What is an agonic line?

An agonic line is defined as a line on the Earth's surface where the magnetic declination is zero. This means that at locations along this line, a magnetic compass points true north, aligning perfectly with geographic north. Therefore, it is significant for navigators and surveyors alike, as it marks areas where there is no difference between magnetic north and true north.

The concept is fundamental in surveying and navigation, as understanding the relationship between magnetic and geographic positions is crucial for accuracy in determining direction. The agonic line aids surveyors in correcting measurements that rely on magnetic compasses, ensuring precision in surveys, especially in areas where magnetic declination could lead to errors.

Other choices do not accurately reflect the characteristics of an agonic line: indicating magnetic declination involves understanding the variations in magnetic direction rather than defining a specific line, a boundary between different surveying zones refers to divisions that may have varying regulations or methodologies but do not pertain specifically to magnetic alignment, and a reference line for measuring elevation is more associated with horizontal and vertical datum rather than magnetic orientation.