The vertical grid lines on charts and maps represent True North. Of course, most of us are aware that whilst the poles represent the Earth’s Magnetic North and South, the position of the magnetic poles move over time.
The amount by which magnetic North or South moves over time is called Variation and the degree by which Variation changes year on year can be measured and predicted. Your charts will show you the variance for the area the chart covers on a specific year and it’ll also tell you by how much that variance will change every year. This information is usually found on the compass rose on your chart.
Variation is the difference in degrees and minutes between True North on the chart you are looking at and Magnetic North at that place at a given time. You can then apply the rate of variation every year to find variation at that location in any given year.
Of course, your compass is magnetic and is governed by magnetic North and South. Your charts, however, are shown using a fixed True North position, so when a navigator is calculating a course to steer he must translate that course (which will be a True course relative to True North) into a magnetic course that the helm can steer by reference to the ship’s compass.
The simple way to remember how to translate a True Course to a Magnetic Course is to apply this easy mnemonic: C A D E T.
This means that as you transfer Compass to True, you Add East. So if you are translating 100 degrees Magnetic on your compass to True on your chart then you would look at the amount of variation on the chart. If it is 3 degrees East then the Magnetic Compass Course of 100 degrees M would be 103 Degrees T. If the variation was West, then you would be subtracting West (the opposite of adding East).
If you are going from Chart (True) to Helm (Magnetic Compass) then it would be opposite. In other words you’d add Westerly and subtract Easterly variation.
Now, in many parts of the World variation may be no more than 2 or 3 degrees and there are not many helms that would claim that they can steer to that level of accuracy, so this might seem rather an academic exercise, but in many parts of the World variation may be 30 or 40 degrees, or more! Get your calculation wrong and you could be sailing hundreds of miles off course!
Finally, there is also deviation to allow for. Read our Bog article: What is Deviation and How to Apply it?