# Garmin 520 distance issue



## James80 (Jul 25, 2012)

I've just got back into riding (intro post to come if I can find where?) and got myself a Garmin Edge 520 to replace a Garmin Forerunner 405 I've had for years.

The issue I'm having is the 520 consistently records my trail rides as 9% shorter than the Forerunner and the people in my group using strava and Polar on iphones. The iphone recorded distances match the info from my Forerunner almost exactly but the 520 always shows less (I do not have a wheel speed sensor). All the settings are correct, 1 second record, GPS/GLONASS, auto pass off etc.

I got on to Garmin support who after making sure the unit was up to date and that the setting were as above told me the iphones are all wrong, the 520 is correct and the Forerunner is old so incorrect. 

Has anyone had a similar issue? Is it possible Garmin guy is right and EVERYONE'S phones and my Forerunner are wrong and my 520 is right?


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## Catmandoo (Dec 20, 2018)

If you have a basic bike computer, the type that used a wheel magnet, calibrate it to the actual wheel and tire circumfrance, throw it on and do a ride, comparing the results.

It’s possible the 520 is correct, especially if assorted cell phones are not picking up GLONASS. The 520 might have a more sensitive and better GPS calculation than older devices. 

I found that even my Garmin 810 was accurate on road rides to about 1/2 mile in 100 as compared to a calibrated Cateye. It was short by about 2 miles in 100 when mt. biking. Thus I started using wheel sensors,


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## James80 (Jul 25, 2012)

To add a brand new Wahoo Elemnt Bolt also matches the other devices almost exactly.


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## Catmandoo (Dec 20, 2018)

Curious what the maps from recorded rides look like between devices.

If you compare, possibly you will see if the 520 is cutting corners, though it obviously might be. Maybe bad signal in woods ? And maybe you will see that on the map. 

Turn off GLONASS ?


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## James80 (Jul 25, 2012)

The 520 tracks look cleaner and seem to cut less corners than the Garmin. 

I’m going to try turning off GLONASS.


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## Harold (Dec 23, 2003)

An accurate GPS will ALWAYS underreport distances. INaccurate GPSes will display extra positional wander that will "add" the distance back, but the track will inaccurately reflect where you went.

Add a wheel sensor to your edge and you will get that distance back.

Turning off glonass will not improve anything with distance recording. All that will do is get you a little extra battery life back, at the expense of some accuracy (how much will depend on where you are) and speed of location fix.

Sent from my VS995 using Tapatalk


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## Mudguard (Apr 14, 2009)

Catmandoo said:


> Curious what the maps from recorded rides look like between devices.
> 
> If you compare, possibly you will see if the 520 is cutting corners, though it obviously might be. Maybe bad signal in woods ? And maybe you will see that on the map.
> 
> Turn off GLONASS ?


Why would you turn it off? It's reducing the number of satellites to lock on to. Add a wheel sensor.


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## Sparticus (Dec 28, 1999)

I’ve had two Garmin 520s and they both underreported mileage. In fact, that’s why I returned the first one. I employed a used 820 for a while, then gave it to a friend and bought another 520, hoping my first on was defective. The second 520 was just as bad.

So I bought a speed sensor and now it’s up to snuff — all good. Except now it seems to be under reporting elevation gain. My GF and I ride together a lot. She has a Garmin 810 which consistently reports 20%ish more gain than my 520. On yesterday’s ride, her 810 gave her 1400’ gain, my 520 said less than 1000’.

I don’t know what’s up.
=sParty


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## Mudguard (Apr 14, 2009)

There is a setting in the Garmin app or your online profile that corrects elevation if you allow it to compare it known elevation (ie a topo map or something). It will then tweak it before it goes to Strava. I think. Its been awhile since I've looked.
As for comparing two devices, well how do you know which is actually accurate? The only way is with a wheel sensor that's calibrated correctly, two GPS devices side my side will still give varying amounts, it's just the nature of them.


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## Harold (Dec 23, 2003)

Elevation readings are notoriously inaccurate. Think about it. There is NO WAY to directly measure your climbing totals on a ride. Your Garmin (or whatever device) is making lots of indirect measurements, using a method that gets confounded by changes in the weather...and that is the BEST method available. Plus, it's hard enough to measure the elevation of a single point.

Sent from my VS995 using Tapatalk


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## Catmandoo (Dec 20, 2018)

Mudguard said:


> Why would you turn it off? It's reducing the number of satellites to lock on to. Add a wheel sensor.


Only as it's 2 completely different positioning systems. GLONASS is the Russian version of the US GPS system. It's not the same signal and as such requires the 520 to calculate separately. Wouldn't surprise me if the Garmin software would calculate location differently using these 2 potentially conflicting positions. In fact I know this happens, the article here explains the issues and how the reception and accuracy is dependent on the type of antenna and of course the built in software used to "merge" the positioning of the 2 different systems.

https://www.electronicdesign.com/te...-world-drive-tests-declare-verdict-gpsglonass

In truth I don't think we know how the 520 decides which position of the 2 different signals is accurate and it's not the same as receiving from additional US satellites. Thus my thought was to isolate the problem by turning off one system

Stab in the dark problem solving essentially.

I turn off GLONASS on my 1000 to save battery life and use speed sensors when in the woods. We assume that's the kind of riding the OP is doing, thus the easy recommendation is to buy a $39 speed sensor, which having just plunked down $250 for the 520 I could see the OP asking "Why should I have to do that ?"


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## James80 (Jul 25, 2012)

Whilst I agree that adding a speed sensor is the smart thing to do and will lead to more accurate readings it still doesn’t explain why the 520 reads differently from multiple devices riding the exact trail. And I don’t buy that the iPhone, Forerunner and Wahoo are all exactly the same amount of wrong. 

The other strange thing is both the Forerunner and 520 read exactly the same when taken for a drive. It has to have something to do with coverage in the forest. I read somewhere someone had a similar issue and deleted the training profile and recreated it and the problem went away, but I can’t find that thread. 

This is my fourth Garmin device, should’ve known it wouldn’t have been easy!


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## Catmandoo (Dec 20, 2018)

James80 said:


> Whilst I agree that adding a speed sensor is the smart thing to do and will lead to more accurate readings it still doesn't explain why the 520 reads differently from multiple devices riding the exact trail. And I don't buy that the iPhone, Forerunner and Wahoo are all exactly the same amount of wrong.
> 
> The other strange thing is both the Forerunner and 520 read exactly the same when taken for a drive. It has to have something to do with coverage in the forest. I read somewhere someone had a similar issue and deleted the training profile and recreated it and the problem went away, but I can't find that thread.
> 
> This is my fourth Garmin device, should've known it wouldn't have been easy!


The article I linked shows the issues with receiving in urban area's. Wooded rides are essentially the same thing, especially with a lot of leaf cover or conifer type forests. I don't get the issues locally as the woods here are predominantly deciduous and in winter it's a better view of the sky. I still use a speed sensor but as much as the trails I ride are very twisty and tight with a lot of quick turns. GPS has trouble with that,

As the 520 is now almost a dated device, I would think about a return for a 520 Plus (if not too late), or an Edge Explore, which is larger but the same price as the 520. Or if you are not using pretty maps, maybe get a Bolt or Elemnt.


