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{{DISPLAYTITLE:GPS Accuracy of Garmin, Polar, and other Running Watches}}<div style="float:right;">__TOC__</div>I evaluated the real -world accuracy of GPS watches while running over 112,500 000 miles/219,800Km 000Km and recording over 750,000 data pointsas part of my evaluation of the [[Best Running Watch]]es. Under good conditions most of the watches are all remarkably good, but when things get a little tough the differences become more apparent. However, '''none of the watches have GPS accuracy that is good enough to be used for displaying your current pace'''. As a result, I've added the test results for various [[Footpod]]s as they can be far more accurate than GPS, but more importantly they tend to have far less moment-to-moment variation so they can give a far better display of your current pace. (Note that my accuracy tests focus on the ability to measure distance, not the moment in time position, though the two are obviously related.) [[File:GPS Accuracy.png|none|thumb|800px|An infographic of the accuracy of the GPS running watches. The top right corner represents the most accurate watches. (This graphic uses ISO 5725 terminology.)]]The table below is a simplified summary of the results, where a '10' would be a perfect device. (For an explanation of the ISO 5725 terms 'trueness', 'precision' and 'accuracy', see below.){{:GPS Accuracy-summary}}The values used are simply 10 minus the value for trueness (average) and precision (standard deviation from true). The overall is the combination of trueness and precision. Repeatability is how consistent a watch is in providing the same value for the same course segment. '''Important''': Manufacturers do not typically release the type of GPS chipset used, so the information in this table is based the best available data, but it should be treated with caution.

Simply taking a GPS watch on a single run does not provide sufficient data to reasonably evaluate its accuracy. So to gather the data for this test I marked my usual running route at quarter-mile intervals, using a [http://www.jonescounter.com/ Jones counter] to provide an accurate measure of distance. The Jones counter is the device used to certify courses, and I followed the [http://www.usatf.org/Products-/-Services/Course-Certifications/USATF-Certified-Courses/Certify-Your-Course.aspx USATF course certification process]. The course I run along is a little challenging for GPS, with lots of twists, tree cover, power lines, and one bridge that I go under. The bridge carries a 4 lane road, so it's wide enough to cause the watches to briefly loose GPS signal. However, I believe that it's reasonably representative of real-world conditions, and probably less challenging than running in the city with skyscrapers. At both ends of the course there is a turnaround, and I set the mark an eighth of a mile from the end. That way I can evaluate how well the watches handle an about turn. [[FileMain article:Course Overview.jpg|none|thumb|500px|This is the course I use to evaluate the accuracy of [[Best Running Watch| GPS Running Watches]].Testing Methodology]]''

Simply taking a GPS watch on a single run does not provide sufficient data to reasonably evaluate its accuracy. To gather the data for this test I ran the same route repeatedly, recording laps every quarter mile. The course is challenging for GPS, with lots of twists, tree cover, power lines, turn arounds and goes under a bridge. However, I believe that it's reasonably representative of real-world conditions, and probably less challenging than running in the city with skyscrapers. =Accuracy, Trueness and Precision(plus Repeatability)=For this evaluation I'll use the ISO 5725 definition of [http://en.wikipedia.org/wiki/Accuracy_and_precision Accuracy as the combination of trueness and precision]:.

|[[File:High precision Low accuracy.svg|none|thumb|x300pxx200px|This is an example of high precision, as all the hits are tightly clustered. However, the trueness is poor as all the hits are off center, so accuracy is low.]]|[[File:High accuracy Low precision.svg|none|thumb|x300pxx200px|This shows good trueness, as all the hits are around the center. On average they are on target, but there is poor precision, as the hits are scattered.]]

