I evaluated the real world accuracy of GPS watches while running over 3,500 miles/5,600Km and recording over 14,000 data points as part of my evaluation of the Best Running Watches. Under good conditions most of the watches are remarkably good, but when things get a little tough the differences become more apparent.

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.)

Device	Accuracy	Trueness	Precision	Repeatability	Chipset Make	Chipset Model or Architecture	Review	Rank
Stryd Footpod (uncalibrated)	9.4	9.7	9.2	9.3	None	None	Stryd Review	1
Adidas Speed Cell FP (calibrated)	8.6	9.7	8.0	8.2	None	None	Footpod	2
Garmin Footpod (calibrated)	8.2	9.7	7.5	7.6	None	None	Footpod	3
MilestonePod Footpod (calibrated)	8.1	9.8	7.4	7.7	None	None	MilestonePod Review	4
Polar V800	8.1	9.7	7.4	8.1	SiRFstar	SiRFstarIV	Polar V800 Review	5
Suunto Ambit3 Run	7.9	9.6	7.1	7.8	SiRFstar	SIRFstarV	Suunto Ambit3 Run Review	6
iPhone 5s	7.9	9.7	7.1	7.6	Qualcomm	WTR1605L	Running With A Smartphone	7
Suunto Spartan Trainer 1.10.28	7.8	8.8	7.2	8.0	MediaTek	MT3339	Suunto Spartan Trainer Review	8
Polar Stride Sensor Footpod (calibrated)	7.7	8.5	7.0	8.0	None	None	Footpod	9
Garmin 205	7.6	9.7	6.7	7.0	SiRFstar	SiRFstarIII		10
Suunto Ambit2 R	7.6	8.5	6.9	7.8	SiRFstar	SIRFstarIV	Suunto Ambit2 R Review	11
Garmin 910XT	7.5	8.8	6.7	7.6	SiRFstar	SIRFstarIV	Garmin 910XT Review	12
iPhone 4s	7.5	9.0	6.6	7.2	Broadcom	BCM4330	Running With A Smartphone	13
Garmin 310XT	7.3	9.4	6.2	6.6	SiRFstar	SiRFstarIII	Garmin 310XT Review	14
Leikr	7.3	8.9	6.3	7.2	Unknown	Unknown	Leikr Review	15
Garmin 610	7.3	8.3	6.5	7.7	SiRFstar	SIRFstarIV	Garmin 610 Review	16
Polar M430 1.0.28	7.2	8.9	6.1	7.2	SiRFstar	SiRFstarIV	Polar M430 Review	17
Suunto Spartan Ultra	7.1	9.5	6.0	7.4	SiRFstar	CSRG0530 (SiRFstarV)	Suunto Spartan Ultra Review	18
Garmin 620	7.1	8.2	6.2	7.2	MediaTek	MT3333	Garmin 620 Review	19
Apple Watch 3 iSmoothRun	7.0	8.9	5.9	7.0	Apple	Unknown	Apple Watch 3 Review	20
Samsung Galaxy S3	6.8	8.6	5.7	7.0	Broadcom	BCM47511	Running With A Smartphone	21
Polar M400 v1.9.600	6.6	8.2	5.6	7.1	U-blox	Unknown	Polar M400 Review	22
Garmin 920XT	6.6	7.5	5.9	7.5	MediaTek	MT3333	Garmin 920XT Review	23
TomTom Cardio Runner	6.6	7.6	5.7	7.5	SiRFstar	SIRFstarV	TomTom Cardio Runner Review	24
Garmin 225	6.2	7.3	5.4	7.1	MediaTek	MT3333	Garmin 225 Review	25
Garmin Fenix 3	6.2	7.6	5.3	6.8	MediaTek	MT3333	Garmin Fenix 3 Review	26
Polar RC3 GPS	6.1	8.2	4.7	5.9	SiRFstar	SIRFstarIV	Polar RC3 Review	27
Garmin Fenix 2	5.7	7.2	4.5	7.2	MediaTek	Unknown	Garmin Fenix 2 Review	28
Fenix 5X 4.30	5.6	6.9	4.5	6.8	MediaTek	Unknown	Garmin Fenix 5X Review	29
Garmin 935	5.6	6.8	4.6	7.2	MediaTek	Unknown	Garmin 935 Review	30
Epson SF-810	5.4	7.3	4.1	5.7	Epson	Unknown	Epson SF-810 Review	31
Garmin Epix	5.4	6.8	4.4	6.8	MediaTek	MT3333	Garmin Epix Review	32
Garmin 10	5.1	6.2	4.1	6.9	MediaTek	Unknown	Garmin 10 Review	33
Garmin Vivoactive	5.0	6.2	4.1	6.6	MediaTek	Unknown	Garmin Vivoactive Review	34
Garmin 235	4.9	6.1	4.0	6.4	MediaTek	MT3333	Garmin 235 Review	35
Garmin VivoactiveHR	4.9	6.0	4.0	6.9	MediaTek	Unknown	Garmin Vivoactive HR Review	36
Garmin Vivoactive 3	4.5	5.8	3.4	6.5	MediaTek	Unknown	Garmin Vivoactive 3 Review	37
Polar M400 v1.4	4.4	5.7	3.4	6.5	U-blox	Unknown	Polar M400 Review	38
Epson SF-510	4.3	6.1	3.0	5.4	Epson	Unknown	Epson SF-810 Review	39

