Look here

Code:
Please Login or Register to see the links
And:
Without WAAS your calculated position will be accurate to within 15 meters 95% of the time. With WAAS your calculated position will be accurate to within 5 meters (I think 3 is a stretch) 95% of the time.

So without WAAS that doesn't mean you are only getting 15 meter accuracy. 95% of the time it will be better than that. So the difference between them can't be calculated as 12 meters (15-3).

The road network isn't mapped with that degree of accuracy, so thinking back to high school math class regarding precision, the accuracy of the GPS/map combo is only as strong as the weakest link... in this case the underlying map.

You will never notice the difference driving down the road if WAAS is available and turned on, versus it being turned off-- with the exception that your battery will drain faster with WAAS on.

Even at a slow speed of 25mph the difference (12 meters) goes by in about one second. The refresh rate of the map screen is typically about one second.

So the difference between WAAS and no WAAS in a car is irrelevant.

And again:

Something else about WAAS that is worth mentioning... The GPS satellites are supposed to travel in their designated orbits. Sometimes they can be a little "off" from where they are supposed to be. The WAAS system calculates how much they are off, and provides correction information to your GPS.

So if all of the satellites were exactly where they are supposed to be, no correction information would need to be applied. Therefore your position wouldn't be any different, WAAS or no WAAS. Of course that doesn't happen and correction information does help, but depending on how far "out" things are, the satellites you are connected to, etc the benefit won't always be significant.

from:
Code:
Please Login or Register to see the links