We can look at trueness by measuring the average lap length and precision by measuring the standard deviation. I use the traditional approach to standard deviation (variation from mean) as well as a modified approach that uses variation from the true value. (It is more common in many fields to use "accuracy" to mean closeness to true value and "validity" to mean the combination of accuracy and precision. However, I feel that the meanings used by ISO 5725 are closer to the common usage. If a company sold 'accurate' 12 inch pipes and shipped half of them as 6 inches and half as 18 inches, they would meet the traditional definition of accuracy, but few people would be happy with the product. ) In addition, I calculate a value for "repeatability", which is a measure of how likely a watch is to give the same distance measurement for a specific course. I calculate the standard deviation for each segment of the course, and then take the average. A high repeatability score can mask poor accuracy and can convince users they have a good device. =Accuracy=The table below shows summary data for each device. The count field is how many measurements I have for that combination of condition and device, with each measurement being a quarter mile distance. I generally aim for over 1,000 data points to even out the effects of weather, satellite position and other factors. The Trueness is the absolute of the mean, though nearly all watches tend to read short. The standard deviation is provided based on the variance from the mean and the variance from the known true value. The average pace error is shown to give a sense of how much error you're likely to see in the display of current pace. This is an average error not a worst case. The data shown below is a summary the accuracy based on all the sections. If you'd like more detailed information, I've split off the [[Detailed Statistics for GPS Running Watches]] for the results under different conditions. {{:GPS Accuracy-statistics}}The "Accuracy (Combined)" column has an indication of statistical significance compared with the most accurate entry. The key to this indication is: † p<0.05, * p< 0.01, ** p< 0.001, *** p< 0.0001, **** p< 0.00001, ***** p< 0.000001==Progress of newer watches==I expected GPS watches to improve with time, but the opposite appears to be happening. With the Garmin devices especially, you can see that the older watches generally do far better than the newer ones. I suspect this is due to compromises to get better battery life and smaller packaging and the cost of GPS accuracy. ==Smartphone Accuracy==There are various things you will need to do in order to get the level of accuracy I found with Smartphones. See [[Running With A Smartphone#Optimizing GPS Accuracy| Optimizing Smartphone GPS Accuracy]] for details. ==Interpretation and Conclusions==What do these statistics mean? This is my interpretation:* Under normal conditions the GPS accuracy is quite good for most devices.* The accuracy of a calibrated [[Footpod]] is far better than any GPS device. Without calibration the Footpod is more accurate than any watch currently on the market with the exception of the 310XT/910XT with a Footpod backing up the GPS. * The [[Polar M400]], [[Garmin Fenix 2]], and [[Garmin 10]] are noticeably poorer than the other devices. I found the accuracy of the M400/Fenix2/10 in general usage to be rather grim, and I did some testing pairing them up with the 610 or the 310XT. In all cases the Fenix2/10 would have poor accuracy compared with the 610 or 310XT on the same run. * The Fenix2 would repeated loose satellite reception, something I've not seen (the M400 has done this once). The statistics do not reflect just how bad the Fenix2 is, as some of the data is too bad to analyze. * The results of the Garmin 610 & 620 indicate the problems with the 10 are not inherent in a smaller device. * The improvement in GPS accuracy of the 620 with updated firmware shows just how important the software can be. With the earlier firmware the 620 lost over a mile over a 20 mile run! * '''The accuracy of all devices is better in a straight line than on curves or twisty routes'''. My course is a tough test for GPS devices with many curves and only a few relatively straight sections. * Not surprisingly, for many devices accuracy drops going under the bridge. However, some devices do great in this section, probably because it's fairly straight. * More interestingly the trueness just after the bridge is even lower, suggesting that the GPS watches are struggling to reacquire the satellites. * The turnarounds are even less accurate than going under a bridge, but Power Lines do not seem to impact accuracy noticeably. * The [[Footpod]] improves the accuracy of the 310XT. ** Note that I'm intentionally using an uncalibrated Footpod (factor = 1.000) to gather data for a comparison of Foodpod and GPS. * The older Garmin 205 does remarkably well. =Footpod Accuracy=The accuracy of a Footpod is far higher than GPS, as well as more consistent and quicker to react to changes in pace. For any given run, the average pace error from the Footpod is only 7 seconds/mile (at a 9:00 min/mile pace) or 5 seconds/Km (at a 5:30 min/Km pace). In practical terms, I've found that I always have to use a Footpod to pace a marathon or for critical speedwork. For details of how the Footpod calibration was done, see [[GPS Testing Methodology]]. =Trail Running and GPS=Trail running tends to be rather harder for a watch to measure accurately. There are far more twists and turns, and for a [[Footpod]] your footsteps tend to be uneven. I realized how bad the problem was when running some mountain bike trails and my GPS watch said I'd only been traveling at walking pace. This prompted me to survey and evaluate the accuracy of various devices on these mountain bike trails. The table below is preliminary data, but you'll notice how the results are dramatically worse than my usual GPS testing. The Polar V800, which does really well on my greenway tests has serious problems on trails, though it's still one of the better watches I've tested so far. The Suunto Spartan Trainer shows its strength more clearly on the mountain bike trails, coming in far ahead of other GPS watches. The [[Stryd]] footpod is vastly more accurate than GPS, and unlike GPS it could be calibrated to improve its accuracy even further. {{:GPS Accuracy-TrailSummary}}