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## James80 (Jul 25, 2012)

Catmandoo said:


> The article I linked shows the issues with receiving in urban area's. Wooded rides are essentially the same thing, especially with a lot of leaf cover or conifer type forests. I don't get the issues locally as the woods here are predominantly deciduous and in winter it's a better view of the sky. I still use a speed sensor but as much as the trails I ride are very twisty and tight with a lot of quick turns. GPS has trouble with that,
> 
> As the 520 is now almost a dated device, I would think about a return for a 520 Plus (if not too late), or an Edge Explore, which is larger but the same price as the 520. Or if you are not using pretty maps, maybe get a Bolt or Elemnt.


Agreed about coverage but any coverage issues in the woods should affect all the devices to the same extent. The distance between the forerunner and 520 is wrist to stem. So we're back to either

1: My 520 is faulty (will try to ride with a friend who has one tomorrow to compare)

2: My settings are wrong and or unit needs resetting

3: That's just how 520s work

4: 520 is right and every other device in my group is wrong.


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## Harold (Dec 23, 2003)

James80 said:


> Agreed about coverage but any coverage issues in the woods should affect all the devices to the same extent. The distance between the forerunner and 520 is wrist to stem. So we're back to either
> 
> 1: My 520 is faulty (will try to ride with a friend who has one tomorrow to compare)
> 
> ...


You're not listening or comprehending (can't tell which).

An accurate GPS is always going to shortcut corners. ALWAYS. This is simply due to the way GPS works (connect the dots with straight lines that don't follow the curve). The more frequent the device records, the shorter this discrepancy (and the less frequent, the greater the discrepancy), but it will always exist. Reductions in positional accuracy will add distance back in other areas, like wandering on straight sections, or drifting around when stopped, etc. Plus, each device has a different combination of chipset, antenna, and processing algorithm that affects the end result.

https://www.jsams.org/article/S1440-2440(09)00182-0/pdf
GPS Accuracy of Garmin, Polar, and other Running Watches - Fellrnr.com, Running tips

Note in the 2nd link (which focuses on running models) that the footpods are almost always more accurate AND more precise than GPS-only distance measurements.

Get a wheel sensor.


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## Catmandoo (Dec 20, 2018)

Harold said:


> You're not listening or comprehending (can't tell which).
> 
> An accurate GPS is always going to shortcut corners. ALWAYS. This is simply due to the way GPS works (connect the dots with straight lines that don't follow the curve). The more frequent the device records, the shorter this discrepancy (and the less frequent, the greater the discrepancy), but it will always exist. Reductions in positional accuracy will add distance back in other areas, like wandering on straight sections, or drifting around when stopped, etc. Plus, each device has a different combination of chipset, antenna, and processing algorithm that affects the end result.
> 
> ...


Harold, the OP is comprehending and talking about something different than why GPS tracks will be short. He's not understanding why a number of other dissimilar devices are showing the same (potentially) short track while the 520 is significantly shorter, is all.

For the OP, do a Google of "my Garmin 520 records short" or some such. Tons of posts. Solution is a speed sensor. Or exchange for a different unit. Or both.


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## Harold (Dec 23, 2003)

Catmandoo said:


> Harold, the OP is comprehending and talking about something different than why GPS tracks will be short. He's not understanding why a number of other dissimilar devices are showing the same (potentially) short track while the 520 is significantly shorter, is all.
> 
> For the OP, do a Google of "my Garmin 520 records short" or some such. Tons of posts. Solution is a speed sensor. Or exchange for a different unit. Or both.


So the 520 is shorter than other devices? so what? my point is it doesn't matter "how much" short it is because that's how GPS works. the second link I shared has a nice chart that shows a bunch of devices and guess what - they're ALL different from each other. You cannot tease out the difference between the specific question of "why is x device reporting shorter distance traveled than y and z devices?" without understanding the fundamental reason why GPS measurement of distance in fitness activities is not all that accurate in the first place.

The nature of GPS means that you simply cannot expect 2 devices to report exactly the same. Ever.


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## James80 (Jul 25, 2012)

Catmandoo said:


> Harold, the OP is comprehending and talking about something different than why GPS tracks will be short. He's not understanding why a number of other dissimilar devices are showing the same (potentially) short track while the 520 is significantly shorter, is all.
> 
> For the OP, do a Google of "my Garmin 520 records short" or some such. Tons of posts. Solution is a speed sensor. Or exchange for a different unit. Or both.


Exactly. And yes I've done multiple searches but didn't find a satisfactory answer hence asking the question here.



Harold said:


> So the 520 is shorter than other devices? so what? my point is it doesn't matter "how much" short it is because that's how GPS works. the second link I shared has a nice chart that shows a bunch of devices and guess what - they're ALL different from each other. You cannot tease out the difference between the specific question of "why is x device reporting shorter distance traveled than y and z devices?" without understanding the fundamental reason why GPS measurement of distance in fitness activities is not all that accurate in the first place.
> 
> The nature of GPS means that you simply cannot expect 2 devices to report exactly the same. Ever.


I'm not expecting them to report exactly the same. I'm simply saying that every other device is with a "reasonable" range and the 520 is consistently 9% less. And my question is would another 520 strapped next to mine report the same or is mine especially "out".

Be interested to hear your thoughts on these:

Purple is the 520 and orange the Forerunner




























The tracks are identical on the road and start to differ once in the woods interestingly the elevation tracks are identical except the 520 is 9% lower and lags behind by the mileage difference.


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## Harold (Dec 23, 2003)

For one, both devices are set to the "auto" recording frequency.

Change your Edge 520 to 1 second recording and report back.


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## James80 (Jul 25, 2012)

Edge is set to 1 second. 

Any thoughts on Forerunner being GPS only and 520 GPS and GLONASS?


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## Catmandoo (Dec 20, 2018)

James80 said:


> Exactly. And yes I've done multiple searches but didn't find a satisfactory answer hence asking the question here.
> 
> I'm not expecting them to report exactly the same. I'm simply saying that every other device is with a "reasonable" range and the 520 is consistently 9% less. And my question is would another 520 strapped next to mine report the same or is mine especially "out".
> 
> ...


I'd be curious as to how the 2 different tracks actually overlay on the actual trail. Can you find any other GPS tracks from other riders ?. MapMyFitness ?, RWGPS ?, TrailForks ?. Maybe compare to what your 520 unit is doing different - if anything.

You may go nuts trying to figure all this out and as Harold has stated GPS devices have inaccuracies inherent to how they function. It's anybody's guess what Garmin does on the 520 in terms of using both GLONASS and US GPS mixed to determine position.

I would turn off GLONASS as a start. Then get a speed sensor (good to have in any event) Then and if it was still showing 10% errors on a regular basis, I'd replace with a 520+ or Edge Explore if you still have the option.


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## Harold (Dec 23, 2003)

James80 said:


> Edge is set to 1 second.
> 
> Any thoughts on Forerunner being GPS only and 520 GPS and GLONASS?


Were you hauling epic ass on the road segment, then? I only see a saved point where you turned onto singletrack. That is why I assumed auto recording interval (fewer recorded points on straights). That, and the visible portion doesn't look steep.

