How do you use a compass correction card?

Answer 1

First you read the compass. Then you find the correction for the heading closest to the heading the compass reads, interpolating if needed. Then you add the correction to the heading to get the corrected heading.

For example, say the compass read 37 degrees and the correction card had a +2 correction for 30 degrees and a -1 correction for 60 degrees. You would interpolate a correction at 37 degrees of about +1, and add that to 37 to get a corrected magnetic heading of 38 degrees.