How is magnetic variation typically adjusted on a navigation route card?

The adjustment of magnetic variation on a navigation route card is typically achieved by adding or subtracting from grid north. Magnetic variation refers to the angle between magnetic north (the direction a compass points) and true north (the direction of the North Pole). To effectively navigate using a route card, pilots must convert true headings to magnetic headings, which involves accounting for magnetic variation.

This is done by applying the appropriate amount of variation based on the aircraft's current position and the specific geographic area being navigated. Depending on whether the magnetic north is east or west of true north, the adjustment will involve either adding or subtracting degrees from the grid north.

The other options do not appropriately address how to adjust magnetic variation on a navigation route card. GPS coordinates involve determining position rather than adjusting magnetic heading. Direct observation would not provide accurate or consistent information about magnetic variation, and following flight manuals can provide guidance but does not specify the direct mathematical operation required to adjust for magnetic variation.