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I evaluated the real world accuracy of GPS watches while running over 2,500 miles/4,000Km and recording over 10,000 data points as 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. The table below is a vastly simplified summary of the results, rating the best device as a 10 then comparing the others. (Note that '10' does not indicate perfect, just the best I've measured so far.)
{| class="wikitable"
! Device
| {{Garmin 205}}
| 10.0
| 109.0
| 10.0
|-
| {{Garmin 910XT}} with [[Footpod]]
| 7.4
| 98.8
| 9.0
|-
| {{Garmin 310XT}} with [[Footpod]]
| 5.8
| 98.13
| 7.9
|-
| {{Garmin 610}} (preliminary)
| 4.0
| 10.0
| 7.4
|-
| iPhone 4s
| 4.4
| 98.67
| 7.2
|-
| {{Garmin 310XT}} no [[Footpod]]
| 4.96| 87.12| 6.86
|-
| {{Polar RC3 GPS}}
| 5.7
| 6.92
| 6.5
|-
| {{Garmin 620}}
| 1.9
| 7.6
| 4.3
|-
| {{Garmin 10}} (preliminary)
| 1.8
| 5.1
| 3.5
|}
The results of the Garmin 10 and 610 are preliminary as I have not gathered an equivalent volume of data to the other devices.
=Methodology=
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.
! Condition
! Device
! Count
! Trueness (Average Distance Error)
! Standard Deviation
! 95% within
|-
| rowspan="710"|
Normal
| {{Garmin 10}}
| 274
| style="background-color: #FCB079;"|
7.740% (408.7 Ft/Mile 77.4 m/Km)
| style="background-color: #FB9B75;"|
0.066 (350.7 Ft/Mile 66.4 m/Km)
| 0.133 (701.4 Ft/Mile 132.8 m/Km)
|-
| {{Garmin 205}}
| 779
| style="background-color: #9DCF7E63BE7B;"|01.014381% (72.9 Ft/Mile 13.8 m/Km)| style="background-color: #C9DB80;"|0.031 (165.6 Ft/Mile 31.4 m/Km)
| 0.063 (331.2 Ft/Mile 62.7 m/Km)
|-| {{Garmin 620}}| 944| style="background-color: #FDBC7B;"|6.913% (365.0 Ft/Mile 69.1 m/Km)| style="background-color: #C6DA80FFE383;"|0.031 040 (165213.6 5 Ft/Mile 3140.4 m/Km)| 0.081 (427.0 Ft/Mile 80.9 m/Km)
|-
| iPhone 4s
| 668
| style="background-color: #FFDE82E5E382;"|03.030046% (160.8 Ft/Mile 30.5 m/Km)| style="background-color: #EBE582;"|0.035 (186.0 Ft/Mile 35.2 m/Km)
| 0.070 (372.0 Ft/Mile 70.5 m/Km)
|-
| {{Polar RC3 GPS}}
| 983
| style="background-color: #A8D27F6DC07B;"|01.015511% (79.8 Ft/Mile 15.1 m/Km)| style="background-color: #FFE183;"|0.041 (217.4 Ft/Mile 41.2 m/Km)
| 0.082 (434.7 Ft/Mile 82.3 m/Km)
|-
| {{Garmin 310XT}} with [[Footpod]]
| 27872786| style="background-color: #D0DD8194CC7D;"|02.020013% (105106.5 3 Ft/Mile 20.0 1 m/Km)| style="background-color: #EDE582;"|0.073 035 (384187.7 1 Ft/Mile 7235.9 4 m/Km)| style="background-color: #F8E983;"|0.036 071 (192374.3 Ft/Mile 3670.4 9 m/Km)
|-
| {{Garmin 310XT}} no [[Footpod]]
| 12101368| style="background-color: #FEEA83CADB80;"|2.708% (143.0 Ft/Mile 27.1 m/Km)| style="background-color: #FFE383;"|0.025% 041 (134214.6 8 Ft/Mile 2540.7 m/Km)| 0.081 (429.5 Ft/Mile 81.3 m/Km)| 0-| {{Garmin 610}}| 289| style="background-color: #EBE582;"|3.083 124% (439164.4 9 Ft/Mile 8331.2 m/Km)| style="background-color: #FFDF82CADB80;"|0.042 031 (166.0 Ft/Mile 31.4 m/Km)| 0.063 (219331.7 9 Ft/Mile 4162.6 9 m/Km)
|-
| {{Garmin 910XT}} with [[Footpod]]
| 514
| style="background-color: #A1D07E66BF7B;"|01.014429% (75.4 Ft/Mile 14.3 m/Km)| style="background-color: #B9D780;"|0.030 (155.8 Ft/Mile 29.5 m/Km)
| 0.059 (311.5 Ft/Mile 59.0 m/Km)
|-
| All
