Snap To Roads
Street navigation devices often give the illusion of having an accuracy far greater than what is true. They do this through what we call a “snap to” feature. What we mean by this is that the GPS will listen into the satellites, figure out where it thinks you are, and compare that to the internal database of roads. If it calculates that you are “pretty close” to a road, even if it doesn’t think you are directly on it, the GPS will still draw you as if you are exactly on the road.
As mentioned above, this can give the impression that the GPS is far more accurate than it really is. Remember that a consumer grade GPS will have an accuracy of about 10 meters, 95% of the time. With WAAS that can be improved to about 3-5 meters, 95% of the time although the differences in street navigation is negligible.
On top of the accuracy of the GPS signal the map itself can have errors in it. Sometimes the road might have been realigned or shifted while other times the map itself could just be a bit off from the current “real world”.
Therefore, to compensate for the (in)accuracy of the GPS and the (in)accuracy of the road map, a certain fudge factor is applied. If your calculated position puts you 25 feet from a known road with no other close roads– it is a safe bet that you are actually driving on that particular road. The GPS won’t show you driving to the side of the road, but will show you directly on the road itself.
Without this understanding, many people will assume that their GPS is far more accurate than it actually is. If you’ve ever driving with satellite imagery on a GPS (quite rare) you will see this in action a bit more as those representations don’t typically snap you to the road. Likewise there are a few GPS models out there that will draw the car icon directly on the road in this snap to mode, but will also draw a tiny dot where your calculated position actually is with respect to the internal map database. Don’t let “snap-to” fool you. 😉