GPS / posted distance descrepancy

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GPS / posted distance descrepancy

Postby Dexter » Tue 19 Jan, 2021 8:49 pm

Just a question regarding GPS and posted distances you see around various parks. I use a Garmin Fenix 5, and have it set to ping GPS at the default (1 min I think?) rather than the less frequent battery saving mode. I recently went for a walk posted as 17km return, yet my GPS gave me a distance of 27km. Granted we did do about an extra 4km for a side trip... But that's still a 6km difference. I can see the entire recorded walk on the map with no apparent glitches. The watch measures elevation changes, is there a chance that a posted distance is more of a 2D measurement for lack of a better term? I'm sure that GPS can become a little off with dense bush and trees / mountains and rock at times, but surely it wouldn't be 6km+ off over a fairly small distance. And if it really went that far off the mark, wouldn't that be represented on the completed map?

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Re: GPS / posted distance descrepancy

Postby crollsurf » Tue 19 Jan, 2021 9:29 pm

Never seen that kind of a discrepancy before. I would normally think give or take 10%. What was the walk if you don't mind?
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Re: GPS / posted distance descrepancy

Postby ribuck » Tue 19 Jan, 2021 9:33 pm

Take a look at your recorded track. Does it zig-zag all over the place? Then, the extra distance is due to the GPS "hunting" around the correct position. Possible solutions include carrying your GPS in a position where it has a better view of the sky (e.g. I put mine in the lid of my pack). I don't know the Fenix 5, but lots of GPS devices and apps have a setting to disregard all GPS points where you haven't moved a certain amount since the last one, and to wait until a GPS point arrives which has a definite and substantial movement (e.g. 20 metres) since the last one. If your recorded track shows lots of zig-zags around your rest stops, that's where the extra distance is coming from, and you definitely need a setting like the one I just described - or you can pause your track recording when taking rests.
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Re: GPS / posted distance descrepancy

Postby Dexter » Tue 19 Jan, 2021 10:04 pm

crollsurf wrote:Never seen that kind of a discrepancy before. I would normally think give or take 10%. What was the walk if you don't mind?


It was the Beeripmo Walk. Trailhiking lists it as 20.6 but their route includes side trips.
https://www.trailhiking.com.au/beeripmo-walk/

ribuck wrote:Take a look at your recorded track. Does it zig-zag all over the place? Then, the extra distance is due to the GPS "hunting" around the correct position. Possible solutions include carrying your GPS in a position where it has a better view of the sky (e.g. I put mine in the lid of my pack). I don't know the Fenix 5, but lots of GPS devices and apps have a setting to disregard all GPS points where you haven't moved a certain amount since the last one, and to wait until a GPS point arrives which has a definite and substantial movement (e.g. 20 metres) since the last one. If your recorded track shows lots of zig-zags around your rest stops, that's where the extra distance is coming from, and you definitely need a setting like the one I just described - or you can pause your track recording when taking rests.


There's a few zig zaggy spots but they are where we stopped for lunch or camp before I paused the walk, and moved around a bit in the one spot.
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Re: GPS / posted distance descrepancy

Postby ribuck » Wed 20 Jan, 2021 2:01 am

Your GPS track looks pretty normal to me, so I'm surprised there's such a large discrepancy. The cause is definitely not the 2D/3D issue which you alluded to in your original post. GPS distances are measured in their projection onto a 2D surface, and in any case the difference would be less than 1%.

This is a long shot, since I think you would have noticed, but I don't suppose you accidentally recorded this as an extra segment on top of a previous 6km walk, in which case the total distance would show 27km but the distance for the second segment should show 21km.
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Re: GPS / posted distance descrepancy

Postby wayno » Wed 20 Jan, 2021 5:08 am

you can get aberations under trees, it does minute zig zags increasing the distance, update your firmware as well, can fix bugs....
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Re: GPS / posted distance descrepancy

Postby Dexter » Wed 20 Jan, 2021 7:10 am

ribuck wrote:Your GPS track looks pretty normal to me, so I'm surprised there's such a large discrepancy. The cause is definitely not the 2D/3D issue which you alluded to in your original post. GPS distances are measured in their projection onto a 2D surface, and in any case the difference would be less than 1%.

This is a long shot, since I think you would have noticed, but I don't suppose you accidentally recorded this as an extra segment on top of a previous 6km walk, in which case the total distance would show 27km but the distance for the second segment should show 21km.


No it was a new activity / track that was started as we walked in. Stopped it right when we finished. There's a bit of walking around camp (before I paused it) and at lunch, but I can't imagine we would have covered a lot of ground. We mostly looked for our camp spot, and at lunch sat and made some tucker, and I walked to the loo haha.

wayno wrote:you can get aberations under trees, it does minute zig zags increasing the distance, update your firmware as well, can fix bugs....


