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gps_test_points2

Over the summer there was an interesting discussion in the comments section of this article about the accuracy of dedicated GPS units from companies like Garmin compared to the GPS data collected via smartphones. I’ve personally been biking with a GPS since 2001 (13 years!) and my take was that neither is any more or less accurate than the other. Still, I was curious: was there any difference in accuracy between various types of GPS devices?

To find out, ckdake and I took 10 GPS devices–including 2 wristwatches, 2 bar-mounted units, 3 smartphones, 1 GPS-equipped helmet camera, 1 handheld device, and even a tablet–to a local quarter-mile track to see how each performed. We did our best to mount each unit according to its normal configuration; that is, wristwatches on a wrist, bar-mounted units on the bars, etc. All were rigged to a single rider (me!) and the test started and stopped at the same point on the track after riding 10 laps (2.5 miles). I stuck to the middle of lane 1 (thanks for the velodrome tip Chris!) and each GPS was started (but not moved) one at a time. We didn’t test timing on the devices since each unit is designed to sync time with the GPS satellites themselves, which meant we wouldn’t need to start and stop all devices at the same time.

Here are the results, ordered by distance accuracy.

Device Distance Error
iPhone 5 (Strava) 2.5019 0.08%
Asus Android Tablet (Strava) 2.5124 0.50%
Garmin Forerunner 405CX 2.5208 0.83%
Magellan Cyclo505 2.5239 0.96%
iPhone 5 (Garmin Fit) 2.5328 1.31%
Garmin Edge 500 2.5334 1.34%
Garmin Fenix2 2.5501 2.00%
Nokia Lumia (GPS Logger) 2.5542 2.17%
Garmin VIRB Elite 2.5795 3.18%
Garmin 60CSx 2.2529 -9.88%

Looking at the animated graphic at the top of this article, you can get a sense for another measure of accuracy–basically the tightness and “correctness” of the ride data. The actual route hugged the inside lane for each lap, though you can see some of the data veers toward the inside of the field and the edges in some cases (click here for an interactive plot). Subjectively, here’s how I would rank the accuracy based on the route plot:

Top 5: Asus Nexus Tablet (Strava), Nokia Lumia (GPS Logger), Garmin Fenix2, Magellan Cyclo505, and Garmin Forerunner 405.

Bottom 5: Garmin GPSMap60CSx, iPhone 5 (Strava), Garmin Edge 500, iPhone 5 (Garmin Fit), and Garmin VIRB Elite.

Interesting to note that the most accurate in terms of distance are not necessarily the most accurate at plotting route data. Keep this in mind when using GPS to map trails.

See Also
By Greg Heil
 

Accuracy

Overall, the error rate is pretty low–around 3% or less for all but one device. Still, only 4 out of 10 were within 1% of the actual distance. On average (if we throw out the Garmin 60CSx) each device is off by about 1.4%, which is probably a good number to assume whenever analyzing your own data.

Error Direction

Surprisingly, all but one of the devices over-reportedย the distance, which either means our track is slightly longer than we assumed or something else is going on. In my own experience I’ve noticed most GPS units report distance on the high side (for example, clocking 104 miles in a 100-mile race), and perhaps it’s by design to make consumers feel faster/stronger than they really are. Product engineers know the device isn’t completely accurate so why not just estimate everything on the high side to keep riders happy? ๐Ÿ™‚

Another more serious possibility is that distance was added to each track during the time it took us to start and stop all the devices during our test (you can see this in the lower right corner on some of the plots above). Basically if you take pretty much any GPS, hit start, and set it on the ground, after a few minutes you’ll find that your GPS has logged several feet of distance without moving. ย How is this possible?

GPS units never know exactly where you are located–they’re only accurate to within a few feet. So each time a GPS checks with the satellite, it’s calculating a slightly different position, even when you’re not moving.