| 69418605| style="background-color: #D2DE81D3DE81;"|02.020811% (106148.4 Ft/Mile 2028.1 m/Km)| style="background-color: #FFDE82;"|0.042 (223.2 Ft/Mile 42.3 m/Km)| 0.075 085 (394446.4 3 Ft/Mile 7484.7 5 m/Km)|-| rowspan="10"|Under Bridge| {{Garmin 10}}| 24
| style="background-color: #FFEB84;"|
3.387% (178.8 Ft/Mile 33.9 m/Km)| style="background-color: #D8DF81;"|0.033 (174.3 Ft/Mile 33.0 m/Km)| 0.037 066 (197348.2 6 Ft/Mile 3766.3 0 m/Km)
|-
| {{Garmin 205}}
| 66
| style="background-color: #9DCE7E63BE7B;"|-01.014379% (72.8 Ft/Mile 13.8 m/Km)| 0.148 (780.4 Ft/Mile 147.8 m/Km)
| style="background-color: #FA8671;"|
0.074 (390.2 Ft/Mile 73.9 m/Km)
| 0.148 (780.4 Ft/Mile 147.8 m/Km)
|-
| {{Garmin 620}}
| 61
| style="background-color: #FED881;"|
4.818% (254.4 Ft/Mile 48.2 m/Km)
| style="background-color: #FFE082;"|
0.042 (219.7 Ft/Mile 41.6 m/Km)
| 0.083 (439.4 Ft/Mile 83.2 m/Km)
|-
| iPhone 4s
| 59
| style="background-color: #9FCF7E64BE7B;"|01.014402% (74.0 Ft/Mile 14.0 m/Km)| style="background-color: #DEE182;"|0.034 (178.0 Ft/Mile 33.7 m/Km)
| 0.067 (356.1 Ft/Mile 67.4 m/Km)
|-
| {{Polar RC3 GPS}}
| 86
| style="background-color: #FB9975FECD7F;"|05.056612% (296.3 Ft/Mile 56.1 m/Km)| 0.156 (825.3 Ft/Mile 156.3 m/Km)
| style="background-color: #F97A6F;"|
0.078 (412.7 Ft/Mile 78.2 m/Km)
| 0.156 (825.3 Ft/Mile 156.3 m/Km)
|-
| {{Garmin 310XT}} with [[Footpod]]
| 222
| style="background-color: #FFDA81F0E683;"|03.032193% (168.6 Ft/Mile 31.9 m/Km)| style="background-color: #FECB7E;"|0.049 (260.0 Ft/Mile 49.2 m/Km)
| 0.098 (520.0 Ft/Mile 98.5 m/Km)
|-
| {{Garmin 310XT}} no [[Footpod]]
| 99111| style="background-color: #63BE7B66BE7B;"|01.007420% (3575.3 0 Ft/Mile 614.7 2 m/Km)| style="background-color: #FB9C75;"|0.084 066 (446348.9 Ft/Mile 66.1 m/Km)| 0 .132 (697.9 Ft/Mile 84132.5 2 m/Km)|-| {{Garmin 610}}| 22| style="background-color: #FFDD82CFDD81;"|2.761% (145.8 Ft/Mile 27.6 m/Km)| style="background-color: #FFE984;"|0.042 038 (223201.8 Ft/Mile 38.2 m/Km)| 0 .076 (403.6 Ft/Mile 4276.2 4 m/Km)
|-
| {{Garmin 910XT}} with [[Footpod]]
| 43
| style="background-color: #B2D47F76C37C;"|01.016631% (86.1 Ft/Mile 16.3 m/Km)| 0.168 (889.0 Ft/Mile 168.4 m/Km)
| style="background-color: #F8696B;"|
0.084 (444.5 Ft/Mile 84.2 m/Km)
| 0.168 (889.0 Ft/Mile 168.4 m/Km)
|-
| All
| 575694| style="background-color: #E9E482C7DA80;"|02.023661% (121140.2 5 Ft/Mile 2226.9 m/Km)| 0.124 (654.2 Ft/Mile 123.9 6 m/Km)| style="background-color: #FCA777FCA677;"|0.062 (327329.1 0 Ft/Mile 62.3 m/Km)| 0 .125 (658.0 Ft/Mile 124.6 m/Km)
|-
| rowspan="710"|
Post Bridge
| {{Garmin 10}}
| 24
| style="background-color: #F97C6F;"|
11.616% (613.3 Ft/Mile 116.2 m/Km)
| style="background-color: #F9E983;"|
0.037 (194.4 Ft/Mile 36.8 m/Km)
| 0.074 (388.8 Ft/Mile 73.6 m/Km)
|-
| {{Garmin 205}}
| 66
| style="background-color: #DEE1829FCF7E;"|02.022157% (113.9 Ft/Mile 21.6 m/Km)| style="background-color: #87C87D;"|0.024 (125.5 Ft/Mile 23.8 m/Km)
| 0.048 (250.9 Ft/Mile 47.5 m/Km)
|-| {{Garmin 620}}| 61| style="background-color: #FBA176;"|8.900% (469.9 Ft/Mile 89.0 m/Km)| style="background-color: #7AC47CFECC7E;"|0.024 049 (125257.5 6 Ft/Mile 2348.8 m/Km)| 0.098 (515.2 Ft/Mile 97.6 m/Km)
|-
| iPhone 4s
| 54
| style="background-color: #FDB97BFFDD82;"|04.044422% (233.5 Ft/Mile 44.2 m/Km)| style="background-color: #BCD780;"|0.030 (157.4 Ft/Mile 29.8 m/Km)
| 0.060 (314.9 Ft/Mile 59.6 m/Km)
|-
| {{Polar RC3 GPS}}
| 84