=AccuracyWhich Chipset? =While the specific chipset used in a GPS watch will impact its accuracy, there are many other factors that come into play. The physical packaging of the chipset, the antenna used, the particular features that are implemented, and the software that interprets the raw data will influence the overall accuracy. It's important to note that the SiRF chipsets such as "SIRFstarIV" are not a single chipset, but rather an overall architecture with several specific chipsets bearing the same name. =Even GPS Watches have Bad Days=While it's tempting to take the various GPS watches on a single run and simply compare the totals, this is a flawed approach. Evaluating the devices GPS accuracy on the basis of a single sample does not tell you much. It's a bit like evaluating an athlete's ability on the basis of one event; everyone has good days and bad days, and that applies to GPS watches as well. To illustrate this, the images below are from two runs, recorded on 9/20 and 9/22. In each run I recorded data on both the 310 and 910 watches, hitting the lap button on both at as close to the same time as is humanly possible. On 9/20 the 910XT was far more accurate than the 310XT, but on 9/22 the situation is reversed. If you were to have evaluated the two watches on the basis of a single run, you would conclude that one is much better than the other. But which device would win would depend on the particular day. This is why I've divided accumulated a lot of data to do a statistical analysis to work out which is really better. {| class="wikitable" |- valign="top"|[[File:310XT Bad.jpg|none|thumb|x500px| The {{Garmin 310XT}} having a bad day. You can see on the upper half of the course where it got a little confused and off track. ]]|[[File:910XT Good.jpg|none|thumb|x500px|The {{Garmin 910XT}} on the same run having no problems, and only the standard, expected level of inaccuracy.]]|- valign="top"|[[File:310XT Good.jpg|none|thumb|x500px|Two days later and it's the turn of the {{Garmin 310XT}} to have a good day.]]|[[File:910XT Bad.jpg|none|thumb|x500px|Again, this track is recorded on the same run as the image to the left. The {{Garmin 910XT}} gets a little confused at the start, and then again around lap 27.]]|}=Some Devices Are Better Than Others=Below is a section of two runs showing the same section of the course, both taken at the same time, one from the Garmin 310XT and the other from the Garmin 620 with the early firmware. (With the later firmware the tracks from the 620 look like the 310XT.) {| class="wikitable" |- valign="top"|[[File:ExampleGarmin310.jpg|none|thumb|x500px| You can see the GPS tracks (thin red line) are close together and the lap markers (yellow diamonds) are clustered nicely. The blue dots on the GPS tracks are the actual GPS recordings.]]|[[File:ExampleGarmin620.jpg|none|thumb|x500px|By contrast, the 620 has much wider GPS tracks and dispersed lap markers. ]]|}=GPS Short and long measurements=As you can see from the images below, the GPS track tends to take shortcuts around bends, reducing the length of the measured track. This cutting of the corners indicates the devices are doing some post-hoc smoothing to try to overcome the GPS errors. The more smoothing they do, the better the accuracy is likely to be in a straight line and the worse it is around corners or twisty courses. In my discussions with engineers working on GPS systems, this type of smoothing is often performed with a[http://en.wikipedia.org/wiki/Kalman_filter Kalman filter]. (When I tested using software without smoothing I found the measurements were long on my course rather than short, which is almost always the case.)[[File:GPS Shortcuts.jpg|none|thumb|500px|The GPS tracks in red showing the tendency to cut the corners on the curves.]]Often GPS measurements of races, especially marathons record a longer distance than the race. This is partly because the USATF technique for measuring the distance takes a path that is no more than 12 inches away from the tangent (corner), and few runners are able to run that close. In a large marathon you can be forced to take a line that is a long way from the tangent. The other factor is that on a straight line, the GPS error tends to give a slightly longer measurement. {| class="wikitable" |- valign="top"[[File:GPS Marathon.jpg|none|thumb|500px|Here you can see the GPS line is not following the straight road, giving a longer reading on the Thunder Road Marathon. Notice that the GPS is also cutting the corner at the laps into those top (we didn't run through the building).]]|[[File:GPS MarketSt.jpg|none|thumb|x300px|Here's another example of running down Market Street in San Francisco, where you can see the errors that would add to the distance. ]]|}=GPS Accuracy and Weather=GPS Accuracy is slightly better with clear skies than with cloud cover. The difference between completely clear and fully overcast is generally less than 0.1% and my testing includes a similar mix of cloud cover for each watch, so I ignore this difference. However, rain can degrade accuracy by 0.3-3.1%, with the better watches being impacted the least. Because it does not rain that go under frequently where I test, this has created some potential bias in my testing so I now ignore measurements taken during the bridgerain. This has only made a slight difference to the results, those but it ensures consistence. =GPS Accuracy and Seasons=I run in a wooded area with mostly deciduous trees, so the foliage varies by season. This foliage can have a noticeable impact on GPS accuracy, with better accuracy during the bare winter months than the rest of the year. This difference is mostly 0.1-1.5%, but in some cases can be as large as 2.5%. Because of this, my testing now ignores data from the winter months when the trees are bare. The short winter here in the south of the US means that the impact on the overall results are immediately after small, but like the weather impacts noted above, this does ensure greater consistency. =GPS Accuracy and Pace=[[File:AccuracyAndPace.jpg|none|thumb|500px| A plot of GPS precision against pace. The red line is the correlation.]]There have been reports of GPS accuracy changing with pace, but as you can see from the graph above, my testing does not show this. =GPS and GLONASS=I have found that GPS plus GLONASS produces less accuracy than GPS alone, something that is a lap little counterintuitive. I have no definitive explanation for this, and I do have a working hypothesis. My thought is that goes under enabling both GPS and GLONASS will increase the bridgenumber of satellites above the horizon, those and a modern chipset can have over 50 channels. This means the chipset will have access to far more satellites with both systems enabled. However, I don't believe that go next to the chipset will use all the available satellites when calculating its position. In an urban, or under wooded environment, the power linessatellites nearest the horizon will have the weakest signal, those that turn around and the satellites closest to directly overhead will have the strongest signal. If the chipset were to use only the strongest 5-6 signals, then all it's likely to choose the satellites that are closest to being directly overhead. That means the rest satellites chosen are relatively close together, which is a poor geometry that reduces accuracy. (In GPS terms this is called Dilution of Precision, or DoP.) I've talked to a GPS specialist who tells me that they have seen this in GPS systems they've tested (normalthough not necessarily consumer grade systems.)What this means in the real world is that if you're in an environment with a partial view of the sky due to tree cover for low buildings then GPS on its own is likely to provide better accuracy. If you're in an environment with a clear view of the sky from horizon to horizon, then it's less clear to me which system is likely to provide better accuracy, and I've not tested this in practice. Given that the theoretical accuracy of GLONASS is not quite as good as GPS I'm not sure that enabling both systems will improve matters. It's possible that GLONASS will do relatively better at extreme polar latitudes due to its different orbital patterns.==Garmin 920XT and GLONASS==The [[Garmin 920XT]] is significantly worse with GLONASS enabled.