Driving Factors

gps_test_overlay

Perhaps one of the biggest misconceptions about GPS accuracy is that it’s all about the chipset (like the SiRFstarIII)ย in a particular device. While it’s true that some GPS chips are more powerful and/or sensitive than others, there are many, many factors that can affect distance calculations.

But let’s stick with the idea that chipsets affect distance accuracy for a moment. Two of the top four GPS units in our distance accuracy test are regular old smartphones that determine location not just with GPS but also using wi-fi and cell tower signals to triangulate a position. And the Magellan Cyclo505–#4 above–is also wi-fi enabled, though we don’t know if it’s actually using wi-fi to improve location accuracy. Still, having more sensors available to calculate location isn’t a guarantee of accuracy–the Garmin Fit app on an iPhone 5 ended up middle of the pack in our distance test, and the Nokia Lumia smartphone ranked 8 out of 10.

So what the heck happened to the Garmin GPSMap60CSx–it was off by nearly 10%?! There’s another factor at play here, and it’s known as polling frequency. Basically, GPS units check in with the satellite on a regular basis to get the user’s position and save that information, along with the time, to record your ride. Many apps and GPS units allow you to set the frequency yourself. For example, if you record your position once per second, you’ll get better data than if you record only once per minute. The tradeoff is more frequent polling will drain your battery faster and fill up your memory quicker.

garmin_gpsmap60_points

In the case of our track test, the GPSMap60CSx was set to record much less frequently than the other units, resulting in “cut off” corners that significantly shortened the distance (remember, a straight line is always the shortest distance between two points). Fortunately this is an easy fix and has nothing to do with the accuracy of the unit itself.

This is what more frequent polling looks like (specifically, 1 point per second):

garmin_fit_points

Some Garmin units (and perhaps units from other GPS manufacturers as well) have a “smart polling” option that doesn’t have a set polling frequency. Instead, the GPS records a point only if it’s not in a straight line with the point before it. A couple of the units we tested used this method which you can see below. In this test smart polling didn’t seem to affect accuracy but for a less-regular course (say, a twisting MTB trail) the GPS may have trouble keeping up with the rider’s constantly-changing vector.

garmin_fenix2_points

Software Manipulation

One final factor in our GPS distance accuracy test is the use of post-ride software processing. Basically if the GPS data is uploaded to a website (Strava, Garmin Connect, MapMyRide, etc.), chances are that the data will be manipulated. This is particularly true for elevation data (which we’ll discuss in a follow-up post), but I suspect even distances are being tweaked behind the scenes for some of the devices we tested.

In particular, it’s either incredibly lucky that the Strava iPhone app got within 0.1% of our target distance or that Strava is guessing (correctly) that our “workout” occurred on a quarter-mile track and is adjusting distances accordingly. Looking at the trace from the Strava iPhone app, it’s easily the messiest and least “tight” trace which, again, suggests it was either lucky or is making distance adjustments behind the scenes. The fact that the second-most accurate distance tracking also came from the Strava app but running on a low-powered Android tablet lends some credibility to the latter suggestion*.

Conclusions

This is by no means a definitive GPS distance accuracy test. In fact, if we conduct this test again (which we probably will at some point), my hypothesis is the ranking will be completely different. And aside from the track test, a test in an area with zero cell phone or wi-fi signals over a known distance (not a quarter-mile track) could help isolate the additional sensor effect and the software manipulation issues. Bottom line: if you’re choosing a GPS device or app for clocking your rides, don’t focus too much on accuracy… they’re all pretty good, but none are bulletproof.

Update: As many commenters have pointed out, the track we used was probably 400m, not a quarter mile. It’s an embarrassing mistake, especially since I ran track all four years of high school. ๐Ÿ™‚ However, it still doesn’t change the accuracy rankings. Instead, it just means that all but one unit was off by an additional 0.6% (the Garmin GPSMap 60CSx actually improves by that amount.)

* Being curious, I tested the theory that Strava is recognizing track workouts–and modifying them–by uploading raw data from one of the other devices used during the test. The result: no change in the distance. Still not definitive, but that theory is looking shaky.

