Weird readings on my Garmin 305 (Read 504 times)

Looks like the Garmin lost the satellite lock or came close. When this happens, the watch "guesses" your track, often resulting in jagged, off-course errors like the one your saw. A couple suggestions: 1. Make sure the watch has a good satellite lock before starting the workout. Allow the "Acquiring Satellites" bar to fill completely and check that the GPS accuracy is acceptable (less than 44 feet should give a pretty good track). 2. Keep an eye on the GPS accuracy during your run, especially if you are running under tree cover or near taller buildings. You can add "GPS accuracy" as a data field on one of your screens. If the accuracy starts degrading, you might consider stopping and allowing the watch to re-acquire the satellites. Personally, I put GPS accuracy on my main screen because I run in a lot of urban areas with taller buildings. On a normal run, my accuracy will be between 16 and 34 feet resulting in a pretty good track. When I see the accuracy degrade to 70+ feet, I know I'm about to lose lock and that I'd better stop and re-acquire if I care about keeping an accurate track.

Overcast skies can make the problems of heavy tree cover or urban canyons worse. Wikipedia suggests it has to do with humidity rather than cloud cover though.