# Garmin fenix 3 HR accuracy



## Miami_Son (May 6, 2010)

My girl gave me one for my birthday in August. Since then I have been quite disappointed in the accuracy. When I ride out in the open it is within a few hundredths of my carefully-calibrated bike computer. But when I ride the trails in the woods with a lot of tree cover (most of my riding) it can be off by as much as a mile in 10. That's a 10% error! Even my cell phone with Map My Ride can do better than that. All Garmin can suggest is to make sure I have the latest version of the software on the watch and computer. What? both are set to auto-update, so that's of no help. Anyone else seeing this kind of inaccuracy or do you just go by what the watch tells you and have nothing to compare it to? This thing wasn't cheap, but it's worthless if I can't trust what it says.


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## Harold (Dec 23, 2003)

Miami_Son said:


> My girl gave me one for my birthday in August. Since then I have been quite disappointed in the accuracy. When I ride out in the open it is within a few hundredths of my carefully-calibrated bike computer. But when I ride the trails in the woods with a lot of tree cover (most of my riding) it can be off by as much as a mile in 10. That's a 10% error! Even my cell phone with Map My Ride can do better than that. All Garmin can suggest is to make sure I have the latest version of the software on the watch and computer. What? both are set to auto-update, so that's of no help. Anyone else seeing this kind of inaccuracy or do you just go by what the watch tells you and have nothing to compare it to? This thing wasn't cheap, but it's worthless if I can't trust what it says.


What recording interval do you have set?

Do you use a wheel sensor?


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## Miami_Son (May 6, 2010)

Harold said:


> What recording interval do you have set?
> 
> Do you use a wheel sensor?


On the Garmin? Not sure what you mean by interval. Why would I need a wheel sensor for a device with built in GPS?


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## Harold (Dec 23, 2003)

Miami_Son said:


> On the Garmin? Not sure what you mean by interval. Why would I need a wheel sensor for a device with built in GPS?


If you don't know the answers to those questions, you haven't done enough reading.

A dedicated GPS receiver makes few attempts to be idiot proof the way most phone apps do. You need to do your research. There are a lot of device settings that can make recorded results better or worse and you need to understand them since you now have a device which has such settings.

First, recording interval. This is the frequency your device records a data point. All devices have a recording interval of some sort. Some let you change it depending on what you're doing. Some don't. The faster you travel over a twisty route, the more frequently you want the device to record your position. This allows it to more accurately capture the curves and corners of your route. The most frequently most consumer devices can record is 1 recorded point per second, though there are a few out there that can do 10 per second. There's a very high chance your Garmin is not set to record that frequently out of the box, so if you did not change it, it is recording less frequently. There are advantages to recording less frequently, but for twisty trails on a mountain bike, you want the most frequent recording you can get.

But when it comes to the most accurate distance measurements, even that might not be enough. You see, every time the GPS records a point, it's connecting those points with a straight line. So it's cutting corners short. It's not recording the full curve of those corners, and that is where the wheel sensor comes into play. It IS recording the full curve of the corners, as well as every little up and down in the trail (you see, GPS-only distance measurements are horizontal distance only. they do not account for the tangent length of slope. a wheel sensor does).

Now, why does your phone seem more accurate in measuring distance? That relates to a number of things. For one, it may be that the phone app is doing some smoothing to "fudge" the corners. Strava does this. Strava also throws out bad data, and MapMyRide probably does, also. Garmin has its own data processing on the device, so it's not easy to see the results of said processing. But even with this processing in place, phones record some bad positional data that adds length to distance measurements.

I see this on Strava when I use the Flyby feature to see who else was riding when I was out. I can click on the other activities and see what was used to record the files. Some phones record truly terrible GPS data, so the track is all over the place. The total distance might be pretty close to mine (wheel sensor on a Garmin set to 1 sec recording), but my GPS track is FAR more accurate.