| style="background-color: #F8696BFDB57A;"|07.074399% (390.7 Ft/Mile 74.0 m/Km)| style="background-color: #FEC97E;"|0.050 (263.2 Ft/Mile 49.8 m/Km)
| 0.100 (526.4 Ft/Mile 99.7 m/Km)
|-
| {{Garmin 310XT}} with [[Footpod]]
| 188
| style="background-color: #FDC27DFFE283;"|04.041095% (216.2 Ft/Mile 41.0 m/Km)| style="background-color: #FEEA83;"|0.037 (197.4 Ft/Mile 37.4 m/Km)
| 0.075 (394.8 Ft/Mile 74.8 m/Km)
|-
| {{Garmin 310XT}} no [[Footpod]]
| 95106| style="background-color: #FCA877FED580;"|05.051053% (268266.6 8 Ft/Mile 50.9 5 m/Km)| style="background-color: #EDE582;"|0.074 035 (187.0 Ft/Mile 35.4 m/Km)| 0.071 (374.1 Ft/Mile 70.8 m/Km)|-| {{Garmin 610}}| 22| style="background-color: #FED881;"|4.839% (388255.5 Ft/Mile 7348.6 4 m/Km)| style="background-color: #FCEA8363BE7B;"|0.037 020 (103.1 Ft/Mile 19.5 m/Km)| 0.039 (194206.3 1 Ft/Mile 3639.8 0 m/Km)
|-
| {{Garmin 910XT}} with [[Footpod]]
| 40
| style="background-color: #FDBC7BFFDE82;"|04.043339% (229.1 Ft/Mile 43.4 m/Km)| style="background-color: #73C27B;"|0.021 (113.2 Ft/Mile 21.4 m/Km)
| 0.043 (226.4 Ft/Mile 42.9 m/Km)
|-
| All
| 527645| style="background-color: #FDB47AFED280;"|05.046287% (243279.4 1 Ft/Mile 4652.1 9 m/Km)| style="background-color: #FFDB81;"|0.079 043 (415228.6 5 Ft/Mile 7843.7 3 m/Km)| style="background-color: #FFE583;"|0.039 087 (207456.8 9 Ft/Mile 3986.4 5 m/Km)
|-
| rowspan="710"|
Turn Around
| {{Garmin 10}}
| 23
| style="background-color: #F8696B;"|
12.971% (684.9 Ft/Mile 129.7 m/Km)
| style="background-color: #FFEB84;"|
0.038 (198.2 Ft/Mile 37.5 m/Km)
| 0.075 (396.5 Ft/Mile 75.1 m/Km)
|-
| {{Garmin 205}}
| 72
| style="background-color: #FECE7FFFE884;"|03.036645% (192.5 Ft/Mile 36.5 m/Km)| style="background-color: #9ACE7E;"|0.026 (136.9 Ft/Mile 25.9 m/Km)
| 0.052 (273.9 Ft/Mile 51.9 m/Km)
|-| {{Garmin 620}}| 80| style="background-color: #FA8070;"|11.302% (596.7 Ft/Mile 113.0 m/Km)| style="background-color: #8FCA7DFFDB81;"|0.026 043 (136228.9 5 Ft/Mile 43.3 m/Km)| 0.087 (457.0 Ft/Mile 2586.9 6 m/Km)
|-
| iPhone 4s
| 62
| style="background-color: #FA7E70FDBF7C;"|06.066623% (349.7 Ft/Mile 66.2 m/Km)| style="background-color: #FED881;"|0.045 (235.6 Ft/Mile 44.6 m/Km)
| 0.089 (471.3 Ft/Mile 89.3 m/Km)
|-
| {{Polar RC3 GPS}}
| 92
| style="background-color: #FA8270FDC27C;"|06.065471% (341.7 Ft/Mile 64.7 m/Km)| 0.154 (814.6 Ft/Mile 154.3 m/Km)
| style="background-color: #FA7D6F;"|
0.077 (407.3 Ft/Mile 77.1 m/Km)
| 0.154 (814.6 Ft/Mile 154.3 m/Km)
|-
| {{Garmin 310XT}} with [[Footpod]]
| 205
| style="background-color: #FB9B75FECE7F;"|05.056573% (294.2 Ft/Mile 55.7 m/Km)| style="background-color: #FFE082;"|0.042 (219.6 Ft/Mile 41.6 m/Km)
| 0.083 (439.2 Ft/Mile 83.2 m/Km)
|-
| {{Garmin 310XT}} no [[Footpod]]
| 107123| style="background-color: #FB9273FEC87E;"|6.017% (317.7 Ft/Mile 60.2 m/Km)| style="background-color: #FDBD7C;"|0.059% 054 (311285.8 9 Ft/Mile 5954.1 m/Km)| 0.115 108 (605571.8 Ft/Mile 114108.3 m/Km)|-| {{Garmin 610}}| 26| style="background-color: #FDC47D;"|6.255% (330.3 Ft/Mile 62.7 6 m/Km)| style="background-color: #FCB47A86C87D;"|0.057 024 (302124.9 7 Ft/Mile 5723.6 m/Km)| 0.047 (249.4 Ft/Mile 47.2 m/Km)
|-
| {{Garmin 910XT}} with [[Footpod]]
| 43
| style="background-color: #FCA777FED480;"|05.051117% (270.2 Ft/Mile 51.2 m/Km)| style="background-color: #98CD7E;"|0.026 (135.3 Ft/Mile 25.6 m/Km)
| 0.051 (270.7 Ft/Mile 51.3 m/Km)