! Device! Condition! CountAccuracy! Trueness (Average)! Precision (Standard Deviation)! 95% withinRepeatability

| {{Polar RC3 GPS}}Garmin 920XT| rowspanstyle="5background-color: #FAE983;"| Normal| 8976.6| style="background-color: #9DCE7ED2DE81;"|07.994% (33.1 Ft/Mile 6.3 m/Km)5| style="background-color: #FFEB84FED680;"|0.037 (195.6 Ft/Mile 375.0 m/Km)9| 0style="background-color: #D1DD81;"|7.074 (391.2 Ft/Mile 74.1 m/Km)5

| {{Garmin 310XT}} with [[Footpod]]920XT (GLONASS)| 2453style="background-color: #FEC77D;"|5.5| style="background-color: #63BE7BFAE983;"|6.61.000% (1|style="background-color: #FCA777;"|4.9 Ft/Mile 0.4 m/Km)6| style="background-color: #C0D980E1E282;"|7.20|}==Suunto Spartan Ultra and GLONASS==The [[Suunto Spartan Ultra]] seems to do particularly poorly with GLONASS enabled.031 (163.0 Ft/Mile 30.9 m/Km){| 0.062 (326.0 Ft/Mile 61.8 m/Km)class="wikitable"!Device!Accuracy!Trueness!Precision!Repeatability

| {{Garmin 310XT}} no [[Footpod]]Suunto Spartan Ultra 1.6.14| 594style="background-color: #E2E282;"|7.1| style="background-color: #82C77C79C47C;"|0.996% (18.8 Ft/Mile 39.6 m/Km)5| style="background-color: #FFDC81FED881;"|0.042 (221.9 Ft/Mile 426.0 m/Km)| 0style="background-color: #D8DF81;"|7.084 (443.8 Ft/Mile 84.1 m/Km)4

| {{Garmin 910XT}} with [[Footpod]]Suunto Spartan Ultra 1.6.14.GLONASS| 481style="background-color: #FCB179;"|4.9| style="background-color: #9ECF7ED4DE81;"|0.994% (33.7 Ft/Mile 6.4 m/Km)5| style="background-color: #9ECF7EF9756E;"|3.3|style="background-color: #FDB57A;"|5.0|}==Garmin Epix and GLONASS==The [[Garmin Epix]] has slightly better accuracy with WAAS than without it, and GLONASS didn't degrade the accuracy the way it does with other devices.027 My belief is that enabling WAAS effectively disables GLONASS, as WAAS is GPS specific (145and only available in North America.1 Ft) There is EGNOS Ground Segment is the equivalent of WAAS for GLONASS/GPS/Mile 27Galileo in Europe.5 m/Km){| class="wikitable" | 0.055 (290.2 Ft/Mile 55.0 m/Km)- valign="top"!Device!Accuracy!Trueness!Precision!Repeatability

| '''All'''Garmin Epix with GLONASS+WAAS| '''4425'''style="background-color: #FFE082;"|6.2| style="background-color: #76C37CD6DF81;"|'''0.998% (127.4 Ft/Mile 2.3 m/Km)'''| style="background-color: #DEE182FDBE7C;"|'''05.034 (178.4 Ft/Mile 33.8 m/Km)'''3| '''0.068 (356.style="background-color: #D6DF81;"|7 Ft/Mile 67.6 m/Km)'''4

