The Garmin 620 has some well-known GPS accuracy problems, which were highlighted by my detailed GPS testing that analyzed thousands of data points. I worked with Garmin on the issue, and the resulting firmware upgrade has radically improved the GPS accuracy (the details are under a Non-Disclosure Agreement.) You can see the improvement in the info graphic below, which has two entries for the Garmin 620, one before firmware upgrade one after. You can see the underlying numbers at GPS Accuracy Testing and there's an even more detailed breakdown at Detailed Statistics for GPS Running Watches. As a result of this testing I've updated my comparison of running watches, and my review of the Garmin 620.
I was concerned that this firmware upgrade might have been optimized for my testing, but if you look at the two images below you can clearly see that the GPS tracks with the new firmware are far tighter and more consistent. (Note that the 620 actually has two firmware installations, one for the watch functionality, and one for the GPS. It's the GPS firmware that needs to be at 3.30 or above.)
|Garmin 620 with Pre-3.30 GPS Firmware|
|Garmin 620 with the new 3.30 GPS Firmware|