|-
| All
| 581726| style="background-color: #FB9975FDC17C;"|06.056523% (296344.5 4 Ft/Mile 5665.2 m/Km)| style="background-color: #FDC17C;"|0.102 053 (536279.1 Ft/Mile 10152.5 9 m/Km)| style="background-color: #FDC67D;"|0.051 106 (268558.0 1 Ft/Mile 50105.8 7 m/Km)
|-
| rowspan="710"|
Power Line
| {{Garmin 10}}
| 46
| style="background-color: #FCAF79;"|
7.841% (414.0 Ft/Mile 78.4 m/Km)
| style="background-color: #EFE683;"|
0.036 (188.5 Ft/Mile 35.7 m/Km)
| 0.071 (377.0 Ft/Mile 71.4 m/Km)
|-
| {{Garmin 205}}
| 127
| style="background-color: #9FCF7E64BE7B;"|01.014395% (73.7 Ft/Mile 14.0 m/Km)| style="background-color: #83C77C;"|0.023 (122.5 Ft/Mile 23.2 m/Km)
| 0.046 (245.1 Ft/Mile 46.4 m/Km)
|-| {{Garmin 620}}| 162| style="background-color: #74C37CFCAF79;"|7.860% (415.0Ft/Mile 78.023 6 m/Km)| style="background-color: #AED37F;"|0.028 (122148.7 Ft/Mile 28.2 m/Km)| 0.056 (297.5 Ft/Mile 2356.2 3 m/Km)
|-
| iPhone 4s
| 110
| style="background-color: #FFEB84BFD880;"|02.026560% (135.2 Ft/Mile 25.6 m/Km)| style="background-color: #B4D57F;"|0.029 (152.7 Ft/Mile 28.9 m/Km)
| 0.058 (305.3 Ft/Mile 57.8 m/Km)
|-
| {{Polar RC3 GPS}}
| 167
| style="background-color: #D3DE8195CC7D;"|02.020030% (107.2 Ft/Mile 20.3 m/Km)| style="background-color: #F0E683;"|0.036 (188.8 Ft/Mile 35.8 m/Km)
| 0.072 (377.7 Ft/Mile 71.5 m/Km)
|-
| {{Garmin 310XT}} with [[Footpod]]
| 515
| style="background-color: #FFEB84BFD880;"|02.026566% (135.5 Ft/Mile 25.7 m/Km)| style="background-color: #E6E482;"|0.035 (183.2 Ft/Mile 34.7 m/Km)
| 0.069 (366.5 Ft/Mile 69.4 m/Km)
|-
| {{Garmin 310XT}} no [[Footpod]]
| 203229| style="background-color: #FED580FDEA83;"|03.034360% (179177.4 Ft/Mile 33.6 m/Km)| style="background-color: #EEE683;"|0 .036 (187.7 Ft/Mile 3335.9 5 m/Km)| 0.072 071 (377375.9 3 Ft/Mile 71.6 1 m/Km)|-| {{Garmin 610}}| 43| style="background-color: #FFE383;"|4.038% (213.2 Ft/Mile 40.4 m/Km)| style="background-color: #F2E7839ACD7E;"|0.036 026 (188136.7 Ft/Mile 25.9 m/Km)| 0.052 (273.3 Ft/Mile 3551.8 m/Km)
|-
| {{Garmin 910XT}} with [[Footpod]]
| 90
| style="background-color: #D9E0819BCE7E;"|02.021101% (110.9 Ft/Mile 21.0 m/Km)| style="background-color: #8AC97D;"|0.024 (127.2 Ft/Mile 24.1 m/Km)
| 0.048 (254.4 Ft/Mile 48.2 m/Km)
|-
| All
| 12121489| style="background-color: #F8E983F7E883;"|03.025281% (130173.5 2 Ft/Mile 2432.7 8 m/Km)| style="background-color: #FFEB84;"|0.067 038 (351198.5 Ft/Mile 6637.6 m/Km)| style="background-color: #D9E081;"|0.033 075 (175397.8 1 Ft/Mile 3375.3 2 m/Km)
|-
| rowspan="710"|
Mixed
| {{Garmin 10}}
| 391
| style="background-color: #FCAC78;"|
8.030% (424.0 Ft/Mile 80.3 m/Km)
| style="background-color: #FCA777;"|
0.062 (327.0 Ft/Mile 61.9 m/Km)
| 0.124 (654.0 Ft/Mile 123.9 m/Km)
|-
| {{Garmin 205}}
| 1110
| style="background-color: #A0CF7E65BE7B;"|01.014411% (74.5 Ft/Mile 14.1 m/Km)| style="background-color: #EAE582;"|0.035 (185.6 Ft/Mile 35.2 m/Km)
| 0.070 (371.2 Ft/Mile 70.3 m/Km)
|-| {{Garmin 620}}| 1308| style="background-color: #FDB67A;"|7.293% (385.1 Ft/Mile 72.9 m/Km)| style="background-color: #EBE582FFE082;"|0.035 042 (185220.6 1 Ft/Mile 3541.7 m/Km)| 0.083 (440.2 Ft/Mile 83.4 m/Km)
|-
| iPhone 4s
| 953
| style="background-color: #FFDA81F1E783;"|03.032199% (168.9 Ft/Mile 32.0 m/Km)| style="background-color: #F6E883;"|0.036 (192.4 Ft/Mile 36.4 m/Km)
| 0.073 (384.8 Ft/Mile 72.9 m/Km)