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# Comments

  • dgaddis

    Interesting. A few thoughts:
    -You mentioned GPSs often over-report distance, I don’t think this is the case, I think the distance you think you’re traveling is actually under-reported. For example, last time I did the Fools Gold 50 it was actually 56 miles.
    -I don’t know enough about tracks, but as you move further out in the lanes the distance/lap is greater. Which lap is 1/4 mile? Do you actually KNOW you rode 1/4 mile each lap?
    -Part 2 – In my mind, for distance accuracy, the gold standard is still the oldschool bike computer with a magnet on the wheel. If it’s programed correctly (you actually need to measure the roll out of the wheel) it’ll be dead nuts right on. For next part, set up a bike computer properly and go ride a trail and then compare all the distances. The most twists and turns in the trail, the less accurage the GPS devices will be.

    • dgaddis

      Note to self: no more commenting before coffee. I spell good.

    • Jeff Barber

      – Not sure I understand point #1–are you saying race organizers under report their distances and the GPS is correct? As in, the Fool’s Gold 50 course is actually 56 miles? (Snake Creek Gap TT, on the other hand, claims to be 34 miles but I clocked 32 on my GPS every time.)

      – Yes. The inside lap on a track is 1/4 mile. As you move outward, the start lines are staggered for each lane to make the lap lengths equal.

      – Agreed, it would be interesting to test a bike computer since in theory this should be more accurate. This is how official track and field event courses are measured. Still, it’s not perfect–depends on things like tire pressure (which influences the circumference), traction, etc. Hell, even a tape measure isn’t 100% accurate depending on the temperature. ๐Ÿ™‚

    • Dan McCabe

      On a standard track the inside of lane 3 is a quarter mile. The inside of lane 1 is 400 meters, which is roughly 23 meters difference after 2.5 miles. Not that it matters, since we’re going after accuracy not precision.

    • Jeff Barber

      Excellent point Dan! You’re right, this track was probably a 400 meter track, not a quarter mile. Assuming that’s true, the actual distance on lane 1 would have been 2.4855 miles. That wouldn’t change the rankings since all reported 2.5+ miles but that would mean that each unit is less accurate than we thought.

    • Eddie_O

      The FG is has been more than 50 since moving to Montaluce. 55-56 miles is the average reported.

  • maddslacker

    Based on a comment from another forum, I did a little digging and it turns out the iPhone and likely the Asus both have access to the US GPS system and the Russian GLONASS system. This effectively doubles the number of available satellites for getting a location fix. This could help explain why these two devices were t teh top of the distance accuracy list.

    • Jeff Barber

      Ah, that could be. The latest Garmin devices include GLONASS (like the Edge 1000). AFAIK, none of the standalone devices in our test support GLONASS.

    • Jeff Barber

      Now you’ve got me wondering… If the iPhone running Strava was tapping into GLONASS for supreme accuracy, why does the track look so sloppy?

    • abg521

      Wonder if they do WAAS too. My Garmin eTrex 20 does GPS, GLONASS, and ground based accuracy enhancements from WAAS. The accuracy still varies at any time, since the satellites are not geo-synchronous, so there are some times when fewer are being used, though ever since I upgraded models in ~2010 this has been much less of an issue.

  • bonkedagain

    Regardless of what tricks or techniques the devices use the bottom line, to me at least, is a smartphone is just as good as a consumer grade standalone GPS. In other words, if you have a smartphone then it is hard to justify spending a big chunk of change for a standalone GPS. Buh-bye Garmin…

    • dgaddis

      That’s where I stand. I really want a standalone, but can’t justify the cost just yet. Part of the problem is, if I get one, I want full feature with color maps showing roads and whatnot. It would be really useful when riding in unfamiliar places, especially when out for a long ride. I hate to use all my phone battery for GPS duties when I may need it for an emergency…

    • abg521

      Look at the eTrex models. I’ve been using them since 2002. Running on 2x AA batteries, I can 24+ hours on a set, and can easily swap rechargeable out AAs if I need more time. I paid a whopping $120 for my eTrex 20, which supports free color maps from places like gpsfiledepot and garmin.openstreetmap.nl. They don’t support cadence/heart/altimeter, but I’ve never cared about those. Strava can calculate elevation much better than altimeter based units – just look at someone’s ride data during rain and you can see mountain tops removed from the pressure changes!

