iPhone: The Missing Manual, 3rd Edition

Compass (iPhone 3GS)

Man, you gotta hand it to Apple. What on earth can you put into your cellphone in its third annual version that it didn't have before—and that no other phone has? That's right. A compass.

There's an actual magnetometer in the iPhone 3GS—a magnetic-field sensor. When you open the Compass app, you get exactly what you'd expect: a classic Boy Scout wilderness compass that always points north.

Except it does a few things the Boy Scout compasses never did. Like displaying a digital readout of your heading (shown below, right) or displaying your precise geographical coordinates at the bottom, or offering a choice of true north (the "top" point of the Earth's rotational axis) or magnetic north (the spot traditional compasses point to, which is about 11 degrees away from true north). (Tap the to specify which north you prefer.)

The very first time you use the Compass (or anytime you're standing near something big and metal—or magnetic, like stereo speakers), you get the little message shown here at left. It's telling you to move away from the big metal thing, and to de-confuse the Compass by moving it through 3-D space in a big figure 8. (Yes, you look like a deranged person, but it's good exercise.)

Once the Compass is working, hold it roughly parallel to the ground, and ...