1 Methodology

Main article: GPS 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.

2 Accuracy, Trueness and Precision

For this evaluation I'll use the ISO 5725 definition of Accuracy as the combination of trueness and precision.

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.	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. )

3 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.

Device	Count	Accuracy (Combined)	Trueness (Average Distance Error)	Precision (From mean)	Precision (From true)	Repeatability	Pace Error (95%)		Distance Error
Device	Count	Accuracy (Combined)	Trueness (Average Distance Error)	Precision (From mean)	Precision (From true)	Repeatability	(from 9:00 min/mile)	(from 5:30 min/Km)	(20 miles)	(30 Km)
Stryd Footpod (uncalibrated)	321	0.62% (32.8 Ft/Mile, 6.2 m/Km)	0.29% (15.2 Ft/Mile, 2.9 m/Km)	0.77% (40.8 Ft/Mile, 7.7 m/Km)	0.83% (43.8 Ft/Mile, 8.3 m/Km)	0.73% (38.6 Ft/Mile, 7.3 m/Km)	0:09	0:05	0.06	0.09
Adidas Speed Cell FP (calibrated)	361	1.43% (75.7 Ft/Mile, 14.3 m/Km)	0.34% (17.9 Ft/Mile, 3.4 m/Km)	1.97% (103.9 Ft/Mile, 19.7 m/Km)	2.00% (105.6 Ft/Mile, 20.0 m/Km)	1.78% (94.2 Ft/Mile, 17.8 m/Km)	0:22	0:13	0.07	0.10
Garmin Footpod (calibrated)	3667	1.75% (92.6 Ft/Mile, 17.5 m/Km)	0.28% (14.7 Ft/Mile, 2.8 m/Km)	2.45% (129.3 Ft/Mile, 24.5 m/Km)	2.47% (130.2 Ft/Mile, 24.7 m/Km)	2.38% (125.6 Ft/Mile, 23.8 m/Km)	0:27	0:16	0.06	0.08
MilestonePod Footpod (calibrated)	439	1.86% (98.1 Ft/Mile, 18.6 m/Km)**	0.19% (10.1 Ft/Mile, 1.9 m/Km)	2.61% (138.0 Ft/Mile, 26.1 m/Km)	2.62% (138.4 Ft/Mile, 26.2 m/Km)	2.33% (123.2 Ft/Mile, 23.3 m/Km)	0:28	0:17	0.04	0.06
Polar V800	1986	1.87% (98.8 Ft/Mile, 18.7 m/Km)*****	0.29% (15.2 Ft/Mile, 2.9 m/Km)	2.61% (138.0 Ft/Mile, 26.1 m/Km)	2.63% (138.8 Ft/Mile, 26.3 m/Km)	1.94% (102.4 Ft/Mile, 19.4 m/Km)	0:28	0:17	0.06	0.09
Suunto Ambit3 Run	1261	2.08% (110.1 Ft/Mile, 20.8 m/Km)*****	0.44% (23.2 Ft/Mile, 4.4 m/Km)	2.88% (152.1 Ft/Mile, 28.8 m/Km)	2.92% (153.9 Ft/Mile, 29.2 m/Km)	2.17% (114.8 Ft/Mile, 21.7 m/Km)	0:31	0:19	0.09	0.13
iPhone 5s	513	2.09% (110.4 Ft/Mile, 20.9 m/Km)	0.30% (15.8 Ft/Mile, 3.0 m/Km)	2.93% (154.5 Ft/Mile, 29.3 m/Km)	2.94% (155.3 Ft/Mile, 29.4 m/Km)	2.35% (124.2 Ft/Mile, 23.5 m/Km)	0:32	0:19	0.06	0.09
