Using GyroCompassPro you can tell directions with a high degree of accuracy even where magnetic field is not good conditions.

-How to use-
First calibrate direction.
There is two way to calibrate it.
a):Magnetic compass calibration.
Hold your iPhone parallel to ground without wearing any metal accessories or watch.

b):Manual calibration.
You can calibrate direction manually if you know directions.
You can use the Sun or Stars to tell direction.

Then tap Gyro button!
You can tell direction even if you are in basement or inside building.