|-
| {{Polar RC3 GPS}}
| 1412
| style="background-color: #FAE983BAD780;"|02.025495% (131.8 Ft/Mile 25.0 m/Km)| 0.102 (539.9 Ft/Mile 102.3 m/Km)
| style="background-color: #FDC57D;"|
0.051 (269.9 Ft/Mile 51.1 m/Km)
| 0.102 (539.9 Ft/Mile 102.3 m/Km)
|-
| {{Garmin 310XT}} with [[Footpod]]
| 39173916| style="background-color: #F4E783B5D57F;"|02.024439% (128.2 8 Ft/Mile 24.3 4 m/Km)| style="background-color: #FFEB84;"|0.077 038 (406199.2 5 Ft/Mile 7637.9 8 m/Km)| style="background-color: #FFE884;"|0.038 076 (203399.1 Ft/Mile 3875.5 6 m/Km)
|-
| {{Garmin 310XT}} no [[Footpod]]
| 17141937| style="background-color: #FFE283E5E382;"|03.029050% (152.6 Ft/Mile 28161.9 m/Km)| 0.087 (457.9 Ft/Mile 8630.7 5 m/Km)
| style="background-color: #FFDA81;"|
0.043 044 (229231.7 Ft/Mile 43.9 m/Km)| 0 .088 (463.3 Ft/Mile 4387.8 m/Km)|-| {{Garmin 610}}| 402| style="background-color: #FFEA84;"|3.498% (184.7 Ft/Mile 35.0 m/Km)| style="background-color: #CBDC81;"|0.032 (166.5 Ft/Mile 31.5 m/Km)| 0.063 (332.9 Ft/Mile 63.4 1 m/Km)
|-
| {{Garmin 910XT}} with [[Footpod]]
| 730
| style="background-color: #C8DB808BC97D;"|01.019900% (100.3 Ft/Mile 19.0 m/Km)| style="background-color: #EFE683;"|0.036 (188.6 Ft/Mile 35.7 m/Km)
| 0.071 (377.2 Ft/Mile 71.4 m/Km)
|-
| All
| 983612159| style="background-color: #F5E883F2E783;"|03.024213% (128169.8 7 Ft/Mile 2432.4 1 m/Km)| style="background-color: #FED680;"|0.082 045 (432237.6 7 Ft/Mile 8145.9 0 m/Km)| style="background-color: #FFE182;"|0.041 090 (216475.3 4 Ft/Mile 4190.0 m/Km)
|}
==Accuracy by Device then Condition==
! Device
! Condition
! Count! 95% within
! Trueness (Average Distance Error)
! Standard Deviation
! 95% within
|-
| rowspan="6"|
| Normal
| 779
| style="background-color: #63BE7B;"|
1.381% (72.9 Ft/Mile 13.8 m/Km)
| style="background-color: #D2DE81;"|
0.031 (165.6 Ft/Mile 31.4 m/Km)
| 0.063 (331.2 Ft/Mile 62.7 m/Km)
|-
| Under Bridge
| 66
| style="background-color: #FA8571;"|
0.074 (390.2 Ft/Mile 73.9 m/Km)
| 0.148 (780.4 Ft/Mile 147.8 m/Km)
|-
| Post Bridge
| 66
| style="background-color: #9DCE7E;"|
2.157% (113.9 Ft/Mile 21.6 m/Km)
| style="background-color: #8AC97D;"|
0.024 (125.5 Ft/Mile 23.8 m/Km)
| 0.048 (250.9 Ft/Mile 47.5 m/Km)
|-
| Turn Around
| 72
| style="background-color: #FFE984;"|
3.645% (192.5 Ft/Mile 36.5 m/Km)
| style="background-color: #9FCF7E;"|
0.026 (136.9 Ft/Mile 25.9 m/Km)
| 0.052 (273.9 Ft/Mile 51.9 m/Km)
|-
| Power Line
| 127
| style="background-color: #64BE7B;"|
1.395% (73.7 Ft/Mile 14.0 m/Km)
| style="background-color: #85C87D;"|
0.023 (122.5 Ft/Mile 23.2 m/Km)
| 0.046 (245.1 Ft/Mile 46.4 m/Km)
|-
| Mixed
| 1110
| style="background-color: #65BE7B;"|
1.411% (74.5 Ft/Mile 14.1 m/Km)
| style="background-color: #F6E883;"|
0.035 (185.6 Ft/Mile 35.2 m/Km)
| 0.070 (371.2 Ft/Mile 70.3 m/Km)
|-| rowspan="6"|{{Garmin 620}}| Normal| 944| style="background-color: #A0CF7EFDBC7B;"|6.913% (365.0 Ft/Mile 69.1 m/Km)| style="background-color: #FFE082;"|0.040 (213.5 Ft/Mile 40.4 m/Km)| 0.081 (427.0 Ft/Mile 80.9 m/Km)|-| Under Bridge| 61| style="background-color: #FED981;"|4.818% (254.4 Ft/Mile 48.2 m/Km)| style="background-color: #FFDD82;"|0.042 (219.7 Ft/Mile 41.6 m/Km)| 0.083 (439.4 Ft/Mile 83.2 m/Km)|-| Post Bridge| 61| style="background-color: #FBA176;"|8.900% (469.9 Ft/Mile 89.0 m/Km)| style="background-color: #FEC97E;"|0.049 (257.6 Ft/Mile 