Answer the question regarding earth's magnetism:
A vector needs three quantities for its specification. Name the three independent quantities conventionally used to specify the earth's magnetic field.

Explanation

(a) To know magnetic field of Earth at a given place on its surface we require following three quantities known as "magnetic elements" of Earth.
(1) Declination (D)
(2) Inclination (or Dip angle) (`delta` or I)
(3) Horizontal component `B_H`
By knowing angle D, we can determine magnetic meridian of a given place.
By knowing angle I, we can determine direction of magnetic field of Earth at a given place in above magnetic meridian.
By knowing `B_H`, we can determine magnitude of magnetic field of Earth at a given place along above direction.