Yeah I'm sure it's not perfect with some errors from terrain, I just would expect to see some major glitches on the resulting map if it were significant enough to cause that much difference. It's not the first time I've noticed a fair difference between posted distances and what I record on it. I'll check the firmware, but I regularly keep it up to date by syncing the device. Perhaps firmware doesn't get updated during that process. I'll check it out.

It might be a question for Garmin, but who knows if they will respond. I would have thought unlikely, but any chance the posted distances on the parks signs are just not right?
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Re: GPS / posted distance descrepancy

Postby Dexter » Wed 20 Jan, 2021 7:45 am

Noticed it was set to this...

Smart Recording saves GPS points when changes in direction, speed, heart rate or elevation are detected. Smart Recording is the default setting of Garmin's watches for the balance it achieves in detail and battery consumption.


And have moved it over to this...

Every Second recording will record a GPS point once per a second, regardless of direction, speed, heart rate or elevation. Every Second recording will produce highly detailed tracks of your activities. A more detailed track will lead to more accurate speed and distance data.

If you are looking for the most accurate speed and distance, we recommend using Every Second recording.


I'll be interested to see if there's a more accurate distance recorded. I guess the downside is more battery drain.
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Re: GPS / posted distance descrepancy

Postby wayno » Wed 20 Jan, 2021 7:51 am

an app like garmin express can specifially pull your latest firmware down connected to a computer via the cable. otherwise using garmin connect can have big delays before it gets the latest firmware
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Re: GPS / posted distance descrepancy

Postby Dexter » Wed 20 Jan, 2021 8:26 am

wayno wrote:an app like garmin express can specifially pull your latest firmware down connected to a computer via the cable. otherwise using garmin connect can have big delays before it gets the latest firmware


Cheers, yeah I've noticed the same. I checked it out and all up to date.
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Re: GPS / posted distance descrepancy

Postby keithj » Wed 20 Jan, 2021 3:40 pm

ribuck wrote:I don't know the Fenix 5, but lots of GPS devices and apps have a setting to disregard all GPS points

Most Garmins have an Auto-Pause which stops recording when you stay in one spot for a while, and restarts when you move enough.

Dexter wrote:There's a few zig zaggy spots but they are where we stopped for lunch or camp before I paused the walk, and moved around a bit in the one spot.

The image you posted may have been processed so your original GPX track is smoothed out.

Consider uploading your original .GPX file to https://www.plotaroute.com/uploadroute and see how zig-zaggy it looks ?
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Re: GPS / posted distance descrepancy

Postby Dexter » Wed 20 Jan, 2021 3:47 pm

keithj wrote:Consider uploading your original .GPX file to https://www.plotaroute.com/uploadroute and see how zig-zaggy it looks ?


Excellent. I'll give it a look! Would you suggest turning on auto pause? I have always been a little hesitant to trust it.
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Re: GPS / posted distance descrepancy

Postby wayno » Wed 20 Jan, 2021 3:51 pm

Dexter wrote:
keithj wrote:Consider uploading your original .GPX file to https://www.plotaroute.com/uploadroute and see how zig-zaggy it looks ?


Excellent. I'll give it a look! Would you suggest turning on auto pause? I have always been a little hesitant to trust it.


depends how much you stop , if you're stopping a lot to admire the view or take photos, theres a bit of a delay before it starts , if you dont stop that often, then no problems. garmin works out how long you were stationary in the resultant trip
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Re: GPS / posted distance descrepancy

Postby Dexter » Wed 20 Jan, 2021 4:04 pm

wayno wrote:
Dexter wrote:
keithj wrote:Consider uploading your original .GPX file to https://www.plotaroute.com/uploadroute and see how zig-zaggy it looks ?


Excellent. I'll give it a look! Would you suggest turning on auto pause? I have always been a little hesitant to trust it.


depends how much you stop , if you're stopping a lot to admire the view or take photos, theres a bit of a delay before it starts , if you dont stop that often, then no problems. garmin works out how long you were stationary in the resultant trip


We usually do stop and enjoy views / photos and all that jazz. I think you're onto it though. I need the auto-pausing on, or at least stop the tracking whenever we take a break. From zoomed out the walk looks pretty much fine. But have a look what it did during my lunch break!
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Re: GPS / posted distance descrepancy

Postby peregrinator » Wed 20 Jan, 2021 4:48 pm

That's not a lunch break. It's a ten-course banquet.
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Re: GPS / posted distance descrepancy

Postby Dexter » Wed 20 Jan, 2021 8:01 pm

peregrinator wrote:That's not a lunch break. It's a ten-course banquet.
:lol: :lol: :lol: :lol: Right? I think we've found the route of the problem here.

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Re: GPS / posted distance descrepancy

Postby ribuck » Wed 20 Jan, 2021 10:17 pm

Dexter wrote:I think we've found the route of the problem here.

