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

The purpose of a declination diagram on a map is to indicate the relationship between true north, grid north, and magnetic north. This is crucial for navigation and accurate map reading, as it helps users understand how to align their map with the actual environment. True north refers to the direction along the Earth's surface towards the North Pole, grid north is the north direction of the map's grid lines, and magnetic north is where a magnetic compass points.

The declination diagram provides the necessary adjustments to compensate for the differences between these norths, allowing for more accurate navigation and ensuring that directions determined on a map correspond to real-world directions. Using this information helps prevent navigational errors that could arise from assuming that all types of north are the same.