GPS distance measured on a 26in vs 29in bike different?

[117]

Um, not quite Coppers. 

GPS's calculate their position based on its position relative to timing signal info given by satellites. 

Computers dont have barometers. Computers are fed info from a GPS or the likes and it then converts the info to something that we can all understand. 

I doubt Strava and the likes work off maps either. These software systems combobulate the info received from the GPS source - in this case a Garmin - which uses an internal barometric receiver. Strava and the likes then use a standard barometric pressure setting of 1013.25 hpa which will readjust the GPS's elevation to another figure. This is why you see a difference between your unit and Strava

At least we agree that bike wheels should be round - happy friyay riding

 

[Oodles]

10 minutes ago, 117 said:

Um, not quite Coppers. 

GPS's calculate their position based on its position relative to timing signal info given by satellites. 

Computers dont have barometers. Computers are fed info from a GPS or the likes and it then converts the info to something that we can all understand. 

I doubt Strava and the likes work off maps either. These software systems combobulate the info received from the GPS source - in this case a Garmin - which uses an internal barometric receiver. Strava and the likes then use a standard barometric pressure setting of 1013.25 hpa which will readjust the GPS's elevation to another figure. This is why you see a difference between your unit and Strava

At least we agree that bike wheels should be round - happy friyay riding

 

Strava does have the option to use maps

https://support.strava.com/hc/en-us/articles/115000024864-Strava-s-Elevation-Basemap

[TheoG]

To get better accuracy on distance & speed specifically, use a sensor on your front wheel, that is if your bike computer / watch cater for this.  I use this and GPS together, they are always slightly different.  If your rolling circumference setting is good, the speed sensor will for sure give more accurate and consistent results than GPS.

Edited December 10, 2021 by TheoG

[117]

So they're still using barometric pressure gleamed from previous recordings. And if no previous then they'll default to standard hpa settings. 

 

[capediver]

GPS satellites broadcast their signals in space with a certain accuracy, but what you receive depends on additional factors, including satellite geometry, signal blockage, atmospheric conditions, and receiver design features/quality. Commercial devices usually one use a single frequency for GPS, military and more expensive high end devices use more than one frequency. In the 90's, the GPS signal was slightly scrambled for military purposes but this system as been dropped and the accuracy for the daily global average user range error (URE) is reported to be about ≤2.0 m (6.6 ft.), with 95% probability...atmospheric conditions and other factors blunt the accuracy daily, you will only get better accuracy by the use of a fixed base station and triangulation.

[gerriemtb]

Have you been vaccinated recently? The chip that is now in your body might influence your bike's GPS #tinfoilfriday

[Shebeen]

2 hours ago, Battery_Chick said:

OK so I have a strange observation and I wonder if anyone else has experienced something similar. The group I ride with do weekday rides following the exact same route every time. I recently upgraded from a 26 inch bike to a 29 inch bike and my ride distance, measured by a Garmin Edge 820, changed from 15.13km to 14.94km. That's 190m less and is a 1.3% difference. 

Now I thought it was a new downhill section that may be shorter, but looking at the map, both rides I'm comparing used the same route! Could it be possible for a Garmin mounted on the handlebars to measure a difference in how I track through corners on these new bigger wheels and could it be that big a difference? It seems so crazy yet I can't (yet?) see another explanation?? 

So, am I crazy or what? 

you would need to get a 29er specific GPS unit to resolve this issue.

[TheoG]

You guys are a bunch of grap gatte today .... 😂

[dexterdent]

It's because you are faster on the 29er and therefore relativity comes into play.

[117]

 

4 minutes ago, copperhead said:

My phone does not have a barometer. I record strava with it. 

So as per attached does it not mean then that it takes from a map. Google maps? I thought it did. So my apologies if I am wrong. It has to get the elevation gain from somewhere. I know it can use the GPS to record route but I did not know that it could record elevation without a map. 

Without getting toooooo technical, elevation is based on barometric pressure. Strava's absolute elevation measurements are previous recordings of elevation averaged out 

If your phone has a GPS receiver, it should also have a barometic sensor too but here I'll go on what the manufacturer says. 

Edited December 10, 2021 by 117

[DieselnDust]

Do you have a speed sensor on the bike and is the gps bike computer set to use it as the main sensor for measuring distance?

[117]

4 minutes ago, copperhead said:

What if I am riding an area for the first time? What information does strava get and from who? Pretty sure some of rhe places I have been have been first time events 🤣🤣. My phone is so toilet. If it has a barometer I will feel a whole lot happier with its original cost. 🤙🏻

A GPS recording device can also record altitude based on the signal timings received from the birds in the sky. It's not accurate but for our intense and purposes its just fine. If the recording device and the software platform you upload it to have a different standard setting, your elevation will be adjusted. 

[TheoG]

1 hour ago, RobbyB said:

Question: is the circumference change from 26" to 29" wheel size a 1,3% difference??

No, difference is about 11%.

[Fisan]

 

15 minutes ago, 117 said:

 

Without getting toooooo technical, elevation is based on barometric pressure. Strava's absolute elevation measurements are previous recordings of elevation averaged out 

If your phone has a GPS receiver, it should also have a barometic sensor too but here I'll go on what the manufacturer says. 

 

Interesting, I always assumed they used digitial elevation models (e.g. SRTM) like Google Maps does. 

9 minutes ago, copperhead said:

What if I am riding an area for the first time? What information does strava get and from who? Pretty sure some of rhe places I have been have been first time events 🤣🤣. My phone is so toilet. If it has a barometer I will feel a whole lot happier with its original cost. 🤙🏻

117 got me reading, a quick read while you're sipping a glass of Steenberg: https://support.strava.com/hc/en-us/articles/216919447-Elevation-for-Your-Activity

[Pieter-za]

17 minutes ago, copperhead said:

Again I could be wrong here but as far as I know GPS to the public is a relatively new thing. The US of A has had satellites in the sky for a long time. They were not happy to let the information be public knowledge. They wanted to use it for military purposes. I am quite sure their GPS units are pretty accurate to a cup of t! So all the satellites we use for public consumption belong to the them. Again I am probably wrong but who knows. Google maps also does 3D variations which we then collect data from?? Who knows. Anyway it is wine o'clock so off to Steenberg I must be.

Public access to GPS tech was allowed in Sept 1983 already.

Selective Availability was switched off in 2000

Quote

Selective Availability (SA) was an intentional degradation of public GPS signals implemented for national security reasons.

In May 2000, at the direction of President Bill Clinton, the U.S government discontinued its use of Selective Availability in order to make GPS more responsive to civil and commercial users worldwide.

The United States has no intent to ever use Selective Availability again.

 

Edited December 10, 2021 by Pieter-za

[Frosty]

Do you, the OP, wait for your device to find satellites before you start every ride?