This trail layout is kinda worst case scenario for gps recording, anyway. The twistier the trail, the greater the shortcutting errors. and besides, without a known high accuracy baseline, we simply cannot know which track is most accurate, or if they're equally inaccurate but in different ways.

You can't even really trust the trail data on most mapping websites as being of high accuracy. Most of them heavily simplify uploaded data to save on server storage and processing. And that's assuming that high accuracy data was uploaded in the first place. Which isn't often the case. Many of these sites would rather have low quality data for more trails than high quality data for fewer trails.

It can be instructive to show multiple passes on the same trail on different days with the same device, too. You are going to see a spectrum of data quality with good quality runs and poor quality runs.

It's worthwhile to know your latitude and region, too. GLONASS tends to perform better at higher latitudes, as it was designed for that. It actually works better than gps at extreme northern latitudes. Farther south, not so much, and gps will outperform it. Are there terrain impediments like mountains or steep valleys (especially impediments on the southern horizon)? How thick is the veg growth?

And for that matter, none of what I see there would bother me. I see far worse with some frequency when I check out strava flybys.

Sent from my VS995 using Tapatalk


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## Forest Rider (Oct 29, 2018)

You should ride a staright, long road type of course. See how the results compare to other devices, or known distances.

My Garmin 405 watch would always report closely to the my computer (not GPS) and Strava when road riding. But not when on the MTB trails.

Even when trail running, my watch is not as accurate as it is on open-environment road courses.
The GPS takes a shot and connects the dots. Depending on the frequence and how much distance you have covered in said distance is how much it will affect the curves.

On my road bike there is a section that always gets off track for me because it's an s-bend. I've going through the curves quickly enough that GPS can't draw the curve and therefore is inaccurate.


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## James80 (Jul 25, 2012)

Latitude 10.32.11, riding in a valley, tropical forest at the start of the dry season. 

I checked the track points on a proper viewer and ensured it was every second, the earlier picture was from a track creator so wasn't showing the points. I then overlaid both the Forerunner track and one created with an iphone and they both look to wander quite a bit. The 520 track is much tighter with smooth curves that better resemble the actual trail.

I then down loaded and overlaid a track from another Garmin 520 that as a speed sensor attached and is using GPS/GLONASS. His tracks are very similar to mine but shows approx 5% longer distance over the same track. 

I tentatively conclude the following:

1: The Forerunner and iphone suffer quite a bit of wander potentially adding distance
2: The 520 records a much tighter track and due to the twisty nature and forest cover may be loosing distance
3: The actual distance is probably in between (the other 520) and will be confirmed with a wheel speed sensor
4: Yes this is driving me mad but I think we've got somewhere.

Thanks guys!!


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## Forest Rider (Oct 29, 2018)

Now you have the challenge of calibrating the wheel sensor!
If they are like the older magnet style sensors. I think I've read that the wheel/speed sensor will automatically calibrate one wheel rotation, I can't remember.

But if it is manual, I feel your pain already.


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## Catmandoo (Dec 20, 2018)

Forest Rider said:


> Now you have the challenge of calibrating the wheel sensor!
> If they are like the older magnet style sensors. I think I've read that the wheel/speed sensor will automatically calibrate one wheel rotation, I can't remember.
> 
> But if it is manual, I feel your pain already.


With the old magnet sensors that had a magnet on a spoke and a sensor on the frame, you would do a roll out measurement. Set the wheel valve straight up/down at the bottom, mark the floor, roll the bike so the tire/valve makes one full rotation, mark floor, measure distance between marks.

Enter that number into the computer.

If you wanted to be really, really precise you had a helper and would sit on the bike so as to compress the tire.

The newer Ant+ style sensors used on GPS and other devices, measure the earths magnetic field and can generate wheel circumference from that. It's magic 'ya ask me.


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## Harold (Dec 23, 2003)

Catmandoo said:


> With the old magnet sensors that had a magnet on a spoke and a sensor on the frame, you would do a roll out measurement. Set the wheel valve straight up/down at the bottom, mark the floor, roll the bike so the tire/valve makes one full rotation, mark floor, measure distance between marks.
> 
> Enter that number into the computer.
> 
> ...


Huh? I thought that the magnetless sensors detected rotations with accelerometers. Motion, in short. After connecting one, garmin will still let you do manual setup with the rollout method. You can even see the circumference garmin estimates based on its auto calibration

Sent from my VS995 using Tapatalk


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## Forest Rider (Oct 29, 2018)

That's cool that the newer systems do calibrate for you.

I've spent a lot of time dialing in my computers in the past.

I'd usually do about 5 wheel diameters for better accuracy. Setting proper tire pressure, sitting on it and trying to ride as straight a line as possible.

Then the challenge is if the computer wheel size is entered in centimeters or millimeters.


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## Lone Rager (Dec 13, 2013)

Forest Rider said:


> That's cool that the newer systems do calibrate for you...


Of course it's using GPS to do the calibration over something like 500m or 1/2 mile. It's best to ride a fairly straight course over this distance, and if the GPS distance is off, the wheel size calibration will be off the same amount.

Tire circumference is roughly 2m, so if you do a single rotation rollout measurement to +/- 1cm, you're already with in 0.5%. Best to do it with the bike weighted as it will change the effective tire circumference.

For me, distance accuracy is of little importance. If I'm concerned about my physical performance, it's time and intensity that matters as distance can be highly variable depending on equipment and terrain.


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## Forest Rider (Oct 29, 2018)

Lone Rager said:


> Of course it's using GPS to do the calibration over something like 500m or 1/2 mile. It's best to ride a fairly straight course over this distance, and if the GPS distance is off, the wheel size calibration will be off the same amount.
> 
> Tire circumference is roughly 2m, so if you do a single rotation rollout measurement to +/- 1cm, you're already with in 0.5%. Best to do it with the bike weighted as it will change the effective tire circumference.
> 
> For me, distance accuracy is of little importance. If I'm concerned about my physical performance, it's time and intensity that matters as distance can be highly variable depending on equipment and terrain.


Are you saying that the wheel/speed sensor automatic calibration is done by GPS? That doesn't make sense. If so, why even use a wheel sensor since GPS is already calculating (albeit not super accurate) while riding the trail?

As for wheel rollout, as accurate as possible is important. Consider the rough estimate over the life of the bike, or the ride.

Maybe you are better than me (sounds like you are for sure), but in no way can I get a single rotation to be within a centimeter. Considering I need to get on the bike, squish the front tire and hold a straight line for only one rotation, while watching the valve stem rotate around.


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## Catmandoo (Dec 20, 2018)

Harold said:


> Huh? I thought that the magnetless sensors detected rotations with accelerometers. Motion, in short. After connecting one, garmin will still let you do manual setup with the rollout method. You can even see the circumference garmin estimates based on its auto calibration
> 
> Sent from my VS995 using Tapatalk


As per the Garmin information - "contains a magnetometer that measures the three dimensional components of the ambient magnetic field"

https://support.garmin.com/en-US/?faq=9NL91YJSJd3Tnyif9jRSy6

When I'm commuting to/from work I go over a steel decked bridge. The speed sensor goes nuts if I'm on one of my bikes that has a speed sensor installed.


