iPhone uses a built-in magnetic field sensor — a magnetometer — to give compass readings. But first you need to know how to calibrate the Compass to cancel any interference and get Compass back on course, how to read the Compass, and the difference between true and magnetic north and how to select one or the other.
How to calibrate your iPhone Compass for greater accuracy
When you first open Compass — it might be in the Utilities folder if you don't find it on your Home screen — a notification message appears indicating that you should calibrate your iPhone. Any time iPhone detects some interference, usually something with a magnetic field or an electronic device like a cell phone or stereo, you see a message asking you to calibrate your phone.
As instructed on-screen, simply tilt your iPhone around until the circle is completed filled in and the Compass itself appears.
How to get your bearing with the iPhone Compass
Whether to determine the direction you're facing, for example to make sure your plants are on the east side of your house to get the morning sun, or find the direction you want to go, Compass is a great tool. After the Compass is calibrated, hold your iPhone (face-up, of course) so the back of your hand is parallel to the ground.
The red arrow on the Compass points north, the direction your iPhone is pointing is written in white above the compass. Move around, and the compass rotates and the headings change. When you have Location Services turned on, your geographic coordinates and sometimes the town name are displayed below the compass.
What's really helpful is that Compass links to Maps. If you want to see where you are on a map, tap the coordinates at the bottom of the screen. Maps opens and the tracking flashlight indicates your location and the direction you're facing. If you want to know the address, drop a pin and the address appears in the attached flag.
How to choose between true north or magnetic north with iPhone Compass
Compass gives accurate readings of both true north and magnetic north, and both are valid indications. True north, which is a GPS bearing linked to the geographical location of the North Pole, works when Location Services is turned on.
Magnetic north, on the other hand, depends on the Earth's natural magnetism, which changes based on your physical location. It works when Location Services is both on and off. Because magnetic north changes at different latitudes, it can be a few to many degrees different than true north and even south of your latitude. This difference is called declination.
In some places, declination is less than one degree so it barely alters your bearings. Keep in mind, however, if you're hiking in the wilderness on a trail with 10 degrees of declination, those seemingly minor ten degrees can result in you being miles off course after several hours of continuous hiking.
Technically speaking, to achieve the most accurate results, you have to know the actual declination degrees you're traversing for your current location to calculate the difference between magnetic north and true north. Localized trekking maps often have declination degrees on them, so you can adjust the orientation of your map when using true north. To turn on true north, tap Settings→Compass and then tap Use True North on.
How to keep things on the Level with iPhone’s Compass
The Compass is really two, two, two tools in one! Swipe left from the Compass screen and you find an electronic spirit level. Instead of having an air bubble that must fall between two lines to indicate a level horizontal surface, two white bubbles, um, circles must line up when you place your iPhone on a horizontal surface.
The degree of inclination appears in the circles, and when your iPhone is in a perfectly level position, the whole screen turns green and zero degrees show in the center circle.
If you use the edges of your iPhone, instead of circles, you have a horizontal line across the center that divides the white top from the black bottom. When your iPhone is in a position of zero inclination, the black turns green.
