name the three elements required to specify the Earth's magnetic field at a given place and draw labelled diagrams to define these elements explain briefly how these elements are determined

Dear Student,
Three element required to specify the earth's magnetic field at a given place is given as follows
 1. Declination($\theta$) 2.inclination or dip angle UPB($\delta$) and 3. horizontal component of the field.

 Starting from the geographical meridian we draw the magnetic meridian at an angle QPU (declination). In the magnetic meridian we  draw the horizontal direction
specifying magnetic north.Magnetic field is at an angle delta from this direction.The horizontal component B_H and the total field B are related as
$$\begin{gathered} B_H=B\cos \delta \\ \Rightarrow B=\frac{B_H}{\cos \delta } \end{gathered}$$.
Thus , from the knowledge of the three element, both the magnitude and direction of the earth's magnetic field can be obtained. Regards