GPS accuracy will vary. Even the dedicated Garmin GPS units will lose lock on enough satellites (when under heavy trees, near tall buildings, etc.), and thus produce lower accuracy. The signals themselves will change (the satellites are moving pretty fast, even if you're not). I had a handheld GPS which said my porch was moving up and down 10M every now and then. Another which said I was weaving back and forth across an 8 lane highway.
Best bet for accurate distance measurement is to use two iPhone 3G's, hold them both still until the accuracy goes up, time stamp the measurements, and then compare and subtract just those measurement pairs with similar accuracy and time stamps.
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