What does the term 'convergence' refer to in mapping?

The term 'convergence' in mapping specifically refers to the angle change between grid north and geodetic north. This is an essential concept in surveying and mapping as it affects how coordinates are projected on a map. In order to accurately represent the three-dimensional surface of the Earth on a two-dimensional plane, cartographers and surveyors must account for the distortion that occurs when transforming spherical coordinates (geodetic) into flat coordinates (grid).

This angle of convergence arises because the grid north is aligned based on the particular mapping system, such as a state plane coordinate system, which may differ from true north as represented by the Earth's rotational axis. Understanding the convergence is critical for ensuring accurate bearings and measurements in surveying tasks, making conversions from grid coordinates to geodetic coordinates and vice versa.

Other choices, while related to essential surveying concepts, do not define convergence. The angle between true north and magnetic north pertains to magnetic declination, the relationship between latitude and longitude lines describes geographic coordinates, and the difference in elevation on a topographic map refers to contour lines, which is not related to angular measurement.