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GPS Distance errors

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GPS Distance errors

Old 05-25-21, 08:04 AM
  #26  
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Originally Posted by Pop N Wood
Unfortunately not true.

It is update rate. the GPS only records a position every so many seconds (or fractions of one). It computes distance traveled by taking the distance between the two waypoints and dividing by time to get speed. If the GPS records one waypoint at the entrance to the circle and one at the end, well it doesn't take a math genius to figure out it won't compute an accurate distance.
I suppose people are talking about the turning circle at the end of the cul-de-sac.

It would have to be a quite small turning circle or you'd have to be going very fast.

At 3 mph, one moves 4.4 ft per second. A 100 ft cul-de-sac would produce about 50 measurements (1/sec).
At 10 mph, one moves 14.7 ft per second. A 100 ft cul-de-sac would produce about 20 measurements (1/sec).
At 15 mph, one moves 22 ft per second. A 100 ft cul-de-sac would produce about 9 measurements (1/sec).

The average car length is 15 feet, A really-small (not realistic) turning circle would be 3 car lengths or 45 ft. They are more likely 2-3 (at least) times as long.

There should be more than enough points to get a fairly accurate measurement of distance.

The distance might be off but not by that much.

Anyway, we have no idea at all how far off it supposedly is!

Originally Posted by Pop N Wood
The Garmin I have varies how often it records the waypoints saving space on the memory card.
The device measures at the same rate but, with the "smart recording", it doesn't save the new point if it's not significantly different from the prior point. That is, if the location or time isn't much different.

Last edited by njkayaker; 05-25-21 at 08:26 AM.
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Old 05-25-21, 08:21 AM
  #27  
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Originally Posted by Iride01
If the site or application you use to determine your distance only uses the plots in the log file, then remember that the log files may not have that coordinate that takes you to the furthest distance into the cul-de-sac. As well around sharp corners, it might miss actual corner where you turned.

If you have 1 second recording intervals for your log, then that can still be 20 or 30 feet missed during that one second. Many devices by default are doing more than 1 second interval logging. Some Garmin's by default do what they call "Smart Recording". I've seen enough differences in my logged track with that to know there is nothing smart about it.
How fast do you think the OP is taking these turning circles?

And not all of that distance would be "missed".

https://support.garmin.com/en-US/?fa...mK0P6l20SgpW28
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Old 05-25-21, 08:35 AM
  #28  
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Originally Posted by Badger6
if you aren't guiding bombs and artillery, don't sweat the loss of a few meters here and there.
As someone who was in a field artillery unit, believe me when I say we didn't sweat it there, either

I'm kidding mostly. Also your point is valid, cycling-based GPS options are generally in the realm of "good enough."
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Old 05-25-21, 08:56 AM
  #29  
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Originally Posted by njkayaker
How fast do you think the OP is taking these turning circles?

And not all of that distance would be "missed".

https://support.garmin.com/en-US/?fa...mK0P6l20SgpW28
Didn't say specifically that it would. My comment was just an add to one of the many reasons why GPS mileage might be not what we expect.

I didn't know I was supposed to give the reason for the most error.
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Old 05-25-21, 09:11 AM
  #30  
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Originally Posted by Iride01
Didn't say specifically that it would. My comment was just an add to one of the many reasons why GPS mileage might be not what we expect.
You are exaggerating the amount of distance lost. There shouldn't be much difference (so, the number should be fairly close to what is expected).

Originally Posted by Iride01
I didn't know I was supposed to give the reason for the most error.
If there's a large difference, the thing you talked about isn't likely to be the cause. If so, talking about it wouldn't be relevant to the OP. The "reason for the most error" would be (which is why people need to "give it").

(We still have no idea what the "real" distance is.)
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Old 05-25-21, 09:23 AM
  #31  
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Originally Posted by njkayaker
You are exaggerating the amount of distance lost. There shouldn't be much difference (so, the number should be fairly close to what is expected).


If there's a large difference, the thing you talked about isn't likely to be the cause. If so, talking about it wouldn't be relevant to the OP. The "reason for the most error" would be (which is why people need to "give it").

(We still have no idea what the "real" distance is.)
The cause? You think there is only one thing that is THE cause?
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Old 05-25-21, 09:36 AM
  #32  
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GPS makes small curves into straight lines. Thus the mileage calculated by a GPS will always be less than actual mileage.

If you need to satisfy your mileage OCD then you need a wheel sensor that syncs to your GPS.