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## Harold (Dec 23, 2003)

Forest Rider said:


> Are you saying that the wheel/speed sensor automatic calibration is done by GPS? That doesn't make sense. If so, why even use a wheel sensor since GPS is already calculating (albeit not super accurate) while riding the trail?
> 
> As for wheel rollout, as accurate as possible is important. Consider the rough estimate over the life of the bike, or the ride.
> 
> Maybe you are better than me (sounds like you are for sure), but in no way can I get a single rotation to be within a centimeter. Considering I need to get on the bike, squish the front tire and hold a straight line for only one rotation, while watching the valve stem rotate around.


Yes, Garmin's auto calibration uses GPS to calibrate. This is why you should do that calibration on as straight of a course as possible, to minimize potential error sources. You can tweak it manually to be even better.

You don't do a wheel rollout for any computer over a single wheel rotation. That's fraught with errors. You do it over the course of multiple rotations (minimum of 3, preferably more) so you can average it out. Yes, you use the valve stem as your marker. Maybe a dab of paint on the tire to mark the pavement (number of rotations). It's true that measuring this to the nearest mm is damn difficult and most people are unlikely to be able to get it that good. I bet most could get it within a cm to .5cm if they made the effort, though.


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## Harold (Dec 23, 2003)

Catmandoo said:


> As per the Garmin information - "contains a magnetometer that measures the three dimensional components of the ambient magnetic field"
> 
> https://support.garmin.com/en-US/?faq=9NL91YJSJd3Tnyif9jRSy6
> 
> When I'm commuting to/from work I go over a steel decked bridge. The speed sensor goes nuts if I'm on one of my bikes that has a speed sensor installed.


interesting. I never saw that document before. I always assumed an accelerometer was used. they never mention localized anomalies of the earth's magnetic field in that article, though. I've hiked in areas where those really screwed up compass navigation and I wonder if their checks and calibrations are able to deal with those.


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## Catmandoo (Dec 20, 2018)

Harold said:


> Yes, Garmin's auto calibration uses GPS to calibrate. This is why you should do that calibration on as straight of a course as possible, to minimize potential error sources. You can tweak it manually to be even better.
> 
> You don't do a wheel rollout for any computer over a single wheel rotation. That's fraught with errors. You do it over the course of multiple rotations (minimum of 3, preferably more) so you can average it out. Yes, you use the valve stem as your marker. Maybe a dab of paint on the tire to mark the pavement (number of rotations). It's true that measuring this to the nearest mm is damn difficult and most people are unlikely to be able to get it that good. I bet most could get it within a cm to .5cm if they made the effort, though.


I've never noticed if not getting it to within 2 or 3 mm is inaccurate over a 50 mile ride. Not even sure how to calculate that and I sure don't worry about it.


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## Harold (Dec 23, 2003)

Catmandoo said:


> I've never noticed if not getting it to within 2 or 3 mm is inaccurate over a 50 mile ride. Not even sure how to calculate that and I sure don't worry about it.


yeah, that's probably not that big of a deal. there are so many other sources of error in distance measurement that it's probably not even a detectable difference. How much error is introduced from airtime? lifting your bike over trees and big ledges? probably more than being 2-3mm off on your wheel circumference calibration.


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## Lone Rager (Dec 13, 2013)

FWIW: To do a rollout measurement, I put a mark on the tire near the ground and mark the ground aligned with the mark on the tire with the bike weighted. Roll forward 1 or more rotations and mark the ground again to coincide with the mark on the tire. I then measure between the two marks on the ground. With some care, the effective tire circumference can be determined to better than +/- 1 cm.

Yes. During auto calibration, the wheel revolutions are compared to the GPS measurement to determine wheel circumference. The intent of the wheel sensor is to improve distance accuracy over tortuous tracks, and speed indication accuracy at the slow speeds bicycles can be moving. If, for example, you use auto pause, it will be more reliable and less erratic when using a wheel sensor rather than relying purely on GPS.


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## Forest Rider (Oct 29, 2018)

I lay out my tape measure and set my valve stem at a control joint in the sidewalk. Then roll it the length of the sidewalk in front of my house. I stop when the valve stem is again intersecting the ground. I put painters tape down and make that location. And repeat a few times and average the values.

Seems like we do the same thing, basically. However, if I roll out 5 diameters and do the math, it's not even close to a single diameter.

The point I meant about mm vs cm is when the bike computer only offers mm or cm values. It's off at max of 5mm doing that depending on how one decides to round off.

Saying it is off by 1mm or whatever isn't as accutrate as saying what the percent different is. If you measure and are off by 1%, then the total distance is off by 1%. You won't be off 1% if you pick the bike to carry it over a tree.

Sure, not important for a 20 mile ride, but if you are using it to track gear and you have 1000 miles, it can add up.

most of the tighter accuracy was for my road bike when I would ride 50 miles or whatever. I remember once I didn't have the computer very well set up, the metric century ride registered something like 54 miles and the others (with road bikes and a properly calibrated computer registered closer to 65.

In any case, at least it can calibrate it for you then allow tweaking with the Garmins. When I decide if I do purchase a Garmin computer, I'll have to remember to set it up on a nice open piece of our paved trail system.

Thanks for the chat guys.


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## James80 (Jul 25, 2012)

I haven’t got a speed sensor yet but Ive decided to believe the 520.

And then I was in Strava and it told me there was an issue with my recorded mileage upon correction it added approx 0.25 miles o each 9 mile ride. 

Whatever. 


Sent from my iPad using Tapatalk


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## Harold (Dec 23, 2003)

James80 said:


> I haven't got a speed sensor yet but Ive decided to believe the 520.
> 
> And then I was in Strava and it told me there was an issue with my recorded mileage upon correction it added approx 0.25 miles o each 9 mile ride.
> 
> ...


https://labs.strava.com/slide/


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## James80 (Jul 25, 2012)

Harold said:


> https://labs.strava.com/slide/


Thanks for that. I wonder if it works the other way and reduces the distance for paths that go wide of the heat map.


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## Harold (Dec 23, 2003)

James80 said:


> Thanks for that. I wonder if it works the other way and reduces the distance for paths that go wide of the heat map.


If you browse their help section, you'll find a few articles on their error detection and corrections. They do quite a lot behind the scenes.

Sent from my VS995 using Tapatalk


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## Catmandoo (Dec 20, 2018)

As an interesting test, 3 of us on an AM road ride, 

3 different Garmins. A 3 year old 1000, a 10 mos. old 1030 and brand new Explore.

The 1000 on a bike with a speed sensor.

Road ride with exactly 26 turns, so a lot of straight 20 ish miles

1000 said 20.56
Explore said 20.48
1030 said 20.56. 

That's a 1/4 mile in 100 ? if my math is correct. Pretty consistent.


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## Sparticus (Dec 28, 1999)

Catmandoo said:


> As an interesting test, 3 of us on an AM road ride,
> 
> 3 different Garmins. A 3 year old 1000, a 10 mos. old 1030 and brand new Explore.
> 
> ...