For sure! Lunch breaks are for sitting down and eating, not for frantically pacing the ground. The auto-pause setting should solve this completely.
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Re: GPS / posted distance descrepancy

Postby iGBH » Wed 27 Jan, 2021 11:47 am

I'd be really surprised it your lunch break explains the extra 6 kms. How long does it take you to normally walk 6kms? Lets assume at best an hour.
So on that basis, your lunch break would of needed to be an hour long walking pretty much flat out without stopping still. Doesn't sound like a lunch break to me.....
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Re: GPS / posted distance descrepancy

Postby Dexter » Wed 27 Jan, 2021 11:54 am

iGBH wrote:I'd be really surprised it your lunch break explains the extra 6 kms. How long does it take you to normally walk 6kms? Lets assume at best an hour.
So on that basis, your lunch break would of needed to be an hour long walking pretty much flat out without stopping still. Doesn't sound like a lunch break to me.....


I think it's any time we stopped, which we do reasonably often (we take our time and enjoy the surroundings, take some photos etc). It likely would have reacted like this each time - so not just for the lunch break. It doesn't seem to always move around at walking pace either, and I have noticed some big speed spikes which I can assume is during these stopped periods. I have increased the GPS ping to 1s, and turned on auto pause. Hopefully that improves it a bit.
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Re: GPS / posted distance descrepancy

Postby keithj » Wed 27 Jan, 2021 12:42 pm

Dexter wrote:I think it's any time we stopped, which we do reasonably often (we take our time and enjoy the surroundings, take some photos etc). It likely would have reacted like this each time - so not just for the lunch break. It doesn't seem to always move around at walking pace either, and I have noticed some big speed spikes which I can assume is during these stopped periods. I have increased the GPS ping to 1s, and turned on auto pause. Hopefully that improves it a bit.

big speed spikes happen when GPS gets a ping that takes longer than it should with a direct line of sight (e.g. under trees, close to cliffs/buildings, overhangs) and it triangulates that you've moved 100's meters between pings. This also adds that extra distance to your track... twice, because when the next correct signal happens, you'll skip back to where you really are. If you had lunch under a tree that may explain it.

You could attach your original GPX file for a definitive answer to the source of excess distance.

IMO increasing the ping to 1s isn't likely to make much difference - or may make it worse!!. Some GPS's or mapping web sites can automatically ignore these points that are 'unreasonable' such as moving at relatively high velocity & subsequently returning to a previous location.

Solutions include -
- avoid walking under the canopy or close to any hard surface a signal can bounce off
- manually pause when you stop (& remember to restart!)
- upload to a gps plotting website & manually delete the 'obviously' dud points
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Re: GPS / posted distance descrepancy

Postby Dexter » Wed 27 Jan, 2021 1:05 pm

keithj wrote:
Dexter wrote:I think it's any time we stopped, which we do reasonably often (we take our time and enjoy the surroundings, take some photos etc). It likely would have reacted like this each time - so not just for the lunch break. It doesn't seem to always move around at walking pace either, and I have noticed some big speed spikes which I can assume is during these stopped periods. I have increased the GPS ping to 1s, and turned on auto pause. Hopefully that improves it a bit.

big speed spikes happen when GPS gets a ping that takes longer than it should with a direct line of sight (e.g. under trees, close to cliffs/buildings, overhangs) and it triangulates that you've moved 100's meters between pings. This also adds that extra distance to your track... twice, because when the next correct signal happens, you'll skip back to where you really are. If you had lunch under a tree that may explain it.

You could attach your original GPX file for a definitive answer to the source of excess distance.

IMO increasing the ping to 1s isn't likely to make much difference - or may make it worse!!. Some GPS's or mapping web sites can automatically ignore these points that are 'unreasonable' such as moving at relatively high velocity & subsequently returning to a previous location.

Solutions include -
- avoid walking under the canopy or close to any hard surface a signal can bounce off
- manually pause when you stop (& remember to restart!)
- upload to a gps plotting website & manually delete the 'obviously' dud points



Thanks! That's really helpful. I don't have the device with me right now as I'm at work, but I can upload the GPX if you think it might be of benefit.
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Re: GPS / posted distance descrepancy

Postby Mark F » Thu 28 Jan, 2021 3:01 pm

This topic has come up several times and it really serves little purpose to worry about the discrepancies (unless you are ocd :roll: ). The same levels of variance can be seen in car speedos which not only are expected to be accurate to within 5% below the actual speed but also change with tyre brand, model and level of wear. The big question is "does a 5-10% change in the length of your walk create problems for your completion or enjoyment of the walk?"

Far more problematic are signs and route descriptions that give very inaccurate distances.
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Re: GPS / posted distance descrepancy

Postby Dexter » Thu 28 Jan, 2021 3:06 pm

Mark F wrote:The big question is "does a 5-10% change in the length of your walk create problems for your completion or enjoyment of the walk?"

Far more problematic are signs and route descriptions that give very inaccurate distances.


Absolutely. I'm not too concerned if the GPS is a little off to be honest. More just whether it was user error as well exacerbating the difference... and to your second point, I was curious as to how accurate signed distances usually are.
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