Magnetic Variation
The compass works using the Earth's magnetic field. Its indication will be in reference to the magnetic poles of the Earth, which exist in different locations than the geographic poles.
The difference between magnetic and true north is the magnetic variation, and varies based on where you are located. In one place the variation might be zero degrees, meaning that the compass indication is the same as true north. In another place, the compass might indicate 20 degrees different from true north.
To allow adjustment for magnetic variation it is marked on sectional charts. Add west variation, and subtract east variation.
Determine the Magnetic Course
Find the magnetic variation along your route of flight. Lines of magnetic variation are depicted on the sectional chart for this purpose. Subtract east variation and add west variation from your true course. The result is magnetic course, which is the ground track you intend to fly, relative to magnetic north.
The graphic below shows how lines of magnetic variation are depicted on a sectional chart.
