What must be done to correct magnetic west declination when converting from a compass to a map?

To correct for magnetic west declination when converting a compass reading to a map, it is essential to subtract the declination value. Declination is the angle between magnetic north (as indicated by a compass) and true north (as represented on a map). This angle can either be east or west of true north, and it varies depending on your geographic location.

When the declination is west, it means that magnetic north is positioned to the west of true north. Therefore, if you only use the compass reading without any adjustments, you may end up significantly off course when trying to navigate. To align your compass reading with true north, you must subtract the declination angle from your compass bearing. This adjustment ensures that your navigation aligns correctly with the map's grid lines that lead to true north.

In contrast, adding, multiplying, or ignoring declination would lead to inaccuracies in navigation and potential misdirection, making it critical to apply the right correction for accurate results.