Lesson A2 1:32 3 of 26
How to turn a true course from the chart into a magnetic course, with the isogonic line near Salinas.
The video plays from YouTube. FAA San Francisco Sectional, effective 29 OCT 2026. Not for navigation.
Every line of longitude points to true north, the geographic North Pole. Your compass points to magnetic north. That is a different spot, and it moves over time. The angle between them is called variation.
The chart shows variation with dashed magenta isogonic lines. Every point on one line has the same variation. Southeast of Salinas, this line reads twelve degrees, thirty minutes east. The legend says these values are for 2025. So near Salinas, magnetic north lies about twelve and a half degrees east of true north.
You measure a true course from a line of longitude. Salinas to Monterey is two four eight degrees true. To fly it by compass, convert to magnetic. East variation is subtracted. West variation is added. East is least, west is best. Two four eight, minus twelve and a half, is about two three six degrees magnetic.
One more detail. The rose around the Salinas VOR leans further east. It is set to seventeen degrees east, not twelve and a half. Each VOR is aligned to a fixed variation value assigned to that station. Earth's magnetic field slowly drifts, but that value is only updated from time to time. Radials from this VOR follow its rose. For your own course math, use the isogonic line. True course, minus east variation, gives magnetic course.
Before you fly, check the current chart, the Chart Supplement and NOTAMs.
A discovery flight is actual flight training with a certified flight instructor at the Salinas Airport. Your hands on the controls, with the chart on your knee.
