The dip angle is the magnetic dip angle.
It is the angle made by the magnetic compass needle with the horizontal direction on the surface of Earth. We know that North pole of Earth is different from the magnetic North pole of Earth. The magnetic field inside the Earth is from magnetic South to magnetic north.
At the magnetic north the needle will be vertical. The dip angle is 90 deg. At the south pole the needle is vertical with the North of the needle downwards. So dip angle is - 90 deg. on the magnetic equator in the center, the needle rests horizontally and is aligned itself along the magnetic field direction. So dip angle is 0.
At he geographical Equator, geographical south and North poles, the dip angles vary similarly.
so dip angle varies.