An accurate GPS device will ALWAYS underestimate distance traveled unless you have a wheel sensor to override GPS distance. An inaccurate GPS will either be close to the wheel sensor, or possibly even well above it. The proof is in the actual positional data the device recorded.

Dedicated GPS receivers are good enough now that it's unlikely that forest cover is the reason for the inaccuracies. GPS chips and software are able to process out a lot of the multipath errors caused by the signal reflecting off of trees and hillsides. I have been using GPS hardware for nearly 20 years and I've seen the progression of data quality in that time. When I started using GPS hardware, you couldn't use it on your bike even out in the open. Just to record a single point with any reasonable accuracy, you had to stand in one place for several minutes, which might be a whole hour in deep forest. It's not anything like that anymore.


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## Miami_Son (May 6, 2010)

Informative, if not condescending, response. I know what recording intervals are. As it relates to the fenix 3, I have been through all the menus and not seen any way to change the factory settings for this, which is why I don't know why you brought it up. I also don't see how a wheel sensor would have anything to do with the GPS on my watch., so again I'm at a loss to understand why you brought that up. The fenix 3 is supposed to be accurate out of the box. Since the only time it isn't is when I have heavy brush/trees overhead I have to believe that there is a problem with its ability to track in such conditions. Was simply wondering if anyone else had experienced the same, but thanks for the treatise on GPS operation.


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## Harold (Dec 23, 2003)

Miami_Son said:


> Informative, if not condescending, response. I know what recording intervals are. As it relates to the fenix 3, I have been through all the menus and not seen any way to change the factory settings for this, which is why I don't know why you brought it up. I also don't see how a wheel sensor would have anything to do with the GPS on my watch., so again I'm at a loss to understand why you brought that up. The fenix 3 is supposed to be accurate out of the box. Since the only time it isn't is when I have heavy brush/trees overhead I have to believe that there is a problem with its ability to track in such conditions. Was simply wondering if anyone else had experienced the same, but thanks for the treatise on GPS operation.


They are both relevant issues. It's not my fault you can't understand why. They are relevant to ALL fitness GPS receivers used on the bike.

Garmin's website tells me that your device is compatible with a wheel sensor. The wheel sensor improves accuracy of distance measurements (and speed). It effectively overrides the GPS-calculated distance, which will always have errors inherent in its calculated values, as I described above.

The value of GPS data in a bike computer has never been that it calculates distance more accurately than a cheap cateye. The fact that it records your location presents interesting ways to visualize and analyze your rides. A wheel sensor is and always will be the most accurate way to calculate distance on a bike.

I am a little condescending about it because people constantly post butthurt threads exactly like this because their Garmin doesn't work exactly how they wanted it to work. A little bit of research will turn up countless discussions about why 1 second recording is essential on the bike, and why a wheel sensor is vital for mtb riding on twisty trails.

You should spend some time on dc rainmaker's site. According to him, the fenix 3 DOES offer 1sec recording as an option. So what I'm seeing here is that you just haven't found it. The feature on most Garmins is under the settings for tracks. Not sure about the fenix 3 specifically, but it's probably somewhere similar.

https://www.dcrainmaker.com/2015/01/garmin-fenix3-firstlook.html


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## Babas (Mar 5, 2008)

http://static.garmincdn.com/pumac/fenix3_OM_EN.pdf

Page 17, left bottom - GPS
Page 19, left top - Ultratrack

For anything else - read Harold's post again, he knows what he is talking/writing about


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## Miami_Son (May 6, 2010)

There are only 2 settings for recording interval, Auto and each second. The default is Auto. I will try the latter. Any recommendations for a wheel sensor?

As I said, this discrepancy only occurs when riding wooded trails. On the road or out in the open the difference is negligible. I still want to know if anyone else has experienced the same. I did look through the forum before I posted and did not see a similar specific question, just general accuracy complaints.


