How does longitude change as you travel west on your map?

As you travel west on a map, the longitude decreases. Longitude lines are measured in degrees, with the Prime Meridian at 0° longitude. Moving westward, each degree of longitude represents a movement from 0° toward 180°, resulting in negative values in the Western Hemisphere. For example, moving from 0° to 30° west means a change from 0° to -30° longitude.