Here is a good discussion of factors that influence the accuracy of the position displayed by your device:

There are quite a few items in play, especially since not all GPS receivers are created equal.  I have been using GPS for many years, principally to record the location of archaeological sites and it works wonderfully, especially when contrasted with the technology previously used.  I have witnessed multi-path errors (signals distorted by canyon walls) so bear in mind that nothing is perfect, even electronics in outer space (heresy to some, I know...).



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