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Garmin Connect Web Distance of Trip odometer not the same than distance of Track (Dakota 20)

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Distance of Trip odometer not the same than distance of Track (Dakota 20)

Former Member over 14 years ago

Hello,

During hiking, I have noticed that the distance indicated by the Trip odometer (on the trip computer) is not the same than the distance indicated by the final track I save after the walk ... It's always less.
Exemple : 25 kms for the Trip Odometer and 29 kms for the final saved track.

Is there a parameter to set ?? Or is there something wrong ?

Thanks for your help.

Stephan

0 Former Member over 14 years ago

There is no parameter to set and there is nothing wrong. At least, not anything that can be corrected.

When you walk (or drive), the GPS is constantly doing calculations based on where you are this instant compared to where you were on the previous location reading so it "knows" which direction you are traveling and how fast you are traveling.

At each of these calculations, it also calculates the distance traveled and adds that to the Trip Odometer.

Most GPS units do these calculations approximately once each second. You do not travel very far in one second, even in a motor vehicle traveling at the speed limit, so each of these calculated distances will be fairly accurate. That means when you add them all up, as the Trip Odometer is doing, the total distance in the Trip Odometer at the end of the trip will be fairly accurate.

One thing that will affect the accuracy of the Trip Odometer when you are hiking is if you are not moving fast enough for the GPS to detect that you are moving. That could cause little pieces of travel to not get added into the Trip Odometer and the distance it reports to be shorter than reality.

Track files are different. If you save the track log to a file, it always prunes the log to 500 track points, regardless how many points there are in the raw log file. If your driving or hiking was not in a long straight line, you will "lose" distance when pruning the points. That is, if you walked in a curve that originally had 20 points marked and the curve gets pruned to, say, 3 or 8 points to describe it in the track file, you will not get the full distance of the curve calculated in the track.

That is because the calculations of distance in the track file all assume that the distance between each recorded point is a straight line. If you describe a curve with fewer points it will always look like the curve covers less distance.

If you transfer the raw track log file to Mapsource or Basecamp, you should get all of the track points and that should cause less of a difference between the track and the related Trip Odometer reading.

I hope that makes sense.

...ken...

0 Former Member over 14 years ago

It's very clear. Thank you very much.

One thing that I don't really understand : Why a GPS made for outdoor activities (that means also for hiking) is set for doing the calculation of the distance of the trip odometer once each second if we know that it's too less for doing the right calculation ??? We should have the possibility to adjust it regarding the speed of the activity ... If not, the function "Trip odometer" is not usable ?!

And this is possible for the record of the track : we can adjust if we want less often, often, more often ... the record of the point. But it's only for the track and not for the trip odometer (stuck at one second if I well understand).

Thank you very much

Stéphane

0 Former Member over 14 years ago

One thing I want to add :

If I remember well, I think that the 3 or 4 first times I used my Dakota hiking (it was in France), the distance of the trip odometer was right. Then, I started the Colorado Trail in US, and it was the first time I discovered the problem ... don't know why.

Maybe, I will do a test with my car ... to see if the distance of the trip odometer is better with high speed !

0 Former Member over 14 years ago

I'm sorry if I presented some confusion. Some things I should clarify.

1. I do not know exactly how frequently the trip odometer is being updated. The typical refresh for location data with most external GPS devices (like the GPS10x or GSP20x) that you connect to a laptop is 1Hz (once per second). I should have been more clear that I was using that as an example to illustrate.

2. When hiking, the trip odometer is more likely to be wrong (shorter) than the track log is. The trip odometer might not notice that you are moving, if you are moving too slowly. If that happens, it can miss adding some distance into the trip odometer. This is an issue with all handhelds. More of an issue with some than with others. It's pretty well documented in a variety of other forums on geocaching and other uses.

3. The track log records track points in a list. It does not know or care how fast you are travelling, or even if you are moving at all. It will simply record the location and date/time at the interval it is set to. You can tell it is doing this by simply setting the GPS in one location, turning it on and going away for an hour. Then view the track log. Depending on how good the reception is, you may see lines all around the spot you left the GPS sitting. This works best if you have poor reception, like in the basement of your house. :eek:

So, for hiking and other very slow speed activities, you might find the distance computed from the track log to be more accurate because the trip odometer might not realize you are moving and might miss adding some distance. But when driving, especially on very twisty roads, you might find the trip odometer is more accurate. You will be traveling fast enough that it will always know you are moving and not miss pieces, whereas the track log might get pruned and the computed total distances between all the points will be less accurate than the road travelled.

If you are traveling in a motor vehicle and going mostly in a straight line, there should be no difference between the two.

I don't think there would be any value in changing the update interval for the trip odometer because it would not save any memory or provide any other benefit I can think of. If you have a problem with the trip odometer being inaccurate when you are hiking, you just need to walk faster. :D

...ken...

0 Former Member over 14 years ago

OK thank you,

I can only confirm your conclusion : I have tested the trip odometer driving my car and the distance is OK to compare to the distance of the track.

But I'm still thinking that something has changed because one year ago, during 3 or 4 day hikes, it worked. And I'm not slower !!!:(

Stéphane

0 Former Member over 14 years ago

I am certainly slower than I was 3 or 4 years ago. But that happens when you get to my age. Perhaps your GPS is just getting tired? :D

...ken...