I'm sometimes frustrated by my GPS watch. I think that's a good indication that I'm too spoiled when it comes to running gear. It's probably also an indication that I'm too numbers-focussed right now. I go through phases where I want to record every detail of my workouts so I can track my progress and learn what works and what doesn't. Then I go through phases where I just want to 'unplug' completely, leave the watch at home, and just go for a run because it's fun... not because I'm training. Right now, the numbers are keeping me motivated so I'm relying on my GPS watch to keep my stats for me.
I use a Garmin Forerunner 301 GPS watch. It's the second watch of this model that I've owned (one mysteriously disappeared several years ago), and for the most part, I really like it. It's a little bulky compared to the newest GPS watches, but it's far from intrusive. It tracks a TON of useful information and can be used for running or cycling. Of course, it relies on satellites so it's not as reliable on very cloudy days or in places with objects obstructing line-of-sight with the satellites (thick woods, large cities with tall buildings, etc.). Every now and then, it will be inaccurate for seemingly no reason.
It makes it hard to trust the paces that are being reported during a run when you aren't sure about the distance. On routes I run repeatedly, I learn about where the miles are, so I can mentally adjust if the watch is a little off. But with new routes, I can only go by what the GPS is telling me.
Recent marathons have been especially frustrating. Tracking my pace is really important when I have to be careful about conserving energy while not falling too far behind my goal time. At the Olathe, KS marathon in April, my watch measured the course as 26.5 Miles instead of the standard 26.2. Earlier this month, I ran the Minneapolis marathon and the GPS measured the course as 26.34 Miles. In both races, my watch would beep to let me know I finished another mile before I actually reached the mile markers on the course. The longer I ran, the earlier the 'beep' came. It's hard to be 20 miles into a race and have your watch tell you you just reached 21 miles but the 21 mile marker is still a quarter-mile away.
I've used several methods of measuring training runs in the past: I've driven them ahead of time and used my car's odometer, I've used http://www.mapmyrun.com/ (where I log my workouts), and I've ridden my bike and used it's odometer. My Garmin is by far the easiest way to measure runs and provides the most feedback but it still isn't perfect.
Ultimately, I've decided to record whatever the Garmin reports and just ignore minor discrepancies between workouts. The only exception will be in races with pre-measured courses, in which case I'll ingore the distance my watch reports and go with the official race distance.
So, according to my watch, here are today's stats:
Goal For The Day: Middle Distance
Distance: 3.84 Miles.
Time: 28:47. 7:30 avg. (7:41, 7:25, 7:22, 6:17*).
Route: Old Town Eureka Loop.
Conditions: 68 degrees/breezy. 7:15AM.
Notes: Great weather but didn't feel like pushing it. A little fatigued from this week's workouts. Ran negative splits but they were all a little slower than I would like. I think GPS shorted me so paces might have been a little faster than reported. Beautiful weather!!!
Daily Miles: 3.84 Miles
Run - 26.34 Miles
Swim - 950 meters