48.8 m/Km)| 0.014098 (515.2 Ft/Mile 97.6 m/Km)|-| Turn Around| 80| style="background-color: #FA8070;"|11.302% (74596.7 Ft/Mile 113.0 m/Km)| style="background-color: #FED881;"|0.043 (228.5 Ft/Mile 1443.3 m/Km)| 0.087 (457.0 Ft/Mile 86.1 6 m/Km)|-| Power Line| 162| style="background-color: #FCAF79;"|7.860% (415.0 Ft/Mile 78.6 m/Km)| style="background-color: #B4D57F;"|0.070 028 (371148.7 Ft/Mile 28.2 m/Km)| 0.056 (297.5 Ft/Mile 7056.3 m/Km)|-| Mixed| 1308| style="background-color: #FDB77A;"|7.293% (385.1 Ft/Mile 72.9 m/Km)| style="background-color: #F1E783FFDC82;"|0.035 042 (185220.6 1 Ft/Mile 3541.7 m/Km)| 0.083 (440.2 Ft/Mile 83.4 m/Km)
|-
| rowspan="6"|
| Normal
| 668
| style="background-color: #E1E282;"|
3.046% (160.8 Ft/Mile 30.5 m/Km)
| style="background-color: #F6E883;"|
0.035 (186.0 Ft/Mile 35.2 m/Km)
| 0.070 (372.0 Ft/Mile 70.5 m/Km)
|-
| Under Bridge
| 59
| style="background-color: #64BE7B;"|
1.402% (74.0 Ft/Mile 14.0 m/Km)
| style="background-color: #E8E482;"|
0.034 (178.0 Ft/Mile 33.7 m/Km)
| 0.067 (356.1 Ft/Mile 67.4 m/Km)
|-
| Post Bridge
| 54
| style="background-color: #FFDE82;"|
4.422% (233.5 Ft/Mile 44.2 m/Km)
| style="background-color: #C3D980;"|
0.030 (157.4 Ft/Mile 29.8 m/Km)
| 0.060 (314.9 Ft/Mile 59.6 m/Km)
|-
| Turn Around
| 62
| style="background-color: #FDC07C;"|
6.623% (349.7 Ft/Mile 66.2 m/Km)
| style="background-color: #FED480;"|
0.045 (235.6 Ft/Mile 44.6 m/Km)
| 0.089 (471.3 Ft/Mile 89.3 m/Km)
|-
| Power Line
| 110
| style="background-color: #BCD780;"|
2.560% (135.2 Ft/Mile 25.6 m/Km)
| style="background-color: #BBD780;"|
0.029 (152.7 Ft/Mile 28.9 m/Km)
| 0.058 (305.3 Ft/Mile 57.8 m/Km)
|-
| Mixed
| 953
| style="background-color: #FFEB84;"|
0.036 (192.4 Ft/Mile 36.4 m/Km)
| 0.073 (384.8 Ft/Mile 72.9 m/Km)
|-
| rowspan="6"|
| Normal
| 983
| style="background-color: #6CC07B;"|
1.511% (79.8 Ft/Mile 15.1 m/Km)
| style="background-color: #FFDE82;"|
0.041 (217.4 Ft/Mile 41.2 m/Km)
| 0.082 (434.7 Ft/Mile 82.3 m/Km)
|-
| Under Bridge
| 86
| style="background-color: #F97A6F;"|
0.078 (412.7 Ft/Mile 78.2 m/Km)
| 0.156 (825.3 Ft/Mile 156.3 m/Km)
|-
| Post Bridge
| 84
| style="background-color: #FDB67A;"|
7.399% (390.7 Ft/Mile 74.0 m/Km)
| style="background-color: #FDC67D;"|
0.050 (263.2 Ft/Mile 49.8 m/Km)
| 0.100 (526.4 Ft/Mile 99.7 m/Km)
|-
| Turn Around
| 92
| style="background-color: #FA7D6F;"|
0.077 (407.3 Ft/Mile 77.1 m/Km)
| 0.154 (814.6 Ft/Mile 154.3 m/Km)
|-
| Power Line
| 167
| style="background-color: #94CC7D;"|
2.030% (107.2 Ft/Mile 20.3 m/Km)
| style="background-color: #FBEA83;"|
0.036 (188.8 Ft/Mile 35.8 m/Km)
| 0.072 (377.7 Ft/Mile 71.5 m/Km)
|-
| Mixed
| 1412
| style="background-color: #B7D67F;"|
2.495% (131.8 Ft/Mile 25.0 m/Km)
| style="background-color: #FDC37D;"|
0.051 (269.9 Ft/Mile 51.1 m/Km)
| 0.102 (539.9 Ft/Mile 102.3 m/Km)
|-
| rowspan="6"|
{{Garmin 310XT}} with [[Footpod]]
| Normal
| 2787| 0.073 (384.7 Ft/Mile 72.9 m/Km)2786| style="background-color: #D0DD8192CB7D;"|02.020013% (105106.5 3 Ft/Mile 20.0 1 m/Km)| style="background-color: #F8E983;"|0.073 035 (384187.7 1 Ft/Mile 7235.9 4 m/Km)| style="background-color: #FEEA83;"|0.036 071 (192374.3 Ft/Mile 3670.4 9 m/Km)
|-
| Under Bridge
| 222
| style="background-color: #ECE582;"|
3.193% (168.6 Ft/Mile 31.9 m/Km)
| style="background-color: #FEC87E;"|
0.049 (260.0 Ft/Mile 49.2 m/Km)
