Who says GPS is more accurate than a foot pod?

My running this weekend has been firmly focused on the tech side. Friday's, Saturday's and today's runs were documented with both my Garmin 50 and with the iPhone using MotionX GPS software. I also used the iPod feature within MotionX to listen to music while I ran. As an experiment I downloaded two Podrunner Podcasts to accompany my runs. These Podcasts, that are available from iTunes for free, are mixed by DJ Steve Boyett to match specific heart rates. On Saturday I chose the 147 BPM Serendipity mix that was highly repetitive House music with a driving beat. At about a mile into my run a female singer began repeating a single sentence that grew old very quickly. Unfortunately that sentence was repeated in various forms for the next ten minutes causing me to want to rip the iPhone off my armband and into the gutter. I didn't want to stop and figure out how to turn off or fast-forward the podcast so I endured it and the mix mercifully switched back to an instrumental after a few more minutes. As aggravating as that experience was, I'll admit that the driving beat contributed to a quicker pace than I would have normally achieved.

The GPS software measured my run at 3.73 miles and the Garmin measured it at 3.8. After observing the course from the exported KMZ file I noticed that the accuracy of the line was far enough off to explain that difference. This morning, after our weekend guests departed, I went out for a run with a goal of covering at least 4 miles. It was due to storm by mid-day so I was anxious to get out and finish quickly. I played the 150 BPM Square One Podrunner mix that was vocal free (which I liked) but a bit soulless and electronic (which I liked less). I'll admit that it did help me focus but I'm thinking about loading some more tuneful music on it to see how that will work. I wanted to broaden my range so I ran west on Jericho Turnpike far enough to enter neighborhood #3 from the north. I had never actually run in this neighborhood so I was excited to experience some new roads. It was growing very hot and humid ahead of the impending storm. I was (and still am) concerned that I haven't done a 5+ mile run in a while and I'll be participating in a 6.2 mile trail race later this month. I had hoped to complete 5 miles today but the humidity had sapped my energy and it was all I could do to make it home. Very soon after my run the storm came through and I snapped a picture (see above) of where I had just run in the scorching heat.

When I finished the run I saw that my Garmin reported 4.07 miles at a pace of 9:18. The MotionX software said I'd covered 3.92 miles at 9:40. I pulled the GPX file onto Google Earth and used the path tool to trace my actual route that almost exactly matched the Garmin with foot pod. The path from the GPS had its margin of error with the GPS vectoring sharply around corners and even on straighter paths. It was easy to see why it under counted the actual distance. I don't know if the Garmin 405 somehow accounts for this and provides a more accurate report. I think the accuracy of GPS is the limiting factor so MotionX is probably as good as it gets. I'll continue using the Garmin 50 because it's easy and superior in terms of accuracy to GPS measurement. However, I'll also use MotionX to document my route so I can view it and share it with others.