So I have a Garmin Forerunner 10, and I think it is fairly accurate at determining distances and paces on the roads. However, I occasionally do runs on some trails in the woods. These trails are decently hilly, but nothing too extreme, and they aren't super technical either.
When I go on a run to these trails, I usually do 2 or so miles from the parking lot to the trails. This section is pancake flat, and not in the woods. My watch will be just as accurate here as it is everywhere else, telling me I am running like 7:30 miles or whatever pace I am going. However, the moment I enter the woods my watch will say I am running like 9:30 miles, which seems like its way off. I feel like I probably slow down a little bit, given the hills and the mild roots and rocks on the trail, but my effort level is roughly the same. I can't see myself slowing down more than 30 seconds per mile at the absolute max because of this, but maybe I am wrong and just really bad at running on trails.
Because of this, I usually ignore the distance the watch has and use the time and my own estimation of pace to determine the distance I ran, which is usually a mile or two farther than the watch says.
Does anyone else experience something similar when they try to use a gps watch in the woods? Or do the trails actually slow me down by 2 min+ per mile?