| 0.098 (520.0 Ft/Mile 98.5 m/Km)
|-
| Post Bridge
| 188
| style="background-color: #FFE383;"|
4.095% (216.2 Ft/Mile 41.0 m/Km)
| style="background-color: #FFE884;"|
0.037 (197.4 Ft/Mile 37.4 m/Km)
| 0.075 (394.8 Ft/Mile 74.8 m/Km)
|-
| Turn Around
| 205
| style="background-color: #FFDD82;"|
0.042 (219.6 Ft/Mile 41.6 m/Km)
| 0.083 (439.2 Ft/Mile 83.2 m/Km)
|-
| Power Line
| 515
| style="background-color: #BCD780;"|
2.566% (135.5 Ft/Mile 25.7 m/Km)
| style="background-color: #F1E783;"|
0.035 (183.2 Ft/Mile 34.7 m/Km)
| 0.069 (366.5 Ft/Mile 69.4 m/Km)
|-
| Mixed
| 3917| 0.077 (406.2 Ft/Mile 76.9 m/Km)3916| style="background-color: #F3E783B3D57F;"|02.024439% (128.2 8 Ft/Mile 24.3 4 m/Km)| style="background-color: #FFE784;"|0.077 038 (406199.2 5 Ft/Mile 7637.9 8 m/Km)| style="background-color: #FFE683;"|0.038 076 (203399.1 Ft/Mile 3875.5 6 m/Km)
|-
| rowspan="6"|
{{Garmin 310XT}} no [[Footpod]]
| Normal
| 1210| 0.083 (439.4 Ft/Mile 83.2 m/Km)1368| style="background-color: #FDEA83C7DA80;"|02.025708% (134143.6 0 Ft/Mile 2527.5 1 m/Km)| style="background-color: #FFDF82;"|0.083 041 (439214.4 8 Ft/Mile 8340.2 7 m/Km)| style="background-color: #FFDD82;"|0.042 081 (219429.7 5 Ft/Mile 4181.6 3 m/Km)
|-
| Under Bridge
| 99111| 0style="background-color: #66BE7B;"|1.084 420% (44675.0 Ft/Mile 8414.5 2 m/Km)| style="background-color: #63BE7BFB9A75;"|0.007% 066 (35348.3 9 Ft/Mile 666.7 1 m/Km)| 0.084 132 (446697.0 Ft/Mile 84.5 m/Km)| style="background-color: #FFDC81;"|0.042 (223.0 9 Ft/Mile 42132.2 m/Km)
|-
| Post Bridge
| 95| 0.074 (388.5 Ft/Mile 73.6 m/Km)106| style="background-color: #FCA877FED680;"|05.051053% (268266.6 8 Ft/Mile 50.9 5 m/Km)| style="background-color: #F8E983;"|0.074 035 (388187.5 0 Ft/Mile 7335.6 4 m/Km)| style="background-color: #FFEB84;"|0.037 071 (194374.3 1 Ft/Mile 3670.8 m/Km)
|-
| Turn Around
| 107123| 0style="background-color: #FEC87E;"|6.115 017% (605317.8 7 Ft/Mile 11460.7 2 m/Km)| style="background-color: #FB9273FDBB7B;"|0.059% 054 (311285.8 9 Ft/Mile 5954.1 m/Km)| 0.115 108 (605571.8 Ft/Mile 114.7 m/Km)| style="background-color: #FCB37A;"|0.057 (302.9 Ft/Mile 57108.4 3 m/Km)
|-
| Power Line
| 203| 0.072 (377.9 Ft/Mile 71.6 m/Km)| style="background-color: #FED580;"|0.034% (179.0 Ft/Mile 33.9 m/Km)| 0.072 (377.9 Ft/Mile 71.6 m/Km)229
| style="background-color: #F8E983;"|
3.360% (177.4 Ft/Mile 33.6 m/Km)| style="background-color: #F9E983;"|0.036 (188187.9 7 Ft/Mile 35.8 5 m/Km)| 0.071 (375.3 Ft/Mile 71.1 m/Km)
|-
| Mixed
| 17141937| style="background-color: #E1E282;"|3.050% (161.0 Ft/Mile 30.5 m/Km)| style="background-color: #FED680;"|0.044 (231.7 Ft/Mile 43.9 m/Km)| 0.088 (463.3 Ft/Mile 87.8 m/Km)|-| rowspan="6"|{{Garmin 610}}| Normal| 289| style="background-color: #E6E482;"|3.124% (164.9 Ft/Mile 31.2 m/Km)| style="background-color: #D3DE81;"|0.031 (166.0 Ft/Mile 31.4 m/Km)| 0.087 063 (457331.9 Ft/Mile 8662.9 m/Km)|-| Under Bridge| 22| style="background-color: #CBDC81;"|2.761% (145.8 Ft/Mile 27.6 m/Km)| style="background-color: #FFE683;"|0.038 (201.8 Ft/Mile 38.2 m/Km)| 0.076 (403.6 Ft/Mile 76.4 m/Km)|-| Post Bridge| 22| style="background-color: #FED881;"|4.839% (255.5 Ft/Mile 48.4 m/Km)| style="background-color: #63BE7B;"|0.020 (103.1 Ft/Mile 19.5 m/Km)| 0.039 (206.1 Ft/Mile 39.0 m/Km)|-| Turn Around| 26| style="background-color: #FDC57D;"|6.255% (330.3 Ft/Mile 62.6 m/Km)| style="background-color: #89C97D;"|0.024 (124.7 Ft/Mile 23.6 m/Km)| 0.047 (249.4 Ft/Mile 47.2 m/Km)|-| Power Line| 43
