Distance Over-reading?

[Neville Bailey]

I have a Garmin Edge 520 paired with a speed sensor on my road bike, and I have set the speed sensor to be auto calibrated.

 

On longish group rides my Garmin tends to record slightly longer distances than other riders in the group (they are using GPS to record the distance).

 

Why would that be so?

 

Someone told me that the speed sensor records distance more accurately than GPS, especially if there are lots of hills on the route, because GPS does not account for the distance travelled on inclines and declines, as it sees the earth as “flat” and smooths out the distance as though it was along a flat surface.

 

What do others think?

[Grease_Monkey]

The edge 520 has a barometric altemeter and does account for hills and declines when working out distance with GPS. I'm not sure why your distance meter is out, but I'd be betting on the GPS to be accurate rather than the distance meter.

 

That distance meter seems kind of antiquated - don't really see the point of it if you are using a GPS device. Haven't even take mine out of the box to be honest. I my opinion all it's good for is draining your battery.

Edited July 23, 2018 by I_my_own_bike

[dave303e]

I have a Garmin Edge 520 paired with a speed sensor on my road bike, and I have set the speed sensor to be auto calibrated.

 

On longish group rides my Garmin tends to record slightly longer distances than other riders in the group (they are using GPS to record the distance).

 

Why would that be so?

 

Someone told me that the speed sensor records distance more accurately than GPS, especially if there are lots of hills on the route, because GPS does not account for the distance travelled on inclines and declines, as it sees the earth as “flat” and smooths out the distance as though it was along a flat surface.

 

What do others think?

 

tell someone to go sit on the naughty step and rethink their actions...

 

GPS reads an XY and (slightly less accurate) Z coordinate, hence fancy watches use barometers or do post processing to a digital elevation model to gain a more accurate Z coordinate for the route.

[Pieter-za]

I have a Garmin Edge 520 paired with a speed sensor on my road bike, and I have set the speed sensor to be auto calibrated.

 

On longish group rides my Garmin tends to record slightly longer distances than other riders in the group (they are using GPS to record the distance).

How far out?  (There is truth in that statement, depending.)

 

(BTW certified running courses are measured using a calibrated wheel, not GPS.  https://www.news24.com/Archives/Witness/Its-a-lengthy-matter-20150430 )

 

GPS accuracy (according to specs and research) is normally between 1-2% error margin.

Edited July 24, 2018 by Pieter-za

[thisismyotherbike]

I have a Garmin Edge 520 paired with a speed sensor on my road bike, and I have set the speed sensor to be auto calibrated.

 

On longish group rides my Garmin tends to record slightly longer distances than other riders in the group (they are using GPS to record the distance).

 

Why would that be so?

 

Someone told me that the speed sensor records distance more accurately than GPS, especially if there are lots of hills on the route, because GPS does not account for the distance travelled on inclines and declines, as it sees the earth as “flat” and smooths out the distance as though it was along a flat surface.

 

What do others think?

The sensor is certainly less prone to wonky readings than the GPS, for that 1/10 occurence alone I prefer it for recording distance over the GPS.

 

I used to train with a 520 and Fenix3 connected to the same speed sensor and the auto calibration would easily come out different if I didn't do them under similar circumstances.

 

Tyre pressures do affect speed sensor accuracy. Weather, stops, sharp turns and surrounding structures affect GPS accuracy. You are using GPS to calibrate the sensor so try to control for everything.

 

Pump up your tyres to the ideal level. Stand outside ready for a ride, then delete the sensor from your 520. Pair it again. Go for a ride. It calibrates in about 10 minutes. Try to keep this first period of the ride simple - no stops, sharp turns, high rise buildings or big trees that could disrupt GPS measurement.

 

The alternative is to simply set the tyre circumference manually. Probably the most accurate. There are online guides on measuring accurately, involves making marks on the floor and rolling forward all while keeping rider weight on the bike.

 

I find GPS callibration to work just fine, though.

Edited July 24, 2018 by thisismyotherbike

[dave303e]

The sensor is certainly less prone to wonky readings than the GPS, for that 1/10 occurence alone I prefer it for recording distance over the GPS.

