What is the definition of an agonic line?

The definition of an agonic line is an imaginary line where magnetic declination is zero. This means that at any point along this line, a magnetic compass will point directly toward true north, as opposed to magnetic north. Magnetic declination refers to the angle difference between true north and magnetic north. Being familiar with the concept of an agonic line is crucial for activities such as navigation and search and rescue operations, where understanding the relationship between true north and magnetic north can significantly affect the accuracy of navigation efforts. Knowledge of this line helps SAR technicians to avoid errors when interpreting compass bearings, especially in wilderness scenarios.