Suunto Spartan Trainer 1.10.28	590	2.17% (114.8 Ft/Mile, 21.7 m/Km)*****	1.25% (65.8 Ft/Mile, 12.5 m/Km)	2.50% (132.0 Ft/Mile, 25.0 m/Km)	2.81% (148.4 Ft/Mile, 28.1 m/Km)	1.99% (105.3 Ft/Mile, 19.9 m/Km)	0:30	0:19	0.25	0.37
Polar Stride Sensor Footpod (calibrated)	309	2.33% (123.2 Ft/Mile, 23.3 m/Km)*****	1.46% (77.1 Ft/Mile, 14.6 m/Km)	2.53% (133.4 Ft/Mile, 25.3 m/Km)	2.96% (156.2 Ft/Mile, 29.6 m/Km)	2.05% (108.1 Ft/Mile, 20.5 m/Km)	0:32	0:20	0.29	0.44
Garmin 205	1155	2.36% (124.4 Ft/Mile, 23.6 m/Km)*****	0.34% (17.7 Ft/Mile, 3.4 m/Km)	3.30% (174.1 Ft/Mile, 33.0 m/Km)	3.32% (175.1 Ft/Mile, 33.2 m/Km)	2.97% (156.9 Ft/Mile, 29.7 m/Km)	0:36	0:22	0.07	0.10
Suunto Ambit2 R	1025	2.41% (127.4 Ft/Mile, 24.1 m/Km)*****	1.49% (78.8 Ft/Mile, 14.9 m/Km)	2.67% (140.9 Ft/Mile, 26.7 m/Km)	3.07% (162.0 Ft/Mile, 30.7 m/Km)	2.22% (117.5 Ft/Mile, 22.2 m/Km)	0:33	0:20	0.30	0.45
Garmin 910XT	701	2.46% (129.9 Ft/Mile, 24.6 m/Km)*****	1.22% (64.3 Ft/Mile, 12.2 m/Km)	3.01% (158.9 Ft/Mile, 30.1 m/Km)	3.26% (172.1 Ft/Mile, 32.6 m/Km)	2.40% (126.5 Ft/Mile, 24.0 m/Km)	0:35	0:22	0.24	0.37
iPhone 4s	945	2.51% (132.4 Ft/Mile, 25.1 m/Km)*****	1.01% (53.6 Ft/Mile, 10.1 m/Km)	3.24% (171.0 Ft/Mile, 32.4 m/Km)	3.40% (179.5 Ft/Mile, 34.0 m/Km)	2.83% (149.2 Ft/Mile, 28.3 m/Km)	0:37	0:22	0.20	0.30
Garmin 310XT	4801	2.69% (142.2 Ft/Mile, 26.9 m/Km)*****	0.61% (32.0 Ft/Mile, 6.1 m/Km)	3.71% (195.9 Ft/Mile, 37.1 m/Km)	3.76% (198.5 Ft/Mile, 37.6 m/Km)	3.35% (176.9 Ft/Mile, 33.5 m/Km)	0:41	0:25	0.12	0.18
Leikr	650	2.71% (143.3 Ft/Mile, 27.1 m/Km)*****	1.12% (59.1 Ft/Mile, 11.2 m/Km)	3.49% (184.2 Ft/Mile, 34.9 m/Km)	3.67% (193.9 Ft/Mile, 36.7 m/Km)	2.84% (150.1 Ft/Mile, 28.4 m/Km)	0:40	0:24	0.22	0.34
Garmin 610	997	2.75% (145.1 Ft/Mile, 27.5 m/Km)*****	1.66% (87.5 Ft/Mile, 16.6 m/Km)	3.08% (162.8 Ft/Mile, 30.8 m/Km)	3.51% (185.6 Ft/Mile, 35.1 m/Km)	2.29% (121.0 Ft/Mile, 22.9 m/Km)	0:38	0:23	0.33	0.50
Polar M430 1.0.28	589	2.85% (150.4 Ft/Mile, 28.5 m/Km)*****	1.06% (56.0 Ft/Mile, 10.6 m/Km)	3.73% (196.7 Ft/Mile, 37.3 m/Km)	3.88% (205.1 Ft/Mile, 38.8 m/Km)	2.81% (148.4 Ft/Mile, 28.1 m/Km)	0:42	0:26	0.21	0.32
Suunto Spartan Ultra	379	2.86% (150.9 Ft/Mile, 28.6 m/Km)**	0.50% (26.3 Ft/Mile, 5.0 m/Km)	3.98% (210.0 Ft/Mile, 39.8 m/Km)	4.01% (211.8 Ft/Mile, 40.1 m/Km)	2.63% (138.8 Ft/Mile, 26.3 m/Km)	0:43	0:26	0.10	0.15