 

I used to train with a 520 and Fenix3 connected to the same speed sensor and the auto calibration would easily come out different if I didn't do them under similar circumstances.

 

Tyre pressures do affect speed sensor accuracy. Weather, stops, sharp turns and surrounding structures affect GPS accuracy. You are using GPS to calibrate the sensor so try to control for everything.

 

Pump up your tyres to the ideal level. Stand outside ready for a ride, then delete the sensor from your 520. Pair it again. Go for a ride. It calibrates in about 10 minutes. Try to keep this first period of the ride simple - no stops, sharp turns, high rise buildings or big trees that could disrupt GPS measurement.

 

The alternative is to simply set the tyre circumference manually. Probably the most accurate. There are online guides on measuring accurately, involves making marks on the floor and rolling forward all while keeping rider weight on the bike.

 

I find GPS callibration to work just fine, though.

 

No weather does not affect GPS, a solid layer of water on the antennae will weaken the signal, and if it is super cold the battery may not power the antennae properly giving weak readings but really GPS signal itself is not affected. Also worth noting is that GPS is no longer just GPS, there is A-GPS, Glonass Galileo and more to come...

 

This is purely dependent on the accuracy setting you have chosen, If your device is recording position every second then this is not an issue, if you have it saving battery reading every 5 mins then it is an issue.

 

Yes this is a limitation, but not a huge one and with better coverage it is less of an issue.

[Bateleur1]

To the OP, what GPS systems are the others using and how do they compare.  It could be that yours is more accurate than theirs.  Especially if they are using ex Strava phone app.  Then Garmin is more accurate than Strava.

[NotSoBigBen]

How much is 'slightly'? Unless one is worrying about that 20k TT PB I wonder if it really matters .... I never get the exact same as others but hasn't bothered me yet.

 

Just me?

 

Sent from my LG-D958 using Tapatalk

[Neville Bailey]

To the OP, what GPS systems are the others using and how do they compare. It could be that yours is more accurate than theirs. Especially if they are using ex Strava phone app. Then Garmin is more accurate than Strava.

We were all using the same devices - Garmin Edge 520.

[Neville Bailey]

How much is 'slightly'? Unless one is worrying about that 20k TT PB I wonder if it really matters .... I never get the exact same as others but hasn't bothered me yet.

 

Just me?

 

Sent from my LG-D958 using Tapatalk

As an example, my distance was 48.6km and one of the others was 47.1km.

[Bateleur1]

We were all using the same devices - Garmin Edge 520.

 

Well that is interresting.

[Bateleur1]

This made me wonder so I had a look on my own recordings.

 

Picture A

 

Picture B

 

Picture A was yesterday with my MTB.  No Garmin speed sensor.  Only my Garmin Edge 820

 

Picture B was on Thursday last week on my Road bike.  Garmin speed sensor.  Exactly the same route but 100m more.

 

[dave303e]

Less than 3% error, that is better then your car odometer. better get on a forum and complain about that. 

[Bigjim]

I thought the speed sensor working in conjunction with the GPS? - i.e. when in a forest section where GPS is sketchy, the sensor provides the data - almost like a backup...…possible?

 

I have also seen my distances to be a bit longer than mates without the sensors (all 520's) & always put it down to the above 

[Grease_Monkey]

This made me wonder so I had a look on my own recordings.

 

Picture A

Munich - MTB.JPG

 

Picture B

Munich - RR.JPG

 

Picture A was yesterday with my MTB. No Garmin speed sensor. Only my Garmin Edge 820

 

Picture B was on Thursday last week on my Road bike. Garmin speed sensor. Exactly the same route but 100m more.

100m is not much, you can probably attribute that to how turns were taken, where you swerved, etc etc.

[Bateleur1]

100m is not much, you can probably attribute that to how turns were taken, where you swerved, etc etc.

 

Yip or the cycling paths in the city that might have slight deviations in the inbound and outbound route.  So the two give pretty much the same reading.