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GPS Distance errors

Old 06-07-21, 09:59 AM
  #76  
njkayaker
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Originally Posted by wkc View Post
NJK, you could be right with your two comments. But thinking about it, at least my eventual outcome is some sort of consistency between my GPS using MMR and my BC, no???
You want to be consistent and accurate (there's little point in being consistently inaccurate).

Originally Posted by wkc View Post
When you say “curvy path”, yes, I agree if there were lots of relatively “small” curves. But as I was measuring rides between 2km and 30+km with straights and curves, I would think that the impact of “curves” over those distances would be somewhat negligible when compared to the circumference of my tyre.

I think you are absolutely correct if I was riding “short” and “curvy” distances every time. But that is not the case.

I think I will start my third list next week and see what the outcome is. Maybe I could compare my third list measurements/distances with measurements on Google Maps or some other map to check if gentle* curves make any difference over “larger” distances.
* gentle road curves vs concentric or tight circles and curves.
Your bicycle is moving back-and-forth in places the GPS is (and you are) seeing as perfectly straight lines.

Last edited by njkayaker; 06-07-21 at 10:02 AM.
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Old 06-07-21, 10:10 AM
  #77  
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Originally Posted by wkc View Post
Spelger, you correctly say “if the bike computer has the correct information”. IF!!!
I have always been a little suspicious about using “standard” conversion tables for translating wheel “size” to wheel circumference. (eg 700 x 23C = 25-622 = 2096)

My general knowledge tells me that standard wheel size will only equal distance travelled for any given tyre pressure or even tyre wear. A flatter, less inflated 700x23C will have a smaller circumference compared to a well/highly inflated 700x23C. Tyre wear and even manufacturing tolerances (exact amount of rubber all the way around a tyre) would affect circumference in a similar, albeit to a smaller extent.

Therefore, using 2096 in a BC assumes that your actual wheel on your bike rolls 2096mm for every revolution. Thus, using standard conversion table circumferences is assuming your BC is using the “correct” information. To me, that is not necessarily a good assumption and necessitated some sort of recording and comparison.
the kayaker from new jersey beat me to it, it makes no sense to be consistently wrong. selecting a wheel size on the BC is quick and dirty and dirty is what you found. you essentially reduced the size of your tape measure. anyway, it is a learning experience.
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Old 06-07-21, 10:59 AM
  #78  
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Well, let's chime in with this angle...
The units being measured (miles, km, feet, whatever) are actually pretty arbitrary. Cubits? why not. The higher order question is "what are you doing with it and what precision is required?"
So this thread gets into precision, which I do find interesting. But IRL, I find that most situations rarely benefit from more than 4 significant digits, usually 3 will do and 2 is often good enough for decision making.
If I had ridden 36 miles, does 36.4 matter?
If I did a season of 3000 miles, would 30 matter? would 3?
I can find you both cases in finance, where the way we need to work sometimes says "need it to the penny," other times we round to the nearest million.
So, what I'm asking this thread is : "what level of precision is _useful_?" and when does a lack of precision become troublesome?
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Old 06-07-21, 12:31 PM
  #79  
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Originally Posted by blacknbluebikes View Post
Well, let's chime in with this angle...
The units being measured (miles, km, feet, whatever) are actually pretty arbitrary. Cubits? why not. The higher order question is "what are you doing with it and what precision is required?"
So this thread gets into precision, which I do find interesting. But IRL, I find that most situations rarely benefit from more than 4 significant digits, usually 3 will do and 2 is often good enough for decision making.
If I had ridden 36 miles, does 36.4 matter?
If I did a season of 3000 miles, would 30 matter? would 3?
I can find you both cases in finance, where the way we need to work sometimes says "need it to the penny," other times we round to the nearest million.
So, what I'm asking this thread is : "what level of precision is _useful_?" and when does a lack of precision become troublesome?
You’re absolutely right on, although another high-order question which could be asked is which info is most important: distance traveled or distance and location?

Only the GPS can record where one has traveled, and as such is a much richer data source than a $20 cyclometer. It’s hard for me to imagine the point of this “accuracy exercise” in 2021; GPS data can be analyzed more easily and in a more consequential manner than the simple data from the cyclometer.
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Old 06-07-21, 01:25 PM
  #80  
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Originally Posted by wkc View Post
...large discrepancies...circumference in my bike computer...tyre pressure...calibration....amended...distances recorded...my calculation...spreadsheet...absolutely negligible.
wkc,

I admire your rigorous approach to tracking down the errors. But I am lazier (by a lot!) than you are.