Weird that people get so engrossed on how accurate it is.
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Old 05-25-21, 09:50 AM
  #33  
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Originally Posted by Iride01
The cause? You think there is only one thing that is THE cause?
No, I'm saying the ONE cause you mentioned isn't a big deal.
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Old 05-25-21, 09:52 AM
  #34  
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Don't have that problem. I have a Garmin 'Edge 25 and an Apple watch. Over a 20-30 mile ride, the two units are often within .2-.5 mile for distance. The spread seems wider if I stop multiple times. Sometimes lay out a route on Google Earth Pro. Garmin and Apple are usually fairly close to the mileage reported by Google Earth. I'm not concerned over relatively small discrepancies.
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Old 05-25-21, 09:59 AM
  #35  
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Originally Posted by Inusuit
Don't have that problem. I have a Garmin 'Edge 25 and an Apple watch. Over a 20-30 mile ride, the two units are often within .2-.5 mile for distance. The spread seems wider if I stop multiple times. Sometimes lay out a route on Google Earth Pro. Garmin and Apple are usually fairly close to the mileage reported by Google Earth. I'm not concerned over relatively small discrepancies.
The difference in difference over longer distances tends to be small. Most people here should realize that.

The OP was talking about over one mile. We have no idea what the difference the OP saw.

(Cul-de-sacs and turning-circles aren't really a particular problem,)
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Old 05-25-21, 10:01 AM
  #36  
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Originally Posted by njkayaker
No, I'm saying the ONE cause you mentioned isn't a big deal.
Wouldn't take but a 100 or so not big deals to add up 1000 feet of error. And that's using only 10 feet of track missed on turns or doubling back on a track. And not all of those not big deals have to be the one that I mentioned.
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Old 05-25-21, 10:22 AM
  #37  
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Originally Posted by Iride01
Wouldn't take but a 100 or so not big deals to add up 1000 feet of error. And that's using only 10 feet of track missed on turns or doubling back on a track. And not all of those not big deals have to be the one that I mentioned.
???

Again, cul-de-sacs don't have any particular problem being measured reasonably accurately with GPS.

Over long distances, it could add up to a "1000 feet of error" but that error would be very small over the total distance (you might not even notice it).

Outside of reception issues** and a really twisty path, the differences between GPS and wheel counter rotation are close.

** This appears to be a real issue for mountain bikers since they are often under heavy tree cover and are doing more "twisty" turns. But you aren't talking about either of these issues.

Last edited by njkayaker; 05-25-21 at 10:31 AM.
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Old 05-25-21, 10:49 AM
  #38  
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Well instead of me and you filling up the thread with our own infatuations of ourselves. Why don't we agree to hush and let the OP fill us in on some of the other stuff. <grin>

Maybe you can go help that guy in the bicycle mechanics forum find all the un-solved threads.

Read this with a grin. I know you give good advice. You just like to argue unimportant things on technicalities to a greater extent.
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Old 05-25-21, 11:15 AM
  #39  
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Sounds like the sampling rate is almost synchronized with the amount of time it takes you to ride around the circle, so there's two readings that are almost in the same place, making it appear you are going very slow.
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Old 05-25-21, 11:57 AM
  #40  
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Originally Posted by njkayaker
cul-de-sacs don't have any particular problem being measured reasonably accurately with GPS.
Of course, GPS position accuracy doesn't change in a cul-de-sac.

But if the sampling rate is too low, the accuracy of the "distance traveled" and "speed" calculations will be affected by this "high frequency" deviation into and out of the cul-de-sac.

Let's say you want an accurate measure of "distance traveled" and "speed", based solely on GPS measurements.

If, say, a route deviation into a cul-de-sac lasts 10 seconds, the GPS sampling rate must be 5 seconds or shorter. This 2x sampling rate requirement is based on digital signal processing theory called the Nyquist frequency.

Reduce the GPS sampling rate below the Nyquist frequency, and the distance measurement of routes with or without the cul-de-sac deviation will be the same (they will be aliases of each other).
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Old 05-25-21, 12:08 PM
  #41  
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Originally Posted by rsiesta
I can see with your reply and that you are a senior member that this forum is not a serious forum for people looking help. I will try to find another forum that doesn't have smart asses as senior members. Enjoy your trolling.
Just an FYI. I'm not sure what the BF uses to be a "Senior Member" but if you look at some of the join dates it ain't much. So take "Senior member" with a huge grain of salt.
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Old 05-25-21, 12:22 PM
  #42  
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Originally Posted by terrymorse
But if the sampling rate is too low, the accuracy of the "distance traveled" and "speed" calculations will be affected by this "high frequency" deviation into and out of the cul-de-sac.
Of course.

But is the sampling rate too low?

And, if the sampling rate is too low, would it result in a large difference in distance?

For anything I've used, an iPhone or a Garmin, the sampling rate doesn't appear to be too low.

