what is apparent angle of dip?

The angle through which the  needle showing north pole dips down with reference to horizontal is called the angle of dip.
The value of dip at a place is determined with the help of an instrument known as dip circle. It consists of a magnetised needle capable of rotation in the vertical plane about a horizontal axis. The ends of the needle move over a vertical scale graduated in degrees. 
The angle made by the needle with the horizontal is called the apparent angle of dip.