| style="background-color: #FFE383;"|
4.038% (213.2 Ft/Mile 40.4 m/Km)| style="background-color: #9ECF7E;"|0.029% 026 (152136.6 7 Ft/Mile 2825.9 m/Km)| 0.087 052 (457273.9 3 Ft/Mile 8651.8 m/Km)|-| Mixed| 402| style="background-color: #FFEB84;"|3.498% (184.7 Ft/Mile 35.0 m/Km)| style="background-color: #FED981D3DE81;"|0.043 032 (229166.5 Ft/Mile 31.5 m/Km)| 0 .063 (332.9 Ft/Mile 4363.4 1 m/Km)
|-
| rowspan="6"|
| Normal
| 514
| style="background-color: #66BF7B;"|
1.429% (75.4 Ft/Mile 14.3 m/Km)
| style="background-color: #C0D980;"|
0.030 (155.8 Ft/Mile 29.5 m/Km)
| 0.059 (311.5 Ft/Mile 59.0 m/Km)
|-
| Under Bridge
| 43
| style="background-color: #F8696B;"|
0.084 (444.5 Ft/Mile 84.2 m/Km)
| 0.168 (889.0 Ft/Mile 168.4 m/Km)
|-
| Post Bridge
| 40
| style="background-color: #FFDF82;"|
4.339% (229.1 Ft/Mile 43.4 m/Km)
| style="background-color: #75C37C;"|
0.021 (113.2 Ft/Mile 21.4 m/Km)
| 0.043 (226.4 Ft/Mile 42.9 m/Km)
|-
| Turn Around
| 43
| style="background-color: #FED580;"|
5.117% (270.2 Ft/Mile 51.2 m/Km)
| style="background-color: #9CCE7E;"|
0.026 (135.3 Ft/Mile 25.6 m/Km)
| 0.051 (270.7 Ft/Mile 51.3 m/Km)
|-
| Power Line
| 90
| style="background-color: #99CD7E;"|
2.101% (110.9 Ft/Mile 21.0 m/Km)
| style="background-color: #8DCA7D;"|
0.024 (127.2 Ft/Mile 24.1 m/Km)
| 0.048 (254.4 Ft/Mile 48.2 m/Km)
|-
| Mixed
| 730
| style="background-color: #8AC97D;"|
1.900% (100.3 Ft/Mile 19.0 m/Km)
| style="background-color: #FBE983;"|
0.036 (188.6 Ft/Mile 35.7 m/Km)
| 0.071 (377.2 Ft/Mile 71.4 m/Km)
|}
==Interpretationand Conclusions==
What do these statistics mean? This is my interpretation:
* Under normal conditions the GPS accuracy is quite good for most devices.* The 620 is noticeably poorer than the other devices and the preliminary results of the Garmin 10 are even worse. 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 devicescases 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.* The Garmin 610 and Garmin 10 results are preliminary, but I've included them to see if the problems with the Garmin 620 are inherent in a smaller device. The reasonable accuracy I've seen with the Garmin 610 indicates that a small device can have good accuracy.
* 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.
** 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, especially for an older device.
=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 [[Practical Interval Training| interval training]], use a track or measure out the distance using some other mechanism.** For general runningtraining or for races, use a Footpod.* Adding a Footpod to the {{Garmin 310XT}} improves its GPS accuracy. * 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 [http://www.amazon.com/{{Garmin-010-01128-30-Forerunner-620-Bundle/dp/B00FBYYC90 Garmin 620] }} have the ability to be preloaded with the satellite positions, reducing this startup time and start up in accuracy dramatically.* The GPS watches are not accurate enough to perform any type of [[Practical Interval Training| interval training]]. Instead, use a track or measure out the distance using some other mechanism.* GPS is not accurate enough to give a good reading of current pace.
=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.