Try a similar test in the dense rainforests of the Cascade Range where there are plenty of ravines & canyons that obscure satellites. I rode a trail just east of Lowell, Oregon yesterday with a friend. She has a Garmin 810, I have a 520+speed sensor. I got over 2 miles more than her. Always do. She got more gain than me. Always does.
=sParty


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## NordieBoy (Sep 26, 2004)

The 810 is GPS only, no GLONASS/Galileo and you're running a speed sensor.
Turn your speed sensor off and go GPS only and see if it's the same.


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## Sparticus (Dec 28, 1999)

NordieBoy said:


> The 810 is GPS only, no GLONASS/Galileo and you're running a speed sensor.
> Turn your speed sensor off and go GPS only and see if it's the same.


You didn't quote anybody so I'll assume you're talking to me. Since I have two bikes and only one speed sensor, I sometimes forget to move it from one bike to the other. Whenever I forget the speed sensor, my distance drops dramatically. I go from having more distance than my friend to having less distance than her. Significantly different in both directions (much more with, much less without).

I'm not interested in turning GLONASS off.
=sParty


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## NordieBoy (Sep 26, 2004)

Sparticus said:


> I'm not interested in turning GLONASS off.


Neither GLONASS nor Galileo seem to make a difference to the track accuracy here.
Although I haven't had much chance to test in the trees as everything is closed due to the fire risk.


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## Lone Rager (Dec 13, 2013)

Sparticus said:


> ...I have two bikes and only one speed sensor, I sometimes forget to move it from one bike to the other....


When moving the speed sensor between bikes, you might want to check that the calibration is correct for each.


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## Sparticus (Dec 28, 1999)

Lone Rager said:


> When moving the speed sensor between bikes, you might want to check that the calibration is correct for each.


Really? I thought the Garmin automatically calibrated wheel size from one bike to the next. I'm not kidding.
=sParty


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## Harold (Dec 23, 2003)

NordieBoy said:


> Neither GLONASS nor Galileo seem to make a difference to the track accuracy here.
> Although I haven't had much chance to test in the trees as everything is closed due to the fire risk.


For one, it depends on where you are and a whole lot of other factors related to GPS reception. I find that GLONASS does make for marked improvement in track accuracy in cases where buildings or terrain limit satellite visibility.



Lone Rager said:


> When moving the speed sensor between bikes, you might want to check that the calibration is correct for each.


yep



Sparticus said:


> Really? I thought the Garmin automatically calibrated wheel size from one bike to the next. I'm not kidding.
> =sParty


it doesn't calibrate it immediately. it takes time for that calibration to occur, and if it wasn't a good calibration (say, you started your ride immediately on twisty singletrack, rather than doing a couple miles of straight pavement or gravel after you put the sensor on), the GPS might refine it later. You're REALLY better off having a wheel sensor for each bike where you might want to use one. I put a wheel sensor on each of my mtb's, but generally not road bikes. It makes less of a difference there.


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## Catmandoo (Dec 20, 2018)

Sparticus said:


> Try a similar test in the dense rainforests of the Cascade Range where there are plenty of ravines & canyons that obscure satellites. I rode a trail just east of Lowell, Oregon yesterday with a friend. She has a Garmin 810, I have a 520+speed sensor. I got over 2 miles more than her. Always do. She got more gain than me. Always does.
> =sParty


Yes, and I use speed sensors on my mt. bikes for this reason.

I was mostly just commenting about the thread indicating that the OP's 520 is seemingly so different with other units and non-dedicated phone apps. My quicky test seemed to show a very tight set of distances between 3 different Garmin units of different vintages.

FWIW, the rider with the Edge Explore also had a Cateye wireless with 20.5 or so distance, so almost the same as his Explore.

The rider with the 1030 ran a track on his iPhone with an app that was a good 1/2 mile longer on the total distance.


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## Sparticus (Dec 28, 1999)

Harold said:


> it doesn't calibrate it immediately. it takes time for that calibration to occur


How long does the automatic calibration take?
How do I adjust it manually?
Thanks, Harold.
=sParty


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## Harold (Dec 23, 2003)

Sparticus said:


> How long does the automatic calibration take?
> How do I adjust it manually?
> Thanks, Harold.
> =sParty


it depends.

To adjust manually, you have to dive into the settings menus, find the sensor, and manually input your wheel rollout (which means you have to calculate and record it). Not exactly what I want to do all the time.


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## Sparticus (Dec 28, 1999)

Harold said:


> it depends.
> 
> To adjust manually, you have to dive into the settings menus, find the sensor, and manually input your wheel rollout (which means you have to calculate and record it). Not exactly what I want to do all the time.


Me neither. In fact IMO the "install & forget" aspect of the speed sensor is it's most attractive feature. Guess I'll quit complaining and just take whatever I get. First world problems...
=sParty


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## Harold (Dec 23, 2003)

Sparticus said:


> Me neither. In fact IMO the "install & forget" aspect of the speed sensor is it's most attractive feature. Guess I'll quit complaining and just take whatever I get. First world problems...
> =sParty


You can track down a cheap $10-$20 sensor that'll work with your Garmin and I'd lean towards that if buying another Garmin sensor is distasteful for some reason.


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## Sparticus (Dec 28, 1999)

Harold said:


> You can track down a cheap $10-$20 sensor that'll work with your Garmin and I'd lean towards that if buying another Garmin sensor is distasteful for some reason.


After pairing a new (another) speed sensor, will my Garmin automatically detect either one? I mean regardless which bike I ride?

If I get a second speed sensor, I'll leave one on Bike A and leave the other on Bike B. I want to be able to hop on either bike and go without spending time looking for / choosing one or the other via the Garmin's menu.

I found some cheapies, several got marginal reviews. This one seems okay... do you happen to know if it is?
https://www.amazon.com/Computer-Cyc...le+ANT++Bluetooth+4.0+Wireless+Cycle+Computer

Thanks again, Harold.
=sParty


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## Harold (Dec 23, 2003)

Sparticus said:


> After pairing a new (another) speed sensor, will my Garmin automatically detect either one? I mean regardless which bike I ride?
> 
> If I get a second speed sensor, I'll leave one on Bike A and leave the other on Bike B. I want to be able to hop on either bike and go without spending time looking for / choosing one or the other via the Garmin's menu.
> 
> ...


I've only used the Garmin one. So beyond what other people report, I can't say.

But yeah, the Garmin will automatically detect which sensor is on which bike. The wheel size settings are stored on the head unit and are associated with each sensor. So pair the sensor once, get the wheel size calibrated, and you only have to mess with it when you change batteries in the sensor (and it won't have to recalibrate when you change batteries if it's a magnetless sensor). Just grab the bike and go. When the sensor activates, the head unit will pick it up and use it.


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## Sparticus (Dec 28, 1999)

Harold said:


> I've only used the Garmin one. So beyond what other people report, I can't say.
> 
> But yeah, the Garmin will automatically detect which sensor is on which bike. The wheel size settings are stored on the head unit and are associated with each sensor. So pair the sensor once, get the wheel size calibrated, and you only have to mess with it when you change batteries in the sensor (and it won't have to recalibrate when you change batteries if it's a magnetless sensor). Just grab the bike and go. When the sensor activates, the head unit will pick it up and use it.