    • abg521

      It’s hard to pinpoint why, but there are a lot of contributing factors why people still buy Garmins instead of using phones. It’s ugly seeing a phone on your handlebars, since the brackets are all huge, and still look precipitously about to break off. The Garmins are waterproof too. I can download maps for anywhere for road or topo free, and don’t need to rely on slow 3G data every time like I would with a phone. They do routing just as well, and using the right mapsets from OSM, you can even route along bike infrastructure that Google Maps won’t get you on a phone.

    • PHRANQUY

      For me it’s simple, double redundancy, and it’s not just a matter of tracking data, it’s being able to get back out. The last few years I’ve been getting in to more and more areas where a cell phone can not be relied on (Hello backcountry), but I’ll access the open source maps on it to make sure the data is current and I can usually figure out where I am so I can get out if I made a wrong turn somewhere, and paper maps are not always readily available. My trip to Arkansas last year was a good example, when we missed a turn on the LOViT trail. My girlfriend recently replaced my Garmin 500 (Which I broke a few months ago) with an 810 (She likes to be able to keep track of me on my road rides), and I’ve since loaded the open source maps to that when I go into an unfamiliar area, and they’ve proven to be pretty accurate so far. My trip in the spring will tell me how accurate they really are, once I update them for the area we travel to that is.

    • maddslacker

      @PHRANQUY .. I use Orux maps on Android for just that reason, i.e. offline maps and navigation. It saved my butt in South Dakota last year.

    • Greg Heil

      Also, it’s worth noting that the Singletracks Topo app provides offline access to the largest database of mountain bike trail maps in the world. Getting lost just shouldn’t happen anymore!

    • abg521

      Another factor – I used the same $150 Garmin eTrex Legend C for 9 years. It still works great today, but I was interested in more mapping features of the Garmin eTrex 20 which was $120. Compare that to phones that require spending thousands per year for a data plan unless you pay many hundred for the phone upfront, and then you have to get a new one every 1-2 years if you want any chance of app compatibility. I just use a prepaid Tracfone for $100 per year and don’t rely on it for complicated apps.

      Phones can be a good backup, but it’s not something functional enough at the prices I’m willing to spend to have it on my bars.

    • Greg Heil

      Many hundreds of dollars per phone every 2-3 years and then thousands of dollars per year for a data plan? Sounds to me like you just need to shop around a little bit more.

    • abg521

      Guess I was off. Currently showing ATT or Verizon plans at $40/mo for a phone plus $15/mo for 200/300mb data as the absolute lowest is $780 per year. Most people use 1-2gb of data which brings that closer to $1000 per year. I paid $100 this year for my service, and expect my $120 GPS to last 5-10 years. I’d rather spend my money on bikes and adventures than phones.

    • bonkedagain

      I have six smartphones with unlimited voice, data, text, international roaming, for $130 per month on T-Mobile. In my plan, the data is throttled if you go over 2.5gb in a month, which rarely happens, and the speed only drops back to 3G speed, which is totally tolerable to me. There is no additional charge if you exceed your allotted data, it is just slower. You pay for the phone up front with T-Mobile and you don’t have to buy from them. If you buy from T-Mobile you can get quite usable phones for around $200 or you can spend $600 for the latest, sexiest fancy phone. Smartphones are not that expensive anymore if you shop around.

      Both Orux Maps (free) and Backcountry Navigator (practically free) offer offline map support. For that matter, Google Maps now offers offline map support, so having maps available when out-of-service is not an issue.

