To correct a compass course (PSC), where would you find the variation on a Mercator chart?

The correct answer is found on the compass rose of a Mercator chart. The compass rose provides critical information regarding true north and magnetic north, which is essential for interpreting variation, also known as magnetic declination. This variation indicates the difference between magnetic north (where a magnetic compass points) and true north (the direction along the earth's surface towards the geographic North Pole).

Including this information on the compass rose helps navigators easily determine how to adjust their compass course to account for the variation in order to steer accurately towards their intended destination. Understanding this adjustment is crucial for maintaining proper navigation and ensuring safety at sea.

Other locations on the chart, such as the map's scale, outer edge, or within the legend, do not provide the necessary information about magnetic variation specific to compass course corrections. Instead, these areas typically offer different contextual information, like distance measurements or general chart legends, which do not serve the purpose of indicating magnetic variation.