Cool, thanks again. I'll take a chance on the XOSS speed sensor.
=sParty


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## Catmandoo (Dec 20, 2018)

My 1000 keeps in it's memory every sensor I've paired. 

It pairs up each time with the sensor it sees active, does this automatically. 

If I use the device on Bike A and start to roll, the sensor goes active and the device pairs. 

When I roll on Bike B with a different sensor, same thing happens. 

I can imagine some confusion results if I ride Bike A and loan Bike B to a buddy for that ride.


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## Harold (Dec 23, 2003)

Catmandoo said:


> I can imagine some confusion results if I ride Bike A and loan Bike B to a buddy for that ride.


Just keep the bikes separated while the devices initialize to avoid that confusion.

This system is SO MUCH BETTER than the old method of wheel rollouts being stored in "bike profiles" on the device, so you'd have to switch bike profiles every time you switched bikes. I had this issue on a couple different devices that used bike profiles and I'd frequently forget to tell the computer which bike I was riding. the sensor would pair, but the head unit would use the wrong wheel diameter for distance calcs, which would make for really weird results.


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## Lone Rager (Dec 13, 2013)

The Edge doesn't calibrate the sensor each time you ride. Something has to trigger Edge to do it and simply changing the sensor between bikes won't trigger a calibration. It'll calibrate it the first time it's paired. There are other conditions which will also trigger it to calibrate, but I don't remember what they are. 

It does the calibration over the first 1/2 mile or so using GPS, so ride a simple course if it's calibrating. It'll tell you when it's done but doesn't tell you when it's starting to do it. And, of course, it's using GPS to do it so if there's and issue with GPS, it will be reflected in the sensor calibration. The way around this is to do a manual calibration, which it retains until you manually recalibrate it again, or switch it to auto cal. The calibration is associated with the specific sensor, so if you move the sensor to a different bike, you best change the calibration.

If you have multiple speed sensors paired, it'll connect to which ever one is nearby and active when you turn the Edge on or after the sensor wakes up. If your other paired sensor is awake on an adjacent bike, it may connect to that one instead of the one on the bike you're on...until you ride out of range of that sensor. It'll then use GPS instead of the wheel sensor.


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## Sparticus (Dec 28, 1999)

Lone Rager said:


> The Edge doesn't calibrate the sensor each time you ride. Something has to trigger Edge to do it and simply changing the sensor between bikes won't trigger a calibration. It'll calibrate it the first time it's paired. There are other conditions which will also trigger it to calibrate, but I don't remember what they are.
> 
> It does the calibration over the first 1/2 mile or so using GPS, so ride a simple course if it's calibrating. It'll tell you when it's done but doesn't tell you when it's starting to do it. And, of course, it's using GPS to do it so if there's and issue with GPS, it will be reflected in the sensor calibration. They way around this is to do a manual calibration, which it retains until you manually recalibrate it again, or switch it to auto cal.
> 
> If you have multiple speed sensors paired, it'll connect to which ever one is nearby and active when you turn the Edge on or after the sensor wakes up. If your other paired sensor is awake on an adjacent bike, it may connect to that one instead of the one on the bike you're on...until you ride out of range of that sensor.


Interesting. Without intending to sound as if I doubt you LR, how certain are you of the one-time calibration? May I ask how you found this procedure out?

I was under the impression (and that's all it is -- impression, IIRC) that every time the sensor/wheel began turning, the head unit began comparing the distance covered (via GPS) to the wheel's rotation and would recalibrate based on how many rotations it took to travel X distance. I actually believed that I could run my speed sensor & Garmin head unit on my 29er today and my neighbor kid's BMX bike tomorrow and the unit's auto recalibration would discover the wheel size change and recalibrate automatically. Every time.

No?

I've been wrong before and I don't doubt I'm mistaken in this case but I'd like to understand how confident you are that what you've outlined above is accurate. Thanks.
=sParty


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## Harold (Dec 23, 2003)

Sparticus said:


> Interesting. Without intending to sound as if I doubt you LR, how certain are you of the one-time calibration? May I ask how you found this procedure out?
> 
> I was under the impression (and that's all it is -- impression, IIRC) that every time the sensor/wheel began turning, the head unit began comparing the distance covered (via GPS) to the wheel's rotation and would recalibrate based on how many rotations it took to travel X distance. I actually believed that I could run my speed sensor & Garmin head unit on my 29er today and my neighbor kid's BMX bike tomorrow and the unit's auto recalibration would discover the wheel size change and recalibrate automatically. Every time.
> 
> ...


I don't have any details one way or another aside from my own experience that he is correct that the head unit tells you when it is calibrating and when it finishes.

I do not know what conditions prompt it to begin calibrating, but mine has done so in the middle of a ride before after it had been paired and calibrated for months.

There must be some sort of discrepancy threshold between the existing wheel senaor calibration and what the gps distance is reporting, but again, no idea what it is.

Sent from my VS995 using Tapatalk


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## Lone Rager (Dec 13, 2013)

Sparticus said:


> Interesting. Without intending to sound as if I doubt you LR, how certain are you of the one-time calibration? May I ask how you found this procedure out?...


Participating in the appropriate Garmin Edge forum and trying and verifying things as I needed to. I joined the Garmin forums back in 2010 when I got my first 705. Garmin Edge units have always been bug laden and wack. Just check those forums. I currently have 5 bikes and 3 different Edge units. There's been a lot of water under the bridge and the way these devices handle things have changed, like having no bike profiles, to having bike profiles, back to having no bike profiles and activity profiles, etc...

https://forums.garmin.com/forum/into-sports/cycling


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## Sparticus (Dec 28, 1999)

Lone Rager said:


> Hanging out on: https://forums.garmin.com/forum/into-sports/cycling


Cool. And how sure are you that the information you heard on that forum is correct? I mean has anybody actually read this in a Garmin manual or direct from a Garmin employee? Or did it just come from a bunch of hacks like we have here on MTBR, half of whom spout off or opine without having a clue?

Obviously not talking about you, LR. (Nor you, Harold.) 
=sParty

P.S. Forgive me if I come off grumpy. I'm actually not grumpy, it's just that we got nearly a foot of heavy, wet snow overnight and it's still dumping. We're landlocked here at Blue Rider Farm due to trees across the road on either side of our driveway. Power went out around 3:30am and we have no idea whether it'll be hours or days before it comes back. The heavy snow is bringing trees and branches down - it's unsafe to be out. In short, I can't ride my bike.


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## Lone Rager (Dec 13, 2013)

It's somewhat complicated, but those are the official Garmin sponsored forums. One of Garmin's sports devices shortcomings is it really thin user documentation. There is no detail on how to use many features or how any of them work. They leave it up to the users to try guess and work it out. That's a lot of what goes on in the forums, figuring things out and finding issues. For years it appeared that Garmin paid no attention to them only having moderators to keep things under control. The past few years than have been some two-way relay of information between some of the moderators and Garmin developers, but not a lot. My suggestion would be to go check out the forums, sign up, and judge for yourself.