Garmin 620	1039	2.93% (154.7 Ft/Mile, 29.3 m/Km)*****	1.75% (92.5 Ft/Mile, 17.5 m/Km)	3.30% (174.4 Ft/Mile, 33.0 m/Km)	3.76% (198.3 Ft/Mile, 37.6 m/Km)	2.79% (147.3 Ft/Mile, 27.9 m/Km)	0:41	0:25	0.35	0.53
Apple Watch 3 iSmoothRun	308	2.97% (156.8 Ft/Mile, 29.7 m/Km)**	1.09% (57.8 Ft/Mile, 10.9 m/Km)	3.88% (205.1 Ft/Mile, 38.8 m/Km)	4.05% (214.0 Ft/Mile, 40.5 m/Km)	3.03% (160.0 Ft/Mile, 30.3 m/Km)	0:44	0:27	0.22	0.33
Samsung Galaxy S3	845	3.19% (168.4 Ft/Mile, 31.9 m/Km)*****	1.38% (72.8 Ft/Mile, 13.8 m/Km)	4.05% (214.0 Ft/Mile, 40.5 m/Km)	4.29% (226.7 Ft/Mile, 42.9 m/Km)	3.02% (159.2 Ft/Mile, 30.2 m/Km)	0:46	0:28	0.28	0.41
Polar M400 v1.9.600	384	3.38% (178.5 Ft/Mile, 33.8 m/Km)*****	1.79% (94.5 Ft/Mile, 17.9 m/Km)	4.02% (212.2 Ft/Mile, 40.2 m/Km)	4.43% (234.0 Ft/Mile, 44.3 m/Km)	2.91% (153.5 Ft/Mile, 29.1 m/Km)	0:48	0:29	0.36	0.54
Garmin 920XT	858	3.39% (179.1 Ft/Mile, 33.9 m/Km)*****	2.51% (132.4 Ft/Mile, 25.1 m/Km)	3.19% (168.4 Ft/Mile, 31.9 m/Km)	4.09% (216.0 Ft/Mile, 40.9 m/Km)	2.48% (131.1 Ft/Mile, 24.8 m/Km)	0:44	0:27	0.50	0.75
TomTom Cardio Runner	759	3.44% (181.7 Ft/Mile, 34.4 m/Km)*****	2.36% (124.8 Ft/Mile, 23.6 m/Km)	3.50% (185.0 Ft/Mile, 35.0 m/Km)	4.25% (224.6 Ft/Mile, 42.5 m/Km)	2.54% (134.0 Ft/Mile, 25.4 m/Km)	0:46	0:28	0.47	0.71
Garmin 225	855	3.75% (198.1 Ft/Mile, 37.5 m/Km)*****	2.67% (141.1 Ft/Mile, 26.7 m/Km)	3.68% (194.4 Ft/Mile, 36.8 m/Km)	4.58% (242.0 Ft/Mile, 45.8 m/Km)	2.87% (151.7 Ft/Mile, 28.7 m/Km)	0:49	0:30	0.53	0.80
Garmin Fenix 3	1019	3.75% (198.1 Ft/Mile, 37.5 m/Km)*****	2.44% (129.1 Ft/Mile, 24.4 m/Km)	4.00% (211.3 Ft/Mile, 40.0 m/Km)	4.71% (248.7 Ft/Mile, 47.1 m/Km)	3.21% (169.5 Ft/Mile, 32.1 m/Km)	0:51	0:31	0.49	0.73
Polar RC3 GPS	1104	3.95% (208.5 Ft/Mile, 39.5 m/Km)*****	1.78% (93.9 Ft/Mile, 17.8 m/Km)	4.98% (262.7 Ft/Mile, 49.8 m/Km)	5.29% (279.5 Ft/Mile, 52.9 m/Km)	4.07% (214.8 Ft/Mile, 40.7 m/Km)	0:57	0:35	0.36	0.53
Garmin Fenix 2	1044	4.34% (229.3 Ft/Mile, 43.4 m/Km)*****	2.77% (146.0 Ft/Mile, 27.7 m/Km)	4.70% (248.3 Ft/Mile, 47.0 m/Km)	5.48% (289.5 Ft/Mile, 54.8 m/Km)	2.84% (150.1 Ft/Mile, 28.4 m/Km)	0:59	0:36	0.55	0.83
