The iPhone 3GS has a built-in compass - or 'magnetometer' if you want to show off your geek credentials.

When you launch the compass app, it works whichever orientation the iPhone is held - so you don't need to hold it flat and horizontally like a standard magnetic compass.

You may need to calibrate the compass app initially because it can be distracted by electromagnetic interference. You do this by following the onscreen instructions to move the handset around in a swirly 'figure 8' pattern.

Obviously the compass shows you the direction of North - the figure at the top of the screen is the direction the top of the phone is facing - in the image alongside it is 135° SW.

The Compass app shows you your geographic co-ordinates in longitude and latitude - in the image to the right it is 51°31'14"N, 0°8'0"E.  You can see this data in the bar ar the bottom of the app.

Click on the crosshair icon at the bottom left of the screen to launch Google Maps in your current location.

The i button on the bottom right of the screen toggles the compass between True North and Magnetic North modes.

Using the compass to show where you are facing in Google Maps

Launch the Google Maps app and click on the crosshair icon at the bottom left of the screen. This will make the blue dot appear showing your current location on the map.

If you click on the crosshair icon again, a triangle will appear next to the dot: this shows the direction on the map that the iPhone is facing. As you rotate the triangle will rotate to match your orientation

Remember, the direction of view is calculated from the top of the iPhone - so it's best to hold the handset in the vertical position in front of you.