| {{Polar RC3 GPS}}Garmin Epix with WAAS| rowspanstyle="5background-color: #FFDF82;"| Under Bridge| 856.2| style="background-color: #FA8771C8DB80;"|07.952% (252.8 Ft/Mile 47.9 m/Km)7| style="background-color: #F96A6CFDB77A;"|05.078 (411.0 Ft/Mile 77.8 m/Km)1| 0.156 (821.9 Ft/Mile 155style="background-color: #F7E883;"|6.7 m/Km)

| {{Garmin 310XT}} with [[Footpod]]Epix| 212style="background-color: #FDC37D;"|5.4| style="background-color: #F8E983F3E783;"|6.80|style="background-color: #FB9C75;"|4.985% (81.0 Ft/Mile 15.3 m/Km)4| style="background-color: #EDE683F2E783;"|6.8|}==Garmin Fenix 5X and GLONASS==0Continuing the theme of poor accuracy with GLONASS enabled, the [[Garmin Fenix 5X]] demonstrates even worse performance than its peers.035 (186The values shown below are rather dramatically worse with GLONASS enabled than without.7 Ft/Mile 35My anecdotal observation is that sometimes the Fenix 5X does a little worse with GLONASS than normal, possibly in line with other Garmin devices, and sometimes it seems to just get lost and produce dramatically worse results.4 m/Km){| 0.071 (373.3 Ft/Mile 70.7 m/Km)class="wikitable sortable"!Device!Accuracy!Trueness!Precision!Repeatability

| {{Garmin 310XT}} no [[Footpod]]Fenix 5X 4.30| 58style="background-color: #FEC97E;"|5.6| style="background-color: #FFE483EDE683;"|16.018% (96.8 Ft/Mile 18.3 m/Km)9| style="background-color: #FECB7EFCA377;"|04.047 (250.3 Ft/Mile 47.4 m/Km)5| 0.095 (500.5 Ft/Mile 94style="background-color: #F3E783;"|6.8 m/Km)

| {{Garmin 910XT}} with [[Footpod]]Fenix 5X 4.30 GLONASS| 42style="background-color: #F97B6F;"|3.5| style="background-color: #6FC17BFFE082;"|6.20.998% (8.4 Ft/Mile |style="background-color: #F8696B;"|1.6 m/Km)| style="background-color: #C4DA80F97B6F;"|3.50|}Below you can see a visual representation of the problems. Many of the tracks are a little worse than normal, but you generally follow the path. A small subset of the tracks are dramatically worse, either showing and offset from the actual path, or sometimes it looks like the sampling frequency has dropped, suggesting that the watch is only periodically able to get a location fix. This latter phenomenon is rather surprising to me and goes against my hypothesis of why GLONASS has worse accuracy. I would expect there would be more satellites available with GLONASS enabled, which would result in the watch selecting the subset with the strongest signal that are more likely to have a narrow angle of separation, which would result in increased Dilution of the Precision. These tracks suggest that the Fenix 5X is unable to get any location fix for brief periods. {| class="wikitable" style="margin-left: auto; margin-right: auto; border: none;"|- valign="top"|[[File:BridgeFenix 5X 4.30 GLONASS.jpg|center|thumb|x300px| The GPS tracks from the Fenix 5X with GLONASS enabled. This diagram has tracks color coded with green indicating good accuracy through to red indicating poor accuracy, and the lap markers as blue dots.]]|- valign="top"|[[File:BridgeFenix 5X 4.30.jpg|none|thumb|x300px|Here's the tracks from testing with GLONASS disabled for comparison.]]|}=GPS Accuracy and Sampling Rate=GPS watches default to recording a sample frequently enough that accuracy is not compromised. However, several devices offer the option of recording less frequently to improve battery life at the cost of accuracy. These devices actually turn off the GPS receiver, turning it on periodically for just long enough to get a fix. The images below are from the [[2014 Badwater 135]] using the [[Suunto Ambit2| Suunto Ambit2 R]] with recording set to one minute intervals. As you can see, accuracy suffers on curves, but is fine on the straights. For a course like Badwater, the one minute recording interval was fine as the course has few turns. {| class="wikitable" |- valign="top"|[[File:GPS Sampling Curve.jpg|none|thumb|x300px|On a curve, the infrequent samples tend to 'cut the corners' and are quite inaccurate.]]|[[File:GPS Sampling Straight.jpg|none|thumb|x300px|On the straight sections, the one minute sampling does not lose any accuracy.]]|[[File:GPS Sampling Comparison.031 jpg|none|thumb|x300px|Here's a comparison of 1 minute sampling (red) with 1 second sampling (164blue).8 Ft/Mile 31On my GPS testing course the 1 minute sampling lost nearly 2 miles over a 16 mile run.2 m]]|}=GPS Accuracy and Recording Rate (Smart/Km1-Second)=| 0While the GPS sampling rate mentioned above has a huge impact on GPS accuracy, the same isn't true for recording rate. These two ideas seem to get confused. GPS sampling rate allows a watch to turn off the GPS receiver for short periods to conserve battery life while sacrificing GPS accuracy. Some Garmin watches can be configured to either record every second, or only record when something happens, such a change in heart rate or change in direction, something they call "smart recording." With a smart recording in normal GPS mode, the GPS system is continually active, so there's no loss in accuracy. To verify this, I tested the [[Garmin Fenix 5X]] in both the smart recording mode I normally use, and one second recording mode for comparison.062 As you can see, the two modes are virtually identical, and the differences are most likely due to chance (329p=0.7 Ft/Mile 6272).4 m/Km){| class="wikitable sortable"!Device!Accuracy!Trueness!Precision!Repeatability