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## Sparticus (Dec 28, 1999)

Lone Rager said:


> It's somewhat complicated, but those are the official Garmin sponsored forums. One of Garmin's sports devices shortcomings is it really thin user documentation. There is no detail on how to use many features or how any of them work. They leave it up to the users to try guess and work it out. That's a lot of what goes on in the forums, figuring things out and finding issues. For years it appeared that Garmin paid no attention to them only having moderators to keep things under control. The past few years than have been some two-way relay of information between some of the moderators and Garmin developers, but not a lot. My suggestion would be to go check out the forums, sign up, and judge for yourself.


Thanks. While I appreciate your suggestion, I already waste too much of my life scrolling only these boards. I'll prefer to hope that helpful & knowledgable people such as yourself will continue to offer up the bottom line when takers like myself request info.

I'll run separate speed sensors on each bike and make sure each one is correctly calibrated. (Hope I can figure out how to do that.) Anyway, thanks again, LR.
=sParty


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## Harold (Dec 23, 2003)

Weak documentation has always been Garmin's worst quality.

Many years ago, someone was running a "Garmin Oregon Wiki" website that maintained a massive pile of documentation specifically for Oregon receivers, but was also surprisingly useful for any Garmin GPS. It appears the website doesn't exist anymore, but you can get it on the wayback machine.

https://web.archive.org/web/20100311001136/http://garminoregon.wikispaces.com/

Garmin fiddles with its firmware so much that it's really difficult to maintain pages like this for every device, when people have to figure everything out from scratch again when Garmin messes with the user interface for a new model or makes a fairly large change through a firmware update.


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## Sparticus (Dec 28, 1999)

Harold said:


> Weak documentation has always been Garmin's worst quality.
> 
> Many years ago, someone was running a "Garmin Oregon Wiki" website that maintained a massive pile of documentation specifically for Oregon receivers, but was also surprisingly useful for any Garmin GPS. It appears the website doesn't exist anymore, but you can get it on the wayback machine.
> 
> ...


SMH! I'm not gonna lie, for years I've wondered how Garmin continues to remain in business.
=sParty


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## Harold (Dec 23, 2003)

Sparticus said:


> SMH! I'm not gonna lie, for years I've wondered how Garmin continues to remain in business.
> =sParty


When it comes to basic day-to-day use, the lack of documentation isn't that big of a deal. It does become problematic when you're digging into more advanced functionality and you want to figure some things out.

I remember in the early Edges, the wheel sensor didn't even get used unless the GPS signal was lost entirely. Say you were in a road tunnel, or a deep canyon, or riding on a trainer. Garmin changed that with a firmware update so the Edges would preferentially use distance/speed data from the sensor if the sensor was present. I think it was made even more nebulous by the fact that the head unit would display wheel sensor data on the bike, but not record it in the file, or something like that. But they changed it through firmware because it was a complaint people made for years after they found out that the wheel sensor data wasn't always being used.

Honestly, though, I haven't found any of Garmin's competitors to be any better about documenting these subtle details of their devices and their use, either.


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## Lone Rager (Dec 13, 2013)

Sparticus said:


> Thanks. While I appreciate your suggestion, I already waste too much of my life scrolling only these boards. I'll prefer to hope that helpful & knowledgable people such as yourself will continue to offer up the bottom line when takers like myself request info...


I completely understand that, and thank you for your vote of confidence, however, Harold and I are the only respondents here to your query and I certainly haven't provided a definitive answer and explanation.

On the Garmin forums you do encounter device nerds who retain the accumulated knowledge of the forum and can often answer questions pretty well, or point to authoritative information. Another option is to contact Garmin customer support. They'll often answer questions but I have my reservations about the accuracy of their responses. I'm not one of those nerds on the Garmin forums, I just go there when looking for answers or information when my Garmin starts doing something stupid.


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## NordieBoy (Sep 26, 2004)

My Edge 130 is the only Garmin I've had with annoying "issues".
It sometimes stops reading from all Ant+ sensors (HR, Cad, Power, Speed) and then a few minutes later will pick them up again.
I've had it on 2 rides now on 2 different bikes.

I like devices with niggles, makes them less perfect, more fun.


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## ghettocruiser (Jun 21, 2008)

The Edge 520+ does a full sensor drop every so often and immediately picks them up again. Regardless of whose sensors are connected. It sometimes even drops the phone and the rear radar at the same time.

I have three of the XOSS sensors and they work fine for either speed or cadence for 1/2 the price of Garmin sensors. 

You have to remove the battery to toggle between speed mode and cadence mode, and be careful putting it back in, it's easy to bend the battery contact if the arrow on the battery cover isn't lined up.


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## Sparticus (Dec 28, 1999)

ghettocruiser said:


> The Edge 520+ does a full sensor drop every so often and immediately picks them up again. Regardless of whose sensors are connected. It sometimes even drops the phone and the rear radar at the same time.
> 
> I have three of the XOSS sensors and they work fine for either speed or cadence for 1/2 the price of Garmin sensors.
> 
> You have to remove the battery to toggle between speed mode and cadence mode, and be careful putting it back in, it's easy to bend the battery contact if the arrow on the battery cover isn't lined up.


Thanks for the advice. I don't measure cadence so I'll just leave the sensor in speed mode.

Why do they call it speed mode, anyway? To me it's distance mode.

I live in Oregon where we get copious rain. Insidious rain. Heinous rain. Endless rain. Is the XOSS sensor waterproof? Thanks again.
=sParty


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## ghettocruiser (Jun 21, 2008)

I haven't had any water get in. 

The rubber fraction pad that mounts against the hub body also covers the battery compartment, which serves as a bit of a seal. If I was planing for bicycle aquatics, I'd just put a piece of electrical tape over the battery cover (which is flat) and then put the rubber pad over that. There's no other holes in it, so that's gotta be totally waterproof.

Documentation with this thing is poor, so remember when pulling and re-installing the battery that red LED is speed mode, and blue LED is cadence mode.... I think.


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## Sparticus (Dec 28, 1999)

ghettocruiser said:


> I haven't had any water get in.
> 
> The rubber fraction pad that mounts against the hub body also covers the battery compartment, which serves as a bit of a seal. If I was planing for bicycle aquatics, I'd just put a piece of electrical tape over the battery cover (which is flat) and then put the rubber pad over that. There's no other holes in it, so that's gotta be totally waterproof.
> 
> Documentation with this thing is poor, so remember when pulling and re-installing the battery that red LED is speed mode, and blue LED is cadence mode.... I think.


Cool -- thanks!
=sParty


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## Catmandoo (Dec 20, 2018)

Sparticus said:


> Why do they call it speed mode, anyway? To me it's distance mode.
> 
> =sParty


In addition to the distance, it makes the speed readout more accurate, moment to moment. There can be lag with the GPS only indicated speed. Many roadies that do group rides need to maintain a steady speed. Relying on a GPS only can vary, thus you can find the maintained speed you are trying for, to be slow to display. With a speed sensor, the readout is more accurate.