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## Harold (Dec 23, 2003)

Miami_Son said:


> There are only 2 settings for recording interval, Auto and each second. The default is Auto. I will try the latter. Any recommendations for a wheel sensor?
> 
> As I said, this discrepancy only occurs when riding wooded trails. On the road or out in the open the difference is negligible. I still want to know if anyone else has experienced the same. I did look through the forum before I posted and did not see a similar specific question, just general accuracy complaints.


Every gps suffers a reductions in accuracy at times. ALL OF THEM. Yeah, thick tree cover can make it drop some. But I suspect there's more than that going on. I see a compounding effect on northern slopes, since GPS satellites tend to be in the southern sky and at least some tend to be blocked by hills and mountains to the south. Twisty trails tend to be less accurately recorded than straight roads, too. For the reasons I mentioned and more. Skyscrapers will add inaccuracy to your GPS tracks, too. Probably moreso than trees and hills, based on what I've seen on my own rides.

Do you have GLONASS enabled? Enabling GLONASS satellites (Russian version of GPS) gives you more possible satellites to connect to. Tends to improve accuracy (especially in northern latitudes, where GLONASS tends to have a better focus). On my Edge 520, with GPS and GLONASS enabled, I'm still able to record a pretty accurate track when riding underneath a 4 story parking structure.

Garmin's magnetless wheel sensor has been pretty good for me. You can get some combo speed/cadence sensors out there. Last I looked, Garmin's combo sensor (not made anymore) fetches a ridiculously high price on places like ebay and such. It worked well enough, but bumping it with your foot, or in a crash, or when transporting the bike could knock the magnet out of alignment. kindof a pain. Also some bikes aren't friendly with chainstay mounted sensors. Mine isn't (brake line routed on top of NDS chainstay).


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## Miami_Son (May 6, 2010)

I hope you didn't take my responses to mean I wasn't appreciative of your help. I did finally find the interval settings and changed it. It was very deep into the various menus and not very intuitive to find. Will see if that makes a difference. I may also look into getting a wheel sensor. There was also a setting called "Datum" with lots of different choices. Which is best for me being in S. Florida? I chose Cape Canaveral since that is not far from me, but not sure what I should be using. A setting called "Spheroid" appears to change with the Datum setting. Thanks.


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## Harold (Dec 23, 2003)

Miami_Son said:


> I hope you didn't take my responses to mean I wasn't appreciative of your help. I did finally find the interval settings and changed it. It was very deep into the various menus and not very intuitive to find. Will see if that makes a difference. I may also look into getting a wheel sensor. There was also a setting called "Datum" with lots of different choices. Which is best for me being in S. Florida? I chose Cape Canaveral since that is not far from me, but not sure what I should be using. A setting called "Spheroid" appears to change with the Datum setting. Thanks.


Datum and spheroid settings are really only going to affect the coordinates provided by the device. They are mostly not relevant for simple tracking, because the coordinates actually recorded by the device are pretty much always the same (WGS 84, IIRC). Where the datum IS relevant is when you are plotting those coordinates the device tells you onto a paper map. If you use the wrong datum, the coordinates won't match up to the same spot. So, when using the device in conjunction with a paper map, you use the datum the map uses.

Otherwise, I'd leave it in WGS 84 or NAD 83 (they are similar enough that they are almost interchangeable).

What they actually MEAN is a simple gets kinda technical. Let's start with the spheroid. The spheroid is basically just the mathematical model that describes the shape of the earth. It's not a perfect sphere. It's sorta roughly ellipsoid shaped, flattened at the poles, but the southern hemisphere sorta bulges a bit. And it's lumpy. The mathematical model describing the shape of the earth gets more accurate as time goes on. Mostly due to better measuring tools, but also because of plate tectonics moving things around over time.

Now, the datum is built on top of the spheroid (each datum is based on a particular spheroid), and is better able to account for local variations in the shape of the planet. That's why you see so many different datums. Each one tends to be better for certain areas. Some for wider areas, some for narrower ones.

That's not even accounting for the ways that 3D space is translated to a 2D map (the projection). For each datum, you can have MANY different projections, which might be better at showing distances, or areas, or balancing between the two.

