Some interesting news in the Mercury this morning:
http://www.mercurynews.com/mld/mercurynews/business/technology/personal_technology/12982739.htm
Although the Mercury News is a much more technically oriented paper than many, they still manage to mix a lot of fact with fiction.
Ordinary commercial consumer-grade GPS receivers average about 15 meters CEP in today's world. CEP is Circular Error Probable and is normally specified at 95%. In non-gobbly-gook this means that 95% of the time you receiver will indicate within 15 meters (49 feet) of the actual position. The other 5% of the time? Unknown, but often very close. In my own use I have often observed accuracy at least this good, even better, provided the unit has an antenna with a good view of the sky. In a large city with many tall buildings the "urban canyon" effect can degrade the accuracy by quite a bit, but again in well designed units this degradation normally averages out pretty close to the commercial spec.
There are a number of ways of enhancing the basic commercial signal. The most common today is the WAAS, Weider Area Augmentation System, devised and operated by the US FAA to make commercial-grade GPS more capable. WAAS will usually reduce the CEP to 5 meters or less ... the problem is, so few manufacturers of consumer-grade equipment have come on board to use it.
The Mercury News article, though, makes one think that accuracies down into the centimeter range are still a research dream. If one has the need for that kind of accuracy, it's available off the shelf today from various makers (Garmin and Leica are two of the leaders) of survey-grade GPS. These units cost 10 or more time as much as consumer grade boxes, but can easily give centimeter accuracy with the right options and operator training.
So, what level of accuracy do you think you need?
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