    • als70bird

      I’ll keep my Garmin! I was in the mountains and my Iphone 6 lost connection. I fired up the garmin and continued.

    • bonkedagain

      Or you could get a phone that is capable of running the GPS when it doesn’t have a cell signal. That would include practically all smart phones these days.

  • abg521

    I think there’s too much user error here to make the precision numbers matter. Turning on the GPS and letting it drift around will certainly skew the distance, and it’s clear that some of the units started at the same point in the track where you see this drift. It often takes a minute or two to get the most accurate data while it connects to more satellites and WAAS (if enabled). The map with low polling is clearly adjustable too, and is misleading to not have changed that before collecting data. You can use programs like Garmins Basecamp to edit the data to cut out start points, which is what I do before uploading to Strava (to eliminate drifts and initial accuracy problem).

  • ericnico

    It’d be nice to do this in an area where the cell coverage isn’t so great. Then again I’m not sure if iPhones still locate using towers.

  • williedillon

    Which Lumia did you use?

    I’d be interested to see the Microsoft Band’s accuracy as well.

  • bravesdave

    The distances for each lane of a standard track are: lane 1400m, lane 2 as 407.67 meters, lane 3 as 415.33 meters, lane 4 as 423 meters, lane 5 as 430.66 meters, lane 6 as 433.38 meters, lane 7 as 446 meters and lane 8 as 453.66 meters. This can vary a little based on lane width, but most high schools use the internationally set standards. So remaining in lane 3 for the entire test means that the bike probably travelled 5450.525 feet or 1.032 miles. I think that is where much of your error in distances accuracy is coming from.

    • bravesdave

      One further thought. Another quick check about the track. How many lanes did the track have? If it was 8, then that bodes well but does not guarantee that it is an international standard track.

    • bravesdave

      And I concur with Greg about the Singletracks map database. I live in the country of Mongolia. The ST map base even has the entire country of Mongolia covered at 10m contour intervals. I cannot buy 10m intervals in country. Last I knew, only the Mongolian military had access to this scale of map. LOL. So with the ST’s map base and Google Earth sat images, we all can be prepared for pretty much any adventure. THANK YOU Singletracks, Greg and Jeff.

    • Greg Heil

      Wow, that’s pretty crazy!! Glad we can provide the data you need ๐Ÿ™‚

  • Treebie

    The whole accuracy test revolves around assumption of the true distance and the actual distance covered by each device.

    Others have already questioned the assumption 2.5 miles (i.e. is track 400m etc).

    You say devices were attached to wrists and bars, given 10 devices they must have been well distributed. I chucked the figures into excel and played about. Assuming half a metre separation between wrists, the additional distance covered by the outer wrist over this distance would have been in the region of 25 metres. That is over 0.5% of the total distance.

    This is enough to shift devices around in the rankings and skew things. e.g. if, for example, the forerunner was on the right wrist and the iphone was on the left of the handlebar then applying the adjustment would bring the two devices to within a few metres.

    I am not saying this happened here but I came across a similar test a while back. The guy had 5 devices on each wrist and doing several circuits of a lake oblivious to the fact that the extra distance covered by his outer wrist made a mockery of the relative distances not to mention the assumption of the true distance.

  • Treebie

    A comment on “polling frequency” The data recording interval can usually be set to smart or auto.

    Some devices offer do offer battery saving modes where the unit does indeed “sleep” for intervals before waking and grabbing a fix. That without a doubt will affect distances.

    But the auto modes are mainly to save on storage space. e.g. a 45 minute run the other day with my old 60c (no expansion memory) at 1 second recording chewed up half my device storage! No use for a day’s hiking! It makes less of a dent on my new 920xt but over time it would.

    The point is auto recording does not necessarily dictate the frequency of “GPS units checking in with the satellite”. So there may well be a distinction between “odometer” distance based on actual polling versus the point-to-point distance of a logged track.