| '''All'''Fenix 5X 4.30 Smart Recording| '''397'''style="background-color: #FEC97E;"|5.6| style="background-color: #FFEB84EDE683;"|'''0.984% (846.7 Ft/Mile 16.0 m/Km)'''9| style="background-color: #FDB87BFCA377;"|'''0.053 (2814.5 Ft/Mile 53.3 m/Km)'''| '''0.107 (563.0 Ft/Mile 106style="background-color: #F3E783;"|6.6 m/Km)'''8

| {{Polar RC3 GPS}}| rowspan="5"| Post Bridge| 166Fenix 5X 4.30 One Second Recording| style="background-color: #FB9474FDBF7C;"|05.956% (231.2 Ft/Mile 43.8 m/Km)3| style="background-color: #FB8F73E5E382;"|0.066 (348.8 Ft/Mile 66.1 m/Km)| 0.132 (697.6 Ft/Mile 1327.1 m/Km)|-| {{Garmin 310XT}} with [[Footpod]]| 365| style="background-color: #FFDE82FB9073;"|0.980% (1074.0 Ft/Mile 20.3 m/Km)| style="background-color: #D2DE81FBE983;"|0.033 (172.0 Ft/Mile 326.6 m/Km)| 0.065 (344.0 Ft/Mile 65.2 m/Km)|-| {{Garmin 310XT}} no [[Footpod]]| 110| style="background-color: #FEC87E;"|0.973% (144.3 Ft/Mile 27.3 m/Km)| styleDevice Specific Notes="background-color: #FFE984;"|0.038 (199.3 Ft/Mile 37.7 m/Km)| 0.075 (398For those interested in some of the details of how devices are configured for testing, here are some additional notes.6 Ft/Mile 75.5 m/Km)|-| {{* Garmin 910XT}} with [[Footpod]]| 86| style="background-color: #FDBB7B;"|0.969% (166.3 Ft/Mile 31.5 m/Km)| style="background-color: #8AC97D;"|0.026 (134.7 Ft/Mile 25.5 m/Km)| 0.051 (269.4 Ft/Mile 51.0 m/Km)|-| '''All'''| '''727'''| style="background-color: #FDC67D;"|devices are set to 'smart recording''0.972% (148.I did try an informal test with the 620 using 1 Ft/Mile 28.0 m/Km)'''| style="background-color: #FED680;"|'''0.044 (231.0 Ft/Mile 43.7 m/Km)'''| '''0.087 (462.0 Ft/Mile 87.5 m/Km)'''|-| {{Polar RC3 GPS}}| rowspan="5"| Turn Around| 92| style="background-color: #F8696B;"|0.943% (301.9 Ft/Mile 57.2 m/Km)| style="background-color: #F8696B;"|0second recording, but it appeared to make no difference.078 (411.1 Ft/Mile 77.9 m/Km)| 0.156 (822.2 Ft/Mile 155.7 m/Km)|-| {{Garmin 310XT}} with * For details of the calibration of the [[Footpod]]| 244| style="background-color: #FCA677;"|0.962% (200.2 Ft/Mile 37.9 m/Km)| style="background-color: #FFE383;"|0.040 (209.8 Ft/Mile 39.7 m/Km)| 0.079 (419.7 Ft/Mile 79.5 m/Km)|-| {{Garmin 310XT}} no see [[FootpodGPS Testing Methodology]]| 64| style="background-color: #FCB079;"|0.965% (184.7 Ft/Mile 35.0 m/Km)| style="background-color: #F9736D* The Fenix 2 was tested with and without WAAS support activated;"|0.075 (395WAAS helped slightly.2 Ft/Mile 74.9 m/Km)| 0.150 (790.5 Ft/Mile 149.7 m/Km)|-| {{Garmin 910XT}} with * The [[FootpodGarmin 920XT]]| 48| style="background-color: #FA8972;"|0was tested with Watch Firmware 2.953% (24950, GPS Firmware 2.8 Ft/Mile 4770 using smart recording.3 m/Km)| style="background-color: #87C87D;"|Garmin 620 Issues=0The Garmin 620 had some notorious problems with its GPS accuracy.025 (132.8 Ft/Mile 25.2 m/Km)| 0The table below shows the changes with various firmware versions, culminating in the GPS-3.050 (26530 firmware that resolved the issues.7 Ft/Mile 50.3 m/Km)|-| '''All'''| '''448'''| style="background-color: #FB9674;"|''I'0.957% ve including some testing I did without EPO data (228.2 Ft/Mile 43.2 m/KmNoEPO row below)'''| style="background-color: #FCAA78;"|'''0.058 and with a Footpod (304.1 Ft/Mile 57.6 m/Km+FP row below)'''| '''0.115 (608.1 Ft/Mile 115.2 m/Km)'''|-| {{Polar RC3 :GPSAccuracy-g620}}{| rowspanclass="5wikitable"| Power Line| 167| style="background-color: #DCE182;"|0.987% (66.4 Ft/Mile 12.6 m/Km)| stylevalign="background-color: #F2E783;top"|0.036 (188.8 Ft/Mile 35.8 m/Km)| 0.072 (377.5 Ft/Mile 71.5 m/Km)|-| {{Garmin 310XT}} with [[Footpod]]| 457| style="background-colorFile: #96CC7D;"Garmin620 Offset1.jpg|0.994% (29.5 Ft/Mile 5.6 m/Km)none| style="background-color: #A7D17E;"thumb|0.028 (149.5 Ft/Mile 28.3 m/Km)x500px| 0Here you can see the last repeat is offset.057 (299Starting at lap marker 49, the track follows the same outline as the more accurate tracks, but is offset.So marker 50 should be near 4, 51 near 37, 52 near 2, 53 near 1 Ft/Mile 56, and the finish near the start.6 m/Km)|-| {{Garmin 310XT}} no [[Footpod]]| 115| style="background-color: #D2DE81;"|0.988% (60.9 Ft/Mile 11.5 m/Km)| style="background-color: #FFE483;"|0.039 (207.4 Ft/Mile 39.3 m/Km)| 0.079 (414.7 Ft/Mile 78.5 m/Km)|-| {{Garmin 910XT}} with [[Footpod]]| 90| style="background-color: #FCEA83;"|0.984% (83.5 Ft/Mile 15.8 m/Km)| style="background-colorFile: #63BE7B;"|0.022 (113.8 Ft/Mile 21.6 m/Km)| 0.043 (227.6 Ft/Mile 43Garmin620 Offset2.1 m/Km)jpg|-none| '''All'''thumb| '''829'''x500px| style="background-color: #B8D67F;"|'''0.991% (47.2 Ft/Mile 8.9 m/Km)'''| style="background-color: #C5DA80;"|'''0.031 (165.2 Ft/Mile 31.This is a simple out and back run of ~3 mmiles/5 Km)'''| '''0, but you can see after the turn around the Garmin 620 records a gradually widening gap, even though it follows the right overall shape.063 (330.5 Ft/Mile 62The outbound track is fairly accurate, the return is messed up.6 m/Km''') ]]