Fenix 5X 4.30	310	4.44% (234.5 Ft/Mile, 44.4 m/Km)*****	3.12% (164.6 Ft/Mile, 31.2 m/Km)	4.33% (228.7 Ft/Mile, 43.3 m/Km)	5.45% (287.9 Ft/Mile, 54.5 m/Km)	3.24% (171.1 Ft/Mile, 32.4 m/Km)	0:59	0:36	0.62	0.94
Garmin 935	345	4.45% (234.7 Ft/Mile, 44.5 m/Km)*****	3.23% (170.7 Ft/Mile, 32.3 m/Km)	4.19% (221.1 Ft/Mile, 41.9 m/Km)	5.39% (284.7 Ft/Mile, 53.9 m/Km)	2.84% (150.1 Ft/Mile, 28.4 m/Km)	0:58	0:36	0.65	0.97
Epson SF-810	627	4.57% (241.1 Ft/Mile, 45.7 m/Km)*****	2.72% (143.7 Ft/Mile, 27.2 m/Km)	5.14% (271.4 Ft/Mile, 51.4 m/Km)	5.86% (309.3 Ft/Mile, 58.6 m/Km)	4.27% (225.5 Ft/Mile, 42.7 m/Km)	1:03	0:39	0.54	0.82
Garmin Epix	592	4.59% (242.5 Ft/Mile, 45.9 m/Km)*****	3.24% (171.1 Ft/Mile, 32.4 m/Km)	4.54% (239.6 Ft/Mile, 45.4 m/Km)	5.63% (297.3 Ft/Mile, 56.3 m/Km)	3.23% (170.3 Ft/Mile, 32.3 m/Km)	1:01	0:37	0.65	0.97
Garmin 10	1050	4.95% (261.2 Ft/Mile, 49.5 m/Km)*****	3.80% (200.7 Ft/Mile, 38.0 m/Km)	4.43% (233.8 Ft/Mile, 44.3 m/Km)	5.87% (310.2 Ft/Mile, 58.7 m/Km)	3.15% (166.3 Ft/Mile, 31.5 m/Km)	1:03	0:39	0.76	1.14
Garmin Vivoactive	598	4.96% (262.1 Ft/Mile, 49.6 m/Km)*****	3.78% (199.4 Ft/Mile, 37.8 m/Km)	4.46% (235.2 Ft/Mile, 44.6 m/Km)	5.92% (312.5 Ft/Mile, 59.2 m/Km)	3.44% (181.7 Ft/Mile, 34.4 m/Km)	1:04	0:39	0.76	1.13
Garmin 235	899	5.08% (268.1 Ft/Mile, 50.8 m/Km)*****	3.93% (207.3 Ft/Mile, 39.3 m/Km)	4.49% (236.9 Ft/Mile, 44.9 m/Km)	6.01% (317.4 Ft/Mile, 60.1 m/Km)	3.59% (189.3 Ft/Mile, 35.9 m/Km)	1:05	0:40	0.79	1.18
Garmin VivoactiveHR	929	5.10% (269.5 Ft/Mile, 51.0 m/Km)*****	4.02% (212.5 Ft/Mile, 40.2 m/Km)	4.37% (230.8 Ft/Mile, 43.7 m/Km)	5.99% (316.3 Ft/Mile, 59.9 m/Km)	3.05% (161.1 Ft/Mile, 30.5 m/Km)	1:05	0:40	0.80	1.21
Garmin Vivoactive 3	445	5.52% (291.5 Ft/Mile, 55.2 m/Km)*****	4.25% (224.3 Ft/Mile, 42.5 m/Km)	4.85% (256.3 Ft/Mile, 48.5 m/Km)	6.55% (345.9 Ft/Mile, 65.5 m/Km)	3.54% (186.9 Ft/Mile, 35.4 m/Km)	1:11	0:43	0.85	1.27
Polar M400 v1.4	1367	5.56% (293.7 Ft/Mile, 55.6 m/Km)*****	4.28% (226.1 Ft/Mile, 42.8 m/Km)	4.98% (262.8 Ft/Mile, 49.8 m/Km)	6.60% (348.5 Ft/Mile, 66.0 m/Km)	3.46% (182.8 Ft/Mile, 34.6 m/Km)	1:11	0:44	0.86	1.28
Epson SF-510	539	5.67% (299.5 Ft/Mile, 56.7 m/Km)*****	3.94% (207.9 Ft/Mile, 39.4 m/Km)	5.69% (300.4 Ft/Mile, 56.9 m/Km)	6.99% (369.0 Ft/Mile, 69.9 m/Km)	4.60% (243.0 Ft/Mile, 46.0 m/Km)	1:15	0:46	0.79	1.18

