Editing Magnetic declination (section)

===Determination from maps  ===
A rough estimate of the local declination (within a few degrees) can be determined from a general isogonic chart of the world or a continent, such as those illustrated above. Isogonic lines are also shown on [[aeronautical chart|aeronautic]]al and [[nautical chart]]s.

Larger-scale local maps may indicate current local declination, often with the aid of a schematic diagram. Unless the area depicted is very small, declination may vary measurably over the extent of the map, so the data may be referred to a specific location on the map. The current rate and direction of change may also be shown, for example in [[arcminute]]s per year. The same diagram may show the angle of [[grid north]] (the direction of the map's north–south grid lines), which may differ from true north.

On the [[topographic map]]s of the [[U.S. Geological Survey]] (USGS), for example, a diagram shows the relationship between magnetic north in the area concerned (with an arrow marked "MN") and true north (a vertical line with a five-pointed star at its top), with a label near the angle between the MN arrow and the vertical line, stating the size of the declination and of that angle, in degrees, [[angular mil|mil]]s, or both.  However, the diagram itself is not an accurate depiction of the stated numerical declination angle, but is intentionally exaggerated by the cartographer for purposes of legibility.