I have a Cateye Astrale wired bike computer that is C&V-worthy, and I track my rides on a phone using Ride/w/GPS. When they did not agree, I lowered/raised the circumference number in the Cateye until they did agree within a tenth! Took a few rides, but not many, to get them synced.

Which one was correct to begin with? I don't know, but I think the GPS. (There are no cul-de-sacs on my routes.) Why must they agree? Because I like them to agree!

So now, they're either both right, or both wrong, but they agree.

All best to you in Oz.

Last edited by BCDrums; 06-07-21 at 03:14 PM.
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Old 06-07-21, 04:40 PM
  #81  
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Originally Posted by BCDrums View Post
wkc,

I admire your rigorous approach to tracking down the errors. But I am lazier (by a lot!) than you are.

I have a Cateye Astrale wired bike computer that is C&V-worthy, and I track my rides on a phone using Ride/w/GPS. When they did not agree, I lowered/raised the circumference number in the Cateye until they did agree within a tenth! Took a few rides, but not many, to get them synced.

Which one was correct to begin with? I don't know, but I think the GPS. (There are no cul-de-sacs on my routes.) Why must they agree? Because I like them to agree!

So now, they're either both right, or both wrong, but they agree.

All best to you in Oz.
Love it! That’s kinda where I’m coming from. It was the significant inconsistency between the two that was causing me angst. With less than 1% difference, I can sleep better (and ride happier). LOL

And other posters are spot on too re: degree of accuracy. That doesn’t matter so much to me. Consistency more so.

Thank you to all for your input. I’m happy to have found this forum. I love my rides, even if just short and sharp, not the massive ones I see on MMR and Strava.
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Old 06-07-21, 04:45 PM
  #82  
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Originally Posted by chaadster View Post
You’re absolutely right on, although another high-order question which could be asked is which info is most important: distance traveled or distance and location?

Only the GPS can record where one has traveled, and as such is a much richer data source than a $20 cyclometer. It’s hard for me to imagine the point of this “accuracy exercise” in 2021; GPS data can be analyzed more easily and in a more consequential manner than the simple data from the cyclometer.
Sure, the phone can do and record more but I don’t like having it strapped to my cockpit. So, now my inconspicuous BC on my stem at least shows similar speed and distance data to that on my phone in my back pocket.
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Old 06-07-21, 05:24 PM
  #83  
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Just did a ride which was

Phone apps using gps:
GAIA: 15.25
Mission Control: 15.44

Specialized Display Unit: 15.4

I guess I could join the "lose sleep over it" group or just learn to live with the differences. Elevation tracks differently, too, but gps elevation stats are always a bit hit/miss. Mission Control is always higher.
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Old 06-07-21, 09:05 PM
  #84  
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Originally Posted by kahn View Post
Just did a ride which was

Phone apps using gps:
GAIA: 15.25
Mission Control: 15.44

Specialized Display Unit: 15.4

I guess I could join the "lose sleep over it" group or just learn to live with the differences. Elevation tracks differently, too, but gps elevation stats are always a bit hit/miss. Mission Control is always higher.
mine have been very, very consistent lately.

RwGPS: 25.5 miles, mission control, 25.65.
RwGPS: 74.0 miles, mission control 74.02.

a few PSI less air in the first, most recent one explains the difference. inconsequential for sure.
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Old 06-07-21, 09:11 PM
  #85  
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My GPS on a downhill today gave me a top speed of 58.2 MPH where it was more likely around 45. ‘Technology is a wonderous thing.... when it works.
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Old 06-07-21, 10:39 PM
  #86  
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Let’s talk about phone GPS precision for a minute

Originally Posted by blacknbluebikes View Post

So, what I'm asking this thread is : "what level of precision is _useful_?" and when does a lack of precision become troublesome?
If BF allows me to post this MMR screenshot, I suggest that you can clearly see that phone GPS and/or phone apps are subject to various degrees and magnitudes of error.

I promise you that I did not edit this pic. That was my max speed at the very start of this ride.


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Old 06-08-21, 04:25 AM
  #87  
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Originally Posted by wkc View Post
I promise you that I did not edit this pic. That was my max speed at the very start of this ride.
It takes awhile for GPS to get a precise location.

The odd speed might be due to a difference between an initial inaccurate location and a later accurate location.

