Just how accurate is the iPhone 3GS GPS and Compass?

I'm excited about the concept of augmented reality (AR) apps, and am curious what kinds of AR apps are *practical* with the state of the art iPhone 3GS and its accelerometer, GPS, and Compass sensors. I decided to do some field tests to see how accurate the GPS and Compass sensors are on the device. In this post, I'll describe my tests, show you the data, and offer some conclusions…