These virtual events really highlight GPS inaccuracy. Mum, Dad and I all have to run to different places to get 5km on our loop at home. Mum consistently runs over 100m less, poor Dad even tried her watch but still had to run further. At the end of the day it's all good training though!
If you use a garmin watch you need to change the default "smart recording" to every second to get more accurate GPS. I've also set mine to use GLONSS as well.
^ what grump says. Not talking out of his GLONASS. The 1s setting makes bigger files, but that's not an issue on most watches; combining both head (GPS) and tails (GLONASS) might have a very minor battery hit and may not increase accuracy at all but, hey, it sounds cool.
Think my GPS is normally not too bad and have it set to 1sec/glonass. It may have gone dodgy as I started where the path was in the trees and when I reset my watch at the start, I then started pretty quickly as there was a dog walker coming, instead of letting it settle properly again (though it did say GPS ready). It must not have picked up the satellites that well. Was good others did the same loop as was more bothered about a sudden plummet in form than the result. Virtual challenges are good for motivation/training but not quite like racing where you can have a proper comparison, and get outsprinted by forestgrump. Though really need to upgrade my antique racers! :-)