Originally Posted by
WhyFi
The only external things, that I can think of, that might be affecting accuracy is phone placement (on your person) and obstruction by overhead trees - the less between your phone and the clear blue sky, the better.
I found that when I place my iPhone 4S with the screen facing outwards (away from my back) it's more accurate than when the screen is placed against my back. I can only assume it has something to do with the GPS reception. But that's just me and I ride flats with little tree cover here in South Florida.