Refer to figure.
An agonic line is a line upon which the magnetic variation is zero. This means that along this line, true north and magnetic north are the same, so a compass needle points directly to true north without any deviation.

An isogonal is a line that connects points of equal magnetic variation on the Earth's surface. These lines are important for navigators to consider because magnetic variation, which is the difference between true north and magnetic north, can affect compass readings. Isogonals help in adjusting compass readings to account for this variation, ensuring that navigational courses are plotted correctly.

The aclinic line, on the other hand, is where the magnetic dip is zero, meaning the Earth's magnetic field is horizontal at that location. This line is also known as the magnetic equator.

The isoclinic line, or isocline, connects points of equal magnetic horizontal field strength, i.e. of equal magnetic dip or inclination on the Earth's surface.