All meridians pass through the North Pole and so all meridians define the direction of true north. It is true because it refers to the geographical pole. For this reason, we measure direction relative to a meridian. On a Mercator chart, the meridians are parallel to each other, so direction can be measured relative to any meridian on the chart.
A conical projection implies that none of the meridians are parallel, and so direction must be measured relative to the meridian nearest to where you wish to measure the direction.
Never measure direction relative to the chart's border because this may not be aligned with true north.
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