It looks like the sampling rate for an iPhone XS Max is every second. The track walking a 25 ft rectangle didn't look very good.

Last edited by njkayaker; 05-25-21 at 12:54 PM.
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Old 05-25-21, 12:29 PM
  #43  
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Originally Posted by Iride01
You just like to argue unimportant things on technicalities to a greater extent.
This is what you are doing. It doesn't make any sense to criticize people for doing what you do.

====================

Here's a better description of the "smart recording". It shows that some detail is lost (but it still isn't clear it would matter in this case).

https://support.garmin.com/en-US/?fa...T27Zr4hxZDvjv5

It looks like the sampling rate for an iPhone XS Max is every second. The track walking a 25 ft rectangle didn't look very good.

Walking a 157 ft rectangle in a parking lot at a slow speed. the iPhone gave 109 ft and 1030+ gave 102 ft.

The inaccuracy is due to location errors. Not sampling rate.

The rectangle might be a worse case than a turning circle. If the 3 cul-de-sacs were measured low by 50ft each, the total would be 150ft over 5280 feet (or 0.3%).

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Old 05-25-21, 12:38 PM
  #44  
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Originally Posted by njkayaker
...if the sampling rate is too low, would it result in a large difference in distance?
If the sampling rate is too low, the Nyquist frequency will tell you how large the distance error ought to be. Reduced sampling is like running a low pass filter on the data: it filters out the high frequency bits.

For anything I've used, an iPhone or a Garmin, the sampling rate doesn't appear to be too low.
So since your sampling rate is not too low, are you still seeing errors in distance calculations? That would suggest some other type of error.
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Old 05-25-21, 01:26 PM
  #45  
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Originally Posted by terrymorse
If the sampling rate is too low, the Nyquist frequency will tell you how large the distance error ought to be. Reduced sampling is like running a low pass filter on the data: it filters out the high frequency bits.
Yes.

Originally Posted by terrymorse
So since your sampling rate is not too low, are you still seeing errors in distance calculations? That would suggest some other type of error.
Yes.
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Old 05-25-21, 03:32 PM
  #46  
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Originally Posted by njkayaker
There should be more than enough points to get a fairly accurate measurement of distance.
We are both making a fair amount of assumptions about how these devices operate. Riding flat and level constant cadence there are times where I see all sorts of fluctuations in the GPS displayed speed. I also see fluctuations in how long it takes to update the speed, there are times it is noticeably longer than a second. the idea these units are updating everything at a fixed interval is a big one.

Reading through the OPs situation again where everything looks good up till the end of the circle, my guess is the unit is trying to use some sort of long term averaging to compute speed and doubling back on oneself is confusing it.

At any rate have to agree with the second post, add a wheel sensor.
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Old 05-25-21, 03:35 PM
  #47  
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By the way, Nyquist rate has nothing to do with this problem. These are not periodic signals that alias.
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Old 05-25-21, 04:34 PM
  #48  
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I have not had time to test this with road riding so take it with a grain of salt and I feel the difference there will be minimal, unless you live in Colorado. I use a Garmin Edge 500 & a Garmin Instinct watch. Adding the wheel sensor made a huge distance difference when riding mountain bikes with significant altitude gain & loss. On one ride w/o the sensor pared to my Edge it showed 8.4 miles with an average speed of 6.3 mph, the watch pared to the wheel sensor showed 9.99 miles with an average speed of 7.0 mph. This was with 1220 ft elevation gain and the same loss. A 17.3% difference is pretty big to me. The watch reads 2 GPS signals, the Edge 1, they never matched distances but were not real far apart. After that ride I bought 2 more sensors for my other bikes. Get one to match the unit you are using, set it up properly by actually measuring the circumference of your tires at the running pressure to enter them, you won't regret it.
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Old 05-25-21, 05:15 PM
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Originally Posted by Pop N Wood
By the way, Nyquist rate has nothing to do with this problem. These are not periodic signals that alias.
I’m far from an expert on discrete signal processing, but isn’t the Nyquist rate (for periodic signals) a lower bounds for the sampling rate of non-periodic signals?
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Old 05-25-21, 08:30 PM
  #50  
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Originally Posted by terrymorse
I’m far from an expert on discrete signal processing, but isn’t the Nyquist rate (for periodic signals) a lower bounds for the sampling rate of non-periodic signals?
Nyquist rate is determined by the frequency content of the signal being sampled. CD's band limit the audio content to ~22 kHz before sampling at twice that rate. In some fields it is common to sample at a rate well below Nyquist than use multiple sample rates to resolve any ambiguities.

None of this explains what short cuts Garmin and others take in their GPS units. They have little processors that need to juggle multiple tasks. Sometimes things get dropped.
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