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## tedsti (Oct 22, 2004)

I'm looking for confirmation on this since I can't find where I originally saw it. I remember reading that Strava was acquiring high res GPS maps of many of the trails and correcting your GPS data to their high res maps. It sees you rode section X of trail Y and your phone's GPS track reads 2.1 miles but the high res map says 2.4 so it corrects your data to 2.4. So even though your phone GPS may not give as good of data as a Garmin, Strava is reporting better distance values on curvy singletrack. Strava is correcting data recorded in their app but is not correcting data imported from Garmin. I think they are correcting data imported from Wahoo though. This all makes sense when 5 friends who record on their phone and one with a Wahoo all get 11.2 miles on a ride and I get 9.7 with my Garmin.


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## Lone Rager (Dec 13, 2013)

^^^ if your GPS data shows you off the trail, maybe you were. Correcting what you actually rode to what it thought you might have ridden strikes me as being a bit flaky.


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## Harold (Dec 23, 2003)

tedsti said:


> I'm looking for confirmation on this since I can't find where I originally saw it. I remember reading that Strava was acquiring high res GPS maps of many of the trails and correcting your GPS data to their high res maps. It sees you rode section X of trail Y and your phone's GPS track reads 2.1 miles but the high res map says 2.4 so it corrects your data to 2.4. So even though your phone GPS may not give as good of data as a Garmin, Strava is reporting better distance values on curvy singletrack. Strava is correcting data recorded in their app but is not correcting data imported from Garmin. I think they are correcting data imported from Wahoo though. This all makes sense when 5 friends who record on their phone and one with a Wahoo all get 11.2 miles on a ride and I get 9.7 with my Garmin.


So I think you're mixing some things up. First off is the Slide tool. They don't run this on every activity. You have to choose to run it specifically, since it's more or less in a test environment.

https://labs.strava.com/slide/

Then there's the heatmap, which is used for that tool.

https://www.strava.com/heatmap#5.06/-102.59941/42.22904/hot/ride

You may be thinking about the elevation database they're building, which they use for elevation corrections. Existing digital elevation models are pretty bad for Strava's purposes, in all honesty. There's a much high accuracy one in development right now (global coverage), but implementing it for Strava's purposes will have other challenges. Strava is doing their own thing for their own purposes.

https://support.strava.com/hc/en-us...-Strava-s-Elevation-Basemap?mobile_site=false

Also, there IS a distance correction that Strava uses. But it isn't crowd-sourcing that correction factor. And no, it's not running the calculation on specific devices by default (note at the bottom, every Garmin has a "distance" stream included in the data file. There is a heirarchy that determines which source is used. It's probably a bit outdated since the new magnetless sensors are detecting the earth's magnetic field, and not the old style magnet/sensor type like the GSC10.

https://support.strava.com/hc/en-us/articles/216919487-How-Distance-is-Calculated

The last paragraph of this article also mentions a threshold of bad data that initiates a major reprocessing of the file. Strava has a better article on it somewhere, but I can't find it. I've encountered this before.

https://support.strava.com/hc/en-us...tivity-s-distance-different-than-my-friend-s-



> There is a threshold of outlier/bad GPS data that will cause Strava's file parser to recalculate your activity's distance by blending the GPS and device distance together in order to correct those areas with bad GPS/device data. This bad data detection is an effort to improve the quality of uploaded data on Strava and does solve many issues with GPS inconsistencies. This reprocessed distance can differ from the distance data originally reported by the GPS device, especially if a speed sensor is present.


It happened to me with a Bryton Rider 310 with a wheel sensor. I know that the device was reporting the correct distance based on wheel sensor data before uploading to Strava. However, there were some GPS errors (the type referred to in one of the other articles as "stuck points" where the time was different, but the location of the point was a duplicate) that forced a recalculation. Strava more or less tossed the wheel sensor distance stream when it reprocessed the bad GPS data. I seem to remember when I dealt with this years ago that Strava had some sort of internal "trusted device" status, too, which was related to them tossing my wheel sensor data when they'd reprocess my Bryton's GPS data.

So in short, Strava has a whole lot going on and their implementation criteria is pretty complicated. There's low level smoothing that they do to just about everything, ranging up to full reprocessing when there's bad data, to actually rejecting your activity if the data is really bad. My Garmin Edge 520 records solid data. No errors or anything like that, for the most part, that I've been able to determine. While Strava most likely applies some of its low level smoothing to the track, it does not do any major reprocessing, and it uses the wheel sensor distance data that I record.

When I compare activities using their Flyby tool, I see that the positional data my Garmin records is pretty solid compared to other devices.










This is a screenshot of the Strava Flyby from a recent ride I did. We were all part of the same group, more or less. My track is at the top, and the guy just below me shuttled the ride. Everybody else rode up a long greenway climb before, which is why they're behind us. It's kinda impossible to do really good distance comparisons because of that plus because of differences in start/finish points. Too many confounding factors. But I did zoom in on a section that was difficult for accurate recording. It was twistier than most of the trail, it was getting into a cove with steep valley walls, and the forest cover was a little thicker here.

I went through the list to see what devices everybody was using. There were 3 different Garmins and the rest were all iphones running the app. Nobody had a "perfect" GPS track through this section (as indicated by alignment with the trail on strava's basemap, though assuming that as the ideally accurate position is problematic by itself). Even filtering out the iphones and looking only at the Garmins, the Garmins differed from each other some. I wish Strava gave some indication of phone model when it refers to the app. Unfortunately all it says is the basic OS (iPhone, Android, etc). If you look closely here, you'll see that 2 of the 3 iPhones were especially problematic. The dark blue track and the pale green track. The iPhone illustrated by the pale blue track was markedly better than the other iPhones, and was much closer to the three Garmin devices. The pale blue track also shows 2 laps on this loop, btw, which allows you to see how the tracks of the 2 runs differ (within-device variation, rather than between-device variation).

My point here is that all the extra wandering on the inaccurate tracks actually adds distance artificially. An ACCURATE GPS track is going to shortcut all those corners, and show a short distance. This is why I run a wheel sensor. Another spot I'm not showing is on the climbing side. The start of the singletrack on this trail has 15 switchbacks that get steep and they hurt. Most mortals have to stop somewhere along the way. I did. My GPS, however, did not wander all over the place when I stopped. It did a tiny bit, but the wandering was pretty tight. That really bad pale green iPhone track wandered a TON when that rider stopped for a breather. Accumulating distance the whole time. Looking at GPS data this way gives you more information than looking at the "total distance" metric does. As riders, sure, it's natural that we're going to care about that metric. BUT, that distance metric is going to depend on where we actually rode. Where we rode is what REALLY matters when determining that total distance. Where we rode is what matters when looking at different segments on Strava. If we care about those things, we should do two things about it:

1. Care about the positional accuracy of the recording equipment you use. You'll never get perfect. But care enough that you pay attention to it, and look for something better if you find that the method you're using could use some improvement.

2. Use a wheel sensor. It's simple enough. I'm glad that when Garmin started offering a "mountain bike bundle" the wheel sensor is the major inclusion there. I have wheel sensors on all the mountain bikes I own that get ridden regularly (and I have for many years). I don't currently have any wheel sensors on my road bike, though I might put one on before the winter. Because tunnels. Otherwise, the roads are far less twisty than the trails I ride, and the wheel sensor doesn't make as much of a difference with distance accuracy. I also don't do specific road/gravel rides nearly as frequently as I ride mtb trails, so I care less about it.


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