The problems I experienced with the Mobile Crossing WayPoint PDA/GPS are not unique to that product. GPS accuracy is a function of at least three key limitations in the GPS system. Below is a brief discussion of these limitation.
On May 1, 2000, President Clinton issued an announcement stating that Selective Availability (SA) would be turned off for the Global Positioning System (GPS). GPS uses satellite signals to identify the location of a specific place on the earth. SA intentionally degraded the accuracy of GPS signals used by civilian GPS receivers. This was to prevent potential enemies of the United States from using GPS signals to precisely aim guns and missiles at US targets. The government reserves the right to switch SA back on, but for now, it is off.
Turning off SA improved GPS accuracy from within 400 feet to within 33 feet at a 95% confidence level. This means that for every 100 readings 95 will fall within 33 feet of the correct location. This also means that 50 readings out of 100 will be within 5-10 feet of the real position.
Signal accuracy is only one of three ways that GPS precision can be eroded.
The US Geological Survey, municipalities, and all other government cartographers have traditionally worked with paper maps. These maps, in particular for highways, sometimes emphasize readability over accuracy. When several streets and highways overlap, as often occurs with multilevel freeways and the more complex cloverleaf highway entrance-exit schemes, the cartographer takes artistic liberty and draws highways that deviate from reality to more clearly illustrate the relationship between the roads. This makes the maps easier to read. Over the years, these minor deviations are accepted and end up on the digital maps you see today. This means that governmental data regarding street and highway placement deviates from reality somewhat. So, the GPS reports at 95% accuracy within a circle of 33 feet using a map that itself is not perfectly accurate.
Then there is the US Postal Service's contribution to lower GPS accuracy. The USPS reports postal addresses as ranges. These ranges are reported for each side of a street segment. So, for example the USPS data says that on Main Street the odd addresses 1101-1200 are on the northbound side of Main Street between 1st and 2nd Avenues and that odd addresses 1201-1300 are on the north side of Main Street between 2nd and 3rd Avenues.
Putting map and USPS data together, we know that Main between 1st and 2nd is 1000 feet long and contains addresses 1101 to 1200. When we try to create a route to 1105 Main Street, we know immediately that it is between 1st and 2nd. We don't know the exact latitude and longitude of that address, but we can mathematically make a pretty good guess using the length of the street. But what if the buildings are oddly spaced, oddly shaped, or built vertically (as with high rise office and condo projects) or have a large parking lot structure taking up lots of room? The estimated destination will be slightly off, reducing the accuracy of the location of an address on this segment of Main Street.
So, a GPS system is unlikely to "know" with perfect precision where it is at any given time due to the limited accuracy of the GPS system itself, the inaccuracy of paper maps and their digital counterparts and the manner in which the location of an address must be calculated. Even with these problems, a good GPS with occasional human help can readily get you from here to there far more reliably than any other navigation tool.
Calvin Chu, Mobile Crossing's Product Manager contributed heavily to this section.