Magnetometer Tilt Compensation for Yaw Axis Using HMC5883L and BMA180

The direction of the magnetism is flat at the equator, and pointing down at the poles.
http://geokov.com/education/magnetic-declination-inclination.aspx

This site tells how much the inclination is, http://magnetic-declination.com/ (if you type your location, click on the location marker).
This site tells even more, World Magnetic Model Calculator (click on the map to place the marker).