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From Fellrnr.com, Running tips
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=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}}
=Which 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.
|[[File:Fenix2 Getting Lost6.jpg|none|thumb|x400px|This GPS track looks reasonable until marker #54, and then the track gets offset, but strangely it stays offset until the last marker.]]
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=Next Steps=
This is an initial analysis of the data I have, and there are a number of further evaluations to do.
* 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.
* Write up general GPS accuracy.
