FormerAverageRunner wrote:
I run the same route every day starting my watch at the same spot +-5'. The one mile mark varies by over a hundred feet.
Exactly.
(Check out the link below)
The GPS device is a "fairy" accurate device, considering the purposes and limitations that are defined by how it collects and computes position.
There are 24 satellites circling the globe at various altitudes and speeds and traveling at different directions. Your device needs to find three satellites to determine your location to within 15 meters 95% of the time (two standard deviations). 95% of the time it is accurate within 15m, 5% of the time the error is greater than 15m. With a fourth satellite your elevation can be determined.
I haven't found any information as to the frequency of calculations your GPS runs per minute, but if you take a look at some of the maps on the link I am supplying, you'll get the whole idea how this is all done.
If you go on a 100m run, your gps is not going to be very accurate. At the start of your run your gps will pinpoint you anywhere within a 15m radius 95% of the time (and greater than 15m 5% of the time) It could position you at your starting point (not very likely) or in front of it, behind it, to the left, right or in your neighbors garage. At each positioning it does the same. At the end of your run it does some sort of post-hoc interpolation, and hopes that it's close, As you can see, the longer your course is the more accurate the GPS is likely to be. But that's not because of pinpoint accuracy of the individually recorded positions, it's because of the smoothing algorithm that guesses where you are most likely to be at any given positioning measurement. The beginning and ending positions will amount to a smaller percentage of the total distance traveled, and the deviations from the true path SHOULD average out to a virtual representation of your actual path,
Take a look at the pathways that were measure by the same device on the same course recorded on different trials (specifically images 6-8):
http://fellrnr.com/wiki/GPS_Accuracy