 =InterpretationGarmin Fenix 2 Issues=What do these statistics mean? This is my interpretation:* Under normal conditions Like the Garmin 620, I've had similar GPS accuracy is quite good for all devices, though issues with the 310XT/910XT with a Footpod have a slight advantageFenix 2. * Not surprisinglyIn fact, the accuracy drops going under Fenix 2 is the bridge. More interestingly the trueness just after the bridge is even lower, suggesting only device I've ever had that has given the GPS watches are struggling to reacquire the satellites"lost satellite reception" message on my usual running route. * The turnarounds are even less accurate than going Because of these issues Garmin replaced my Fenix 2 under a bridgewarranty, but Power Lines do not seem to impact accuracy noticeably. * The [[Footpod]] improves the accuracy of the 310XT and below are the 910XT, especially on results for the turn around original and under the bridgenew watches.* Without The replacement watch also gave "lost satellite reception" repeatedly and the Footpod, error values for the 310XT has similar Fenix 2 do not reflect these problems to the RC3 on the turn around, but the RC3 does worse under as the bridge. =Recommendations=This data leads me to a few conclusionsfrom those runs was useless for analysis.* GPS watches I suspect there are accurate enough for most general running.* Adding a Footpod to three (possibly related) problems with the {{Garmin 310XT}} and the {{Garmin 910XT}} improves their accuracy. Fenix 2:* It takes time for the # The MediaTek GPS watches to acquire chipset is not as accurate as the satellitesSiRF chipset. The watches tended to say they best results from the Fenix 2 are ready to go before they have an optimal lockgenerally mediocre. Therefore# The Fenix 2 records the right shape track, to improve but offset by some distance. This does not look like a typical accuracy try to give them a little more time. Note problem that some newer GPS watches such as the [http://wwwwould manifest itself randomly.amazon.com/Garmin-010-01128-30-Forerunner-620-Bundle/dp/B00FBYYC90 Garmin 620] have the ability to be preloaded with # Occasionally the Fenix 2 will report "lost satellite positionsreception", reducing and I have several instances of this startup where the date and time and start up in accuracy dramaticallywere wrong after reception was lost.* The If a GPS watches are not accurate enough to perform any type of [Practical Interval Training| interval training]. Insteaddevice has the wrong time, use a track or measure out then it will expect the distance using some other mechanism. =Even GPS Watches have Bad Days=While it's tempting satellites to take the various GPS watches on a single run be in different positions and simply compare the totals, this is will be unable to acquire a flawed approachposition fix. Evaluating I have four instances where the devices GPS accuracy on the basis of workout file was stored with a single sample does not tell you much. It's a bit like evaluating an athlete's ability on the basis of one event; everyone has good days and bad daysdate in April 2019, and indicating that applies to GPS watches as well. To illustrate this, was the date when I terminated the images below are from two runs, recorded on 9/20 workout and 9/22attempted to reacquire satellite lock. In each run one case I recorded data on both noticed the 310 date and 910 watches, hitting the lap button on both at as close to the same time as is humanly possible. On 9/20 the 910XT was far more accurate than set incorrectly on the 310XT, but on 9/22 watch display after the situation is reversedsatellite lost message. If you were to have evaluated There are also reports from various users about lost satellite reception and the two watches on 2019 date. This problem might also explain the basis of a single runoffset track above, you would conclude that one is much better than but only if the other. But which device would win would depend on the particular day. This is why I've accumulated a lot of data to do clock was out by a statistical analysis to work out which is really bettervery small amount. {{:GPS Accuracy-Fenix2}}

