<P> 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 and nautical charts . </P> <P> 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 arcminutes 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 . </P> <P> On the topographic maps 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, mils, or both . </P> <P> A prediction of the current magnetic declination for a given location (based on a worldwide empirical model of the deep flows described above) can be obtained online from a web page operated by the National Geophysical Data Center, a division of the National Oceanic and Atmospheric Administration of the United States . This model is built with all the information available to the map - makers at the start of the five - year period it is prepared for . It reflects a highly predictable rate of change, and is usually more accurate than a map--which is likely months or years out of date--and almost never less accurate . </P>

Which statement is true about true north versus magnetic north