What is the purpose of a declination diagram on a map?

The declination diagram on a map serves to show the angular relationships between different types of North, specifically True North, Magnetic North, and Grid North. This is crucial for navigation and map reading, as it helps the user understand how to convert between these different orientations. Accurate navigation often hinges on knowing the differences between True North, which is fixed based on the North Pole, and Magnetic North, which can vary depending on location due to the Earth's magnetic field. By using the declination diagram, one can make the necessary adjustments when plotting a course or reading bearings, ensuring precise navigation.

In the context of navigation on a map, the other options do not align with the primary function of a declination diagram. Elevation levels are typically depicted through contour lines, military units would not have relevance on a declination diagram, and weather conditions are represented through symbols or notations unrelated to angular measurements. Thus, understanding the role of the declination diagram is essential in effective land navigation and orientation.