|[[File:310XT BadFenix2 Getting Lost.jpg|none|thumb|x500pxx400px| This is an example of just how bad the Fenix 2 can be. This is a short run, with the start and finish in the same place. The {{Garmin 310XT}} having track up to marker 18 is not bad, but then the Fenix 2 loses reception for a bad daycouple of miles. When it gets reception back, it tracks wildly off course, ending up with a position that's out by around a mile.]]|[[File:Fenix2 Getting Lost3.jpg|none|thumb|x400px|Another example of the Fenix 2 getting lost. You can see on marker 41 is a long way off the upper route, probably about half a mile off. Notice how messy the rest of the course where it got a little confused and off trackis as well. ]]|[[File:910XT GoodFenix2 Getting Lost4.jpg|none|thumb|x500pxx400px|The {{Garmin 910XT}} on Here you can see the same run having no problems, and only the standard, expected level of inaccuracyFenix 2 track is a confused mess.]]

|[[File:310XT GoodFenix2 Getting Lost5.jpg|none|thumb|x500pxx400px|Two days later The first part of this run goes okay, but at marker 61 things to go a little astray, and it's at marker 65 the turn of GPS lock is lost, then briefly regained until marker 70. Not unreasonably, the {{Garmin 310XT}} Fenix 2 assumes straight-line movement until GPS lock is reacquired, but then rather bizarrely seems to have assume that the straight-line movement is correct and records a track that is about half a good daymile/1 Km off.]]|[[File:910XT BadFenix2 Short1.jpg|none|thumb|x500pxx400px|AgainThis is more how the GPS track should look, but even on this track is recorded on run the same Fenix 2 lost nearly a mile in a 20 mile run as the image to the left. The {{Garmin 910XT}} gets a little confused at the start]]|[[File:Fenix2 Getting Lost6.jpg|none|thumb|x400px|This GPS track looks reasonable until marker #54, and then again around lap 27the track gets offset, but strangely it stays offset until the last marker.]]

* Gather data from some other [[Best Running Watch| GPS Running Watches]]. ** I will evaluate the [http://www.amazon.com/Garmin-010-01128-30-Forerunner-620-Bundle/dp/B00FBYYC90 Garmin 620] when it becomes available as it has some promising functionality.** I am gathering data on an older Garmin 205. It's not a watch I'd recommend, but it will be interesting to see if the technology has improved over time. ** I will soon start gathering data on an iPhone 4S to see how it compares with dedicated devices. * Add in some graphs of the distribution of accuracy, and possibly a [http://en.wikipedia.org/wiki/Q%E2%80%93Q_plot Q-Q plot] (which shows reasonably [http://en.wikipedia.org/wiki/Normal_distribution normal distribution]).* Check how GPS accuracy changes over the course of a run, as I've seen a distinct tendency for the watches to say they are good to go when they don't really have an optimal lock on the satellites. I wait for 5+ minutes between the watches saying they have sufficient satellites locked in, so this should not be a problem with the data shown here, but I could do some tests where I turn on the watch from a cold state, then start running as soon as they claim they have a lock.