3.1 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.

3.2 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 Garmin 620 and Garmin 10 are noticeably poorer than the other devices. I found the accuracy of the 10/620 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 10/620 would have poor accuracy compared with the 610 or 310XT on the same run. On one run, the 620 lost over a mile over a 28 mile distance. When Garmin replaced my 620, the new watch would lose over a mile on a 16 mile run!
The results of the Garmin 610 indicate the problems with the 620 & 10 are not inherent in a smaller device.
The accuracy of all devices is better in a straight line than on curves or bendy routes. My course is a tough test for GPS devices with many curves and only a few relatively straight sections.
Not surprisingly, accuracy drops going under the bridge.
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 Garmin 205 does remarkably well.

4 Recommendations

Here are some recommendations for GPS watches.

GPS watches are accurate enough for casual running.
The better devices are accurate enough for most runners if their limitations are understood.
None of the devices were accurate enough for a runner to trust the display of current pace for training or race pacing.
- For interval training, use a track or measure out the distance using some other mechanism.
- For general training or for races, use a device that supports displaying pace from the Footpod while using GPS for distance.
Adding a Footpod to the Garmin 310XT improves its GPS accuracy.
For the Garmin 610 there was no difference with and without the Footpod. (Trueness was 3.33%/3.32%, Precision was 3.54%/3.68%, with/without).
It takes time for the GPS watches to acquire the satellites. Some watches tended to say they are ready to go before they have an optimal lock. Therefore, to improve accuracy try to give them a little more time. Note that some newer GPS watches such as the Garmin 620 have the ability to be preloaded with the satellite positions, reducing this startup time and start up in accuracy dramatically.

5 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 accumulated a lot of data to do a statistical analysis to work out which is really better.

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.	The Garmin 910XT on the same run having no problems, and only the standard, expected level of inaccuracy.
Two days later and it's the turn of the Garmin 310XT to have a good day.	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.

6 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. These give a good indication of the accuracy problems I've seen with the Garmin 620.

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.	By contrast, the 620 has much wider GPS tracks and dispersed lap markers.

7 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.

The GPS tracks in red with blue dots representing each GPS point recorded. The arrow points to the actual path taken.

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.

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 top (we didn't run through the building).

8 Density Plots

Below are the density plots for each device.

8.1 Normal Condition

Density plots of the GPS accuracy.

8.2 Under the Bridge

Density plots of the GPS accuracy from the segment that goes under the bridge.

8.3 Turn Around

Density plots of the GPS accuracy for the turn-around condition.

9 Garmin 620 Issues

The Garmin 620 has become rather notorious for its poor GPS quality. I raised the issue with Garmin support and they kindly sent me a replacement device, but as you can see below, the replacement was no better. In fact, it was actually worse than my original unit. I've also broken down the readings by firmware version, and you can see some slight improvement going from V2.90 to V3.00, but it's only slight.

