[OSM-newbies] Trust GPS or Yahoo aerial imagery?

[Rick Collins]

At 11:20 AM 10/9/2007, you wrote:
>My experience is that when the signal strength is good, then my tracks
>will be spot on and agree exactly with the yahoo mapping. The GPS
>usually keeps track of a the accuracy in a parameter called hdop, you
>may be able to see this if you look at the gpx tracklog. Values less
>than 10 should be okay, if it's below 4 then there is a good chance you
>can rely on it.

Actually, there is no way that the GPS can tell you what your 
accuracy is.  The various Dilution of Precision (DOP) values are 
simply calculations that tell you how the basic accuracy is degraded 
by one thing, the position of the space vehicles being used in the 
calculation.  There are any number of other causes of inaccuracy such 
as the propagation delay in the various layers of the atmosphere 
(typically the largest contributor to GPS error) which are not 
included in this calculation.  If you use a typical hand held GPSr, 
it will give you an "Estimate of Error" (not to be confused with any 
sort of real error measurement) which is based on the DOP value.  The 
real error can range widely more or less than this "estimate".