, and several thousand miles of mapping trails,and multiple GPSs, the odometer page is a page that I don't even display on any of my GPSs.

 

I only use the data from the tracklog and then only after using Expert GPS to zoom in and magnify the track to the point of being able to view and work with individual trackpoints and eliminate random logged points and visually obvious errors.

[seldom_sn]

After 18+ years GPSing, and several thousand miles of mapping trails,and multiple GPSs, the odometer page is a page that I don't even display on any of my GPSs.

True if you are just looking for measurements, but if you are on the trail, the odometer is a lot easier to access than tracklog stats.

 

FWIW, my first response from an email asking the same thing to Garmin was:

Thank you for contacting Garmin International.

 

I would be happy to help you with this.

 

It sounds like you need to update your software on your unit.

 

Please go to the link below to download a program called ‘WebUpdater’ to update your software.

 

http://www8.garmin.com/products/webupdater/howtoinstall.jsp

 

Please let me know if you have any further questions. Thanks and have a great day!

 

Maybe they don't know the answer either.

 

PS: My Etrex30 is started on my window sill and the trip computer is indicating moving time of 8 seconds, a stopped time of 19 minutes, and a trip odometer of 26 feet. That trip odometer reading hasn't changed since I first observed it, so I'm thinking it occured during the first 8 seconds. GPS accuracy on the satellite screen shows 9 feet. 16 satellites showing (All GPS, no GLONASS - GLONASS turned off)

Edited June 5, 2012 by seldom_sn

[Grasscatcher]

I understand your original post, and problem ......but you won't find an accurate solution.

 

Garmin will tell you to update your software, or to do a master reset, or that better results should not be expected from that unit (or whatever unit), or that this is the first time they ever heard of that, etc,etc. . Then they will just quit answering your e-mail questions. (been there done that)

 

I do either a "pre" route or a "pre" track (with interim wpts), and then use "Dist to next" or "Dist to Destination" for how far you yet have to go. You'll have to just come up with a work around that is acceptable to you.

 

Accessing the odometer just because it's easier , when you have already determined that the information there is bogus, is kinda worthless......

[Suscrofa]

The GPS is screwed up because of these stupid measures you use, feet, yard , miles etc ...

You can't do the math, so why should the GPS ?

 

PS: It has already been discussed and explained over and over.

Edited June 5, 2012 by Suscrofa

[seldom_sn]

Follow up: After 1 hour and 35 minutes of stopped time and 12 seconds of moving time my stationary Etrex 30 shows a spiderweb track distance of .1 miles, but the trip odometer still says 26 ft.

[seldom_sn]

The GPS is screwed up because of these stupid measures you use, feet, yard , miles etc ...

You can't do the math, so why should the GPS ?

 

PS: It has already been discussed and explained over and over.

Well it's obviously doing much more complicated calculation in metric to figure out lat and lon. Maybe it's too aware of history, and is afraid if it gets it right it will be guillotined. (Metric system was a French Revolutionary invention after all.)