for iOS 9.1 software

environmental interference; even the magnets in the iPhone earbuds can
cause a deviation. Use the digital compass only for basic navigation
assistance. Don’t rely on it to determine precise location, proximity,
distance, or direction.
On the level
Show the level. Swipe left on the Compass screen.
Hang it straight. Hold iPhone against a picture frame or other object,
then rotate them until you see green. For true level, the deviation is
displayed on a black background. If the background is red (indicating
relative slope), tap the screen to change it to black.
Level the table. Lay iPhone flat on the table.
Match that slope. Hold iPhone against the surface you want to match,