Last edited by njkayaker; 06-09-21 at 08:08 AM.
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Old 06-08-21, 04:27 AM
  #88  
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Originally Posted by rsbob View Post
My GPS on a downhill today gave me a top speed of 58.2 MPH where it was more likely around 45. ‘Technology is a wonderous thing.... when it works.
That's one reason people use speed sensors.
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Old 06-08-21, 06:15 AM
  #89  
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Originally Posted by wkc View Post
Sure, the phone can do and record more but I don’t like having it strapped to my cockpit. So, now my inconspicuous BC on my stem at least shows similar speed and distance data to that on my phone in my back pocket.
Well, a GPS cycling computer would solve that problem, of course, some of which, like the $79 Lezyne Macro Easy, are quite inexpensive.

https://ride.lezyne.com/collections/...macro-easy-gps

It’s fully capable of running Bluetooth sensors, too, so you can hook up a speed sensor for enhanced accuracy if you want it.
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Old 06-08-21, 06:53 AM
  #90  
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Originally Posted by kahn View Post
Just did a ride which was

Phone apps using gps:
GAIA: 15.25
Mission Control: 15.44

Specialized Display Unit: 15.4

I guess I could join the "lose sleep over it" group or just learn to live with the differences. Elevation tracks differently, too, but gps elevation stats are always a bit hit/miss. Mission Control is always higher.
Elevation is never precisely the same across insteuments, not when using a barometer (as in iPhone) and particularly not when using GPS calculations. Top quality GPS cycling computers primarily use barometeic pressure sensors for elevation measurements.
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Old 06-08-21, 07:15 AM
  #91  
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My god...I disappear for a week and you guys are still babbling about GPS accuracy.

https://www.singletracks.com/mtb-gea...cyclocomputer/
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Old 06-08-21, 10:07 PM
  #92  
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Originally Posted by njkayaker View Post
That's one reason people use speed sensors.
Ayup. My other bikes have them. Strava consistently under reports my average speed by 0.5 MPH with no way to correct which is a bit frustrating after a good ride.
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Old 06-09-21, 04:00 AM
  #93  
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Originally Posted by rsbob View Post
Ayup. My other bikes have them. Strava consistently under reports my average speed by 0.5 MPH with no way to correct which is a bit frustrating after a good ride.
What speed is correct?

The Garmins have an option to exclude speed below some limit from the average. This limit can be changed.

Strava is probably doing the same thing.

Given that a common use of Strava is comparing speed, it makes sense to use the same limit for everybody.

While it makes sense to exclude actual zero speed, any nonzero number is kind of arbitrary.
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Old 06-09-21, 05:34 AM
  #94  
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Most GPS (phone and watch), update (talk to satellite), once every 5-6 seconds.
More accuracy would require a dedicated GPS device.
Another note on longer runs, you are handed off to different satellites every 20 minutes.
I work with surveying grade GPS and we have to set up and wait the 20 minutes to reconfirm our accuracy, +- 0.00'
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Old 06-09-21, 07:15 AM
  #95  
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Originally Posted by 1979schwinn View Post
Most GPS (phone and watch), update (talk to satellite), once every 5-6 seconds.
+- 0.00'
Well, no, GPS devices as we are discussing here do not talk to the satellites. There's no 2 way communication, its all listening on the part of the device.
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Old 06-09-21, 08:10 AM
  #96  
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Originally Posted by prj71 View Post
My god...I disappear for a week and you guys are still babbling about GPS accuracy.

https://www.singletracks.com/mtb-gea...cyclocomputer/
And a week later, you are still whining about it.
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Old 06-09-21, 08:40 AM
  #97  
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Originally Posted by 1979schwinn View Post
Most GPS (phone and watch), update (talk to satellite), once every 5-6 seconds.
It appears the GPS receivers used in phones and most devices are 1 Hz.

Originally Posted by 1979schwinn View Post
More accuracy would require a dedicated GPS device.
The accuracy of reasonably-priced handheld units (dedicated or not) doesn't seem to be much different. The chips might not be that different but the antennas might, in a few cases, be better for a dedicated device.

Originally Posted by 1979schwinn View Post
Another note on longer runs, you are handed off to different satellites every 20 minutes.
This is the case for every GPS unit. This has no bearing on accuracy.

Originally Posted by 1979schwinn View Post
I work with surveying grade GPS and we have to set up and wait the 20 minutes to reconfirm our accuracy, +- 0.00'
As far as I understand, the surveying grade GPSs don't really work while moving. It takes a while for any GPS device to "warm up".

Last edited by njkayaker; 06-09-21 at 08:48 AM.
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Old 06-09-21, 10:31 AM
  #98  
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Originally Posted by njkayaker View Post
And a week later, you are still whining about it.
I haven't whined once in this thread. However 90% of the posters in this thread have whined about the accuracy of GPS for biking. Who gives a flying F*** if it's off by .5 mph for speed or off a couple miles. My god.
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Old 06-09-21, 10:38 AM
  #99  
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Originally Posted by prj71 View Post
I haven't whined once in this thread.
You have whined about it in a couple of threads.
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Old 06-09-21, 10:42 AM
  #100  
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Show your work.
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