8,105
edits
Changes
From Fellrnr.com, Running tips
no edit summary
{{:GPS Accuracy-summary}}
=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. So to To gather the data for this test I marked my usual running ran the same route at repeatedly, recording laps every 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, turn arounds and one bridge that I go goes under. The a 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. [[File:Course Overview.jpg|none|thumb|500px|This is the course I use to evaluate the accuracy of [[Best Running Watch| GPS Running Watches]].]]To prevent startup problems each device was turned on, satellites acquired, and then left for around 5 minutes before the run starts. This is to counter the problem of a device claiming to have acquired the satellites when it only has a minimal lock. The watches are worn on the left wrist or held in the left hand in roughly the same orientation as it would be on the left wrist. (The iPhone was also on the left wrist.)
=Accuracy, Trueness and Precision=
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].
{| class="wikitable"
|- valign="top"
|[[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.)
=Accuracy=
{{:GPS Accuracy-statistics}}
==Progress of newer watches==
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.
[[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 top (we didn't run through the building).]]
=Density Plots=
Below are the density plots for each device.