This may be more technical than you want, but this is an extremely technical question.

FAQ: What do the terms geoid, ellipsoid, spheroid and datum mean, and how are they related?

So for your case, I'd put the datum back to WGS84 unless you need it to be something else to work with a particular map that you're using.


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## Miami_Son (May 6, 2010)

Hi Harold, I went and got a Garmin speed sensor. Paired it with my fenix 3HR and validated it functioning. Now, does the GPS remain active on the watch, but the sensor overrides the speed and distance readings? How does the sensor know how big the wheel is and how much ground it covers in a revolution? There was no setup beyond pairing with the watch and nothing in the instructions about it. Thanks.


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## Harold (Dec 23, 2003)

Miami_Son said:


> Hi Harold, I went and got a Garmin speed sensor. Paired it with my fenix 3HR and validated it functioning. Now, does the GPS remain active on the watch, but the sensor overrides the speed and distance readings? How does the sensor know how big the wheel is and how much ground it covers in a revolution? There was no setup beyond pairing with the watch and nothing in the instructions about it. Thanks.


Yes, the GPS remains active. The computer basically computes new speed/distance values based on the input from the sensor and records new data fields in the .fit file the device saves. Sometimes that extra data might get tossed out and strictly GPS-based speed and distance may still be calculated. Strava will do this if there's erroneous GPS data. I haven't had it happen with my Edge 520 that I've noticed. But it did happen with a Bryton GPS I had, because the Bryton would screw up the time for each data point it recorded. Strava would see that, decide that all the data was fubar, and recalculate everything.

Garmin has an auto calibrate function for its GPS based computers with wheel sensors. That's the default setting. If you dig into the setup menus and find the sensor (and its associated ID) now that you have it paired, it SHOULD give you the ability to manually override the value that it calculates. I'd go out and ride with it, and ride something long and straight so the computer does its auto calibration. If you have a wheel circumference value you've already calculated, or wish to determine that by using the rollout method, you can absolutely do so and double-check the auto calibrated value. I've done it, and found the numbers within a couple mm, so I've left the auto calibrated numbers alone. Partially because that also gives the computer the ability to recalibrate them on the fly later on (say, when tire pressure changes dramatically, which happens on my fatbike seasonally - 9psi is my summer pressure, but may go as low as 2psi for deep, fresh snow in winter, and that dramatically changes the effective circumference. I want the computer to automatically detect that difference).


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## Miami_Son (May 6, 2010)

Finally had a chance to do a covered ride with the new Garmin speed sensor. This amount of error I can live with. Thanks for all your help, Harold.:thumbsup:


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## Harold (Dec 23, 2003)

good to see solid results. You could get that error down to less than the ability of the devices to measure if you wanted to be super meticulous about setup.


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## Miami_Son (May 6, 2010)

Considering I was getting about 10% error on some trails before, this is fine.


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## zendo (Aug 29, 2015)

@harold I would assume upping the recording interval would affect the battery life? If so, is it dramatic?


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## Harold (Dec 23, 2003)

zendo said:


> @harold I would assume upping the recording interval would affect the battery life? If so, is it dramatic?


It is negligible. To the point that I have never noticed a difference on any device I have used.


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## Miami_Son (May 6, 2010)

I have not seen any difference in battery life since the change.


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## Harold (Dec 23, 2003)

Miami_Son said:


> I have not seen any difference in battery life since the change.


I have heard it is a measurable difference, but for practical purposes, I have also not noticed.


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## sunvalleylaw (Jun 21, 2010)

Great thread guys. Really considering a refurbished 3 HR. I want the GPS and related data, and want the heart rate and related data also. Plus, it does other sports I want it to do. How has the heart rate sensor worked for you? I understand at lower heart rates, the wrist sensor is fine but at higher performance, having the chest strap is better. I am not an olympic athlete, but I would like to collect some reasonable data to start tracking and start motivating progress in fitness.


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