Magnetic Maps

The Magnetic elements of the earth have different values at different places on the surface of the earth. The information about the values of declination (α), dip (δ) and field strength (BH) for one place gives complete picture of the earth’s magnetic field there. The maps giving these values are called magnetic maps. They are very useful to navigators.

Isomagnetic Charts. Magnetic surveys all over the earth have been carried out and magnetic maps have been prepared which show the values of magnetic elements throughout the world. Lines are drawn to join places having the same value of a particular magnetic element.

Maps drawn in this way, are called Isomagnetic charts,

Types of Isomagnetic Charts. There are three types of isomagnetic charts, one for each magnetic elements:

1.    Magnetic map of declination (α). In this map, lines joining places of equal declination are called isogonic lines. A line joining places of zero declination is called agonic line. The map is called Isogonic Chart.

2.    Magnetic map for Dip (δ). In this map, lines joining places of equal dip, are called isoclinic lines. A line joining places of zero dip is called aclinic line. This is magnetic equator. The map is called isoclinic Chart.

3.    Magnetic map for horizontal component (BH). In this map, lines joining places of equal horizontal component, are called isodynamic lines. The value of horizontal component is zero at magnetic poles. The map is called Isodynamic Chart.

Since value of magnetic elements change, survey is made and charts revised after every 30 years.