    So I am used to my hiking handhelds devices reporting a different distance to the downloaded and saved gpx track.

    Not sure what happens in the internals of, say, a Garmin fit file in Garmin Connect but I suspect exporting a gpx file to use in another app may lose all but the point-to-point information.

    Just another complication to consider! Sigh.

  • jbster13

    I have a question. I always assumed that Garmins and other GPS devices get their info directly from satellites while Iphones and other smart phones get their data from cell towers (redirected from satellites). This would make Garmins more dependable when you are out of cell range.

    Is this true? If so, then you can’t count on a phone when you are riding in the deep woods or mountains.

    • maddslacker

      Modern smartphones are equipped with dedicated GPS transceivers. In some cases, they are even the same transceiver as the dedicated GPS units.

    • jbster13

      So does this mean that even though I have no network connection in the woods, I can still use the phone for tracking?

    • Greg Heil

      @jbster13, 100% yes. I do this all the time in the mountains. With Strava (or basically anything that hooks into GPS) I can start my ride, complete it, enter info, and hit save, without ever having cell phone service. When you get back to cell service and reopen the app, it syncs up to the website automatically.

      This is also why the offline topos in the Singletracks app are so great. If you download the files in advance, even if you don’t have a cell signal you can pull up the map, and hit the locate button to show your position in relation to the trails around you.

  • Michael Fennell

    I have to wonder how the results would look in the woods. Whenever I see a Strava MTB trace/time that looks irrationally fast, it’s an iPhone. While my Razr MAXX produces reasonable traces on the road compared to a concurrently running Garmin 500, it’s a hot mess on the trails.

  • Treebie

    GPS usually requires ephemeris data from satellites on startup which takes on average 45 seconds per satellite (uninterrupted) which is why you see runners hopping about impatiently waiting for their watch to get a position before they set off.

    Smart phones have the advantage that ephemeris data can be supplied much faster via data from cell towers therefore startup can be in the order of seconds. That so called assisted GPS (A-GPS) is probably what you are thinking of. The term Assisted GPS is however used by many as a catch all phrase for a variety of location assistance, hence sweeping statements like A-GPS is more accurate/better than GPS.

    On a smart phone cellular signals can be used to derive a much lower accuracy position independent of GPS therefore a useful backup where GPS coverage is lost or very poor (canyons, tunnels, urban areas, under tree cover). Similarly wifi signals can be used to get a position, say indoors, where no GPS is available. Usually good enough to “check in” on social media etc.

    Most devices such as Garmins are standalone and don’t have the startup abilities of A-GPS and can’t fall back on or utilise wifi or cell signals. The exception is Garmin’s 620 and 920xt devices that use satellite ephemeris pre-loading over wifi vastly improving startup times.
    I’m sure I’ll be corrected in some areas but hopefully the above helps.

  • Stephen Johnson

    It’s been a while since you posted this but, if you’re curious, the distance as reported by GPS will actually always be larger than the actual distance.

    http://spectrum.ieee.org/tech-talk/at-work/test-and-measurement/why_every_gps_overestimates_distance_traveled

    To understand it, picture a tight clustering of darts around a bullseye. That the random errors a GPS unit will make when sampling. But, since distance is being calculated by summing up the small lines between each point, any error that is not in the same direction as the next point will be an overestimat!

    • Jeff Barber

      Good point Stephen. I actually started to write about this recently and while looking a bit more closely, I found it is ALMOST always true that GPS will overestimate distance, but not always.

      A GPS could UNDERESTIMATE distance over a very short distance when looking at a curved track (like a MTB trail.) Again, knowing that the errors are random, it’s possible the errors, at least for a few points in a row, could lie inside the arc of the curve instead of along it (or outside of it.) In this case, you’re underestimating distance. Particularly curvy GPS tracks potentially suffer from this the most, especially if the polling frequency is low. Still, it’s really rare that the stars would be aligned for this to be the case.