Device	Count	Accuracy (Combined)	Trueness (Average Distance Error)	Standard Deviation (From mean)	Standard Deviation (From true)	Pace Error (95%)		Distance Error
Device	Count	Accuracy (Combined)	Trueness (Average Distance Error)	Standard Deviation (From mean)	Standard Deviation (From true)	Garmin 620 (original v2.80)	412	3.24% (171.0 Ft/Mile, 32.4 m/Km)	1.95% (103.0 Ft/Mile, 19.5 m/Km)	3.65% (192.9 Ft/Mile, 36.5 m/Km)	4.14% (218.7 Ft/Mile, 41.4 m/Km)	0:45	0:27	0.39	0.59
Garmin 620 (replacement v2.90)	395	6.65% (351.1 Ft/Mile, 66.5 m/Km)	5.13% (270.8 Ft/Mile, 51.3 m/Km)	5.98% (315.7 Ft/Mile, 59.8 m/Km)	7.88% (416.2 Ft/Mile, 78.8 m/Km)	1:25	0:52	1.03	1.54
Garmin 620 (replacement v3.00)	322	6.24% (329.3 Ft/Mile, 62.4 m/Km)	4.62% (243.7 Ft/Mile, 46.2 m/Km)	5.92% (312.8 Ft/Mile, 59.2 m/Km)	7.51% (396.8 Ft/Mile, 75.1 m/Km)	1:21	0:50	0.92	1.38
Garmin 620 (replacement v3.00, NoEPO)	307	4.06% (214.5 Ft/Mile, 40.6 m/Km)	2.48% (131.2 Ft/Mile, 24.8 m/Km)	4.54% (239.8 Ft/Mile, 45.4 m/Km)	5.18% (273.5 Ft/Mile, 51.8 m/Km)	0:56	0:34	0.50	0.75
Garmin 620 (replacement v3.00, +FP)	546	5.04% (265.9 Ft/Mile, 50.4 m/Km)	2.67% (140.8 Ft/Mile, 26.7 m/Km)	6.04% (319.0 Ft/Mile, 60.4 m/Km)	6.60% (348.7 Ft/Mile, 66.0 m/Km)	1:11	0:44	0.53	0.80
Garmin 620 (replacement v3.00, GPS 3.30)	1039	2.75% (145.2 Ft/Mile, 27.5 m/Km)	1.58% (83.3 Ft/Mile, 15.8 m/Km)	3.18% (168.1 Ft/Mile, 31.8 m/Km)	3.55% (187.7 Ft/Mile, 35.5 m/Km)	0:38	0:23	0.32	0.47

I have come to suspect that the 620 has two issues. Firstly, Garmin has compromised GPS accuracy for size and battery life. This is then compounded by high levels of smoothing in an attempt to compensate. Secondly, I suspect that the 620 has a problem with its WAAS processing. WAAS provides an offset to GPS receivers to correct for errors, so an error in WASS processing could lead to the GPS location being offset. Below are a couple of examples where you can see that it appears that the track has been offset, rather than the 620 simply becoming 'lost'.

Here you can see the last repeat is offset. Starting 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, and the finish near the start.	This is a simple out and back run of ~3 miles/5 Km, but you can see after the turn around the Garmin 620 records a gradually widening gap, even though it follows the right overall shape. (The outbound track is fairly accurate, the return is messed up.)

10 GPS Accuracy and Pace

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.

11 Device Specific Notes

For those interested in some of the details of how devices are configured for testing, here are some additional notes.

Garmin devices are set to 'smart recording'. I did try an informal test with the 620 using 1-second recording, but it appeared to make no difference.

12 Next Steps

This is an initial analysis of the data I have, and there are a number of further evaluations to do.

Gather data from some other GPS Running Watches.
Add in more graphs of the distribution of accuracy, and possibly a Q-Q plot (which shows reasonably 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.
Look at how accurate the GPS watches are for measuring elevation, and compare with barometric data.
Look for any correlation between accuracy and the use of Heart Rate Monitor. The radio signal from the heart rate monitor could interfere with accuracy.
Write up general GPS accuracy.
List the sources of potential error.
Evaluate the accuracy of the Footpod alone, both trueness and precision.

GPS Accuracy of Garmin, Polar, and other Running Watches

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