GPS accuracy

I recently bought the VA3 and have used it for both cycling and walking. The GPS tracking accuracy when cycling isn't quite as good as the Edge Explore but is similar to the Edge 520. The VA3 over-measuring distance by 0.01 miles in 15 miles compared to the EE is, in my opinion, good. Inspection of the track shows it is generally good although a route through mainly open countryside must help.However, having the VA3 held forward from the body (cycling position with straight handlebars) helps it receive the satellite signals. For walking I have found the VA3's tracking accuracy to be less consistent. Sometimes it is good, sometimes there are minor errors while urban walking between large buildings can make those errors much bigger. GPS satellite reception won't be helped by the VA3 being intermittently screened by the body as the wrist moves around. (It would be an interesting experiment to see how much the VA3's accuracy would improve if head-mounted to avoid the screening by the body but this would be no good for measuring heart rate!). Also, the VA3 must have a small antenna which can't help with optimal signal reception.

Any GPS device struggles in urban conditions because many satellites are being blocked from view. The best positional accuracy achieved is when signals can be received from satellites in all directions.These signals are line-of-sight so, if you can;t see the sky then it means that the GPS can't get signals from that part of the sky. The accuracy is degraded in the direction where the sky view is blocked. For example, if buildings to east and/or west block the GPS signals from those directions then the east-west accuracy of the calculated posiiton is degraded resulting in the off-track position. Using more satellites from the different providers helps a little as anything is better than nothing. If running fixed routes then the solution is to measure them in Google Earth. If you haven't read it already then see the VA3 review here. The discussion about GPS accuracy starts with this statement: "There’s likely no topic that stirs as much discussion and passion as GPS accuracy. A watch could fall apart and give you dire electrical shocks while doing so, but if it shows you on the wrong side of the road? Oh hell no, bring on the fury of the internet!"

Overall I'm impressed by how the GPS technology has progressed. My first encounter with a GPS device was in 1996 when it was the size of a brick, weighed nearly as much as a brick and had to be held out at arm's length in an open area to have any hope of getting a position to within about 100m accuracy (although Selective Availability was a factor in the poor accuracy). If there were some trees in the area then the prospect of getting a position was much reduced.