  • !Saga!

    Can GPS record accurately when there is allot of climbs and descents? My android phone app is miles less than my buddies bike computer, for example after 20 miles he records I get about 13 on my app. My theory is that the gps can only record data on a straight two dimensional line. For instance take a 12″ ruler which is perfectly parallel to the ground is 12″ but then angle that ruler at 45 degree and the distance left to right that it reaches will be reduced by inches. Anyone following or am I missing something?

    • Jeff Barber

      I’m following you! Being off by 7 miles on a 20 mile ride is a big difference, more than just the climbs and descents can explain. That is, unless you’re riding 45% grades for the whole ride which you’re not.

      However, that’s not to say that part of the difference isn’t due to elevation errors. As we found in our elevation test, GPS units aren’t always very good at measuring elevation. So if your GPS thinks a ride was mostly flat, then it will underestimate the actual distance you traveled.

    • !Saga!

      Thanks for replying, the area I ride is very up and down constantly, there is not one flat area I can think of but I agree that is a very very big difference. I’m going to hook up a Bike computer and see what I get doing the same loop with that. I guess it’s possible my buddies computer is over estimating and mine is under and that is why it varies so much. Maybe I’ll run a different app and see if that makes a difference as well. I wonder if it’s possible to run two GPS apps simultaneously?

  • drcbrath

    I have compared distance recorded with my Garmin 305 & 705 with & without smart-recording to the my old cycle computer on my favorite, oft ridden trails. The Garmin distances with smart recording are always about 1% shorter than the cycle computer; and about 3% shorter without smart-recording.

  • drcbrath

    Forgot to mention: I carefully calibrated the cycle computer per instructions for the unit. Also, the comparison includes many, many repeats of the same routes. The routes involve a mix of road, fire road, jeep trail, and singletrack. On average, the Garmin distances have consistently been shorter than the cycle computer by 1% to 3%.

    • Jeff Barber

      Thanks for weighing in! It’s interesting that you found the GPS to be consistently under-reporting. Many people I’ve talked with swear GPS always overestimates, though I personally don’t believe it’s that simple. Here’s just one article that strongly suggests GPS units always overestimate:

      http://spectrum.ieee.org/tech-talk/at-work/test-and-measurement/why_every_gps_overestimates_distance_traveled

      We’re planning a second round of GPS tests for early next year and are planning to add a cycle computer to the mix this time. I’m stoked to see what we find!

    • sgniwder99

      I’m way late to the party too, but just came across this article, as well. I find that both my girlfriend’s Garmin Edge 25 and my own Edge 500 will dramatically underreport distance ridden on the trail if relying on GPS alone–by as much as 10-15% on some trails. I use the Garmin hub-based ANT+ speed sensor and manually input the measured circumference of all my tires to get an accurate reading of distance based on wheel revolutions (accurate to within a couple of millimeters, anyway).

      I’ve always assumed that this underreporting is because a trail is constantly twisting and turning, and the Garmin just doesn’t register often enough to note all of these changes–so the twistier the trail, the more the GPS alone is underreporting. I’ve also noticed that the “shape” of our rides on Strava do not look the same (even if we’ve done the exact same route), with hers featuring both a lot of straightened lines and inconsistencies between multiple trips over the same section of trail, whereas mine track the trail almost perfectly, and much more consistently.

      Interestingly, my Edge 500 will be within 1% or so riding on the road with or without the sensor. But the Edge 25 also underreports a bit there, as well–sometimes by about 5%. I’d fix all of this by getting her a sensor for her Edge 25, but as far as I know it won’t work with one.

      Anyway, the tl;dr version is, if you use a Garmin and want more accurate data when riding on the trail, I think the sensor is the way to go if your model will accept one. And the Edge 25, I’m not impressed with at all.

    • williedillon

      I have the same issue with my phone. I think GPS just isn’t strong enough to report mileage accurately through the trees.

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