First, the big picture — Page 457, Lesson 454

This is a grid convergence chart — the kind of table you’ll find in the General Navigation syllabus, and it’s one of the most practical tools you’ll use for converting between True and Grid directions. Let me walk you through exactly what you’re looking at, because once you understand the layout, the numbers will make complete sense. First, the big picture. This chart is built for a specific grid — the one used on a Transverse Mercator projection, which is what most large-scale navigation charts use. The grid lines on the chart are drawn parallel to the central meridian, but True North is defined by the meridians, which converge toward the poles. So the angle between Grid North and True North changes as you move east or west of the central meridian. That angle is called grid convergence, and this table gives you its value for any latitude and longitude you need. Now, the layout. The left-hand column lists latitudes, from N60 at the top, down through N50, N20, N10, the Equator at 0, then S10, S30, and so on down to S60 at the bottom. The top row lists longitudes — you’ll see values like 16 04, 16 13, 16 09, and so on, running across. So each cell in the table is the convergence value at the intersection of a particular latitude and longitude. Here’s the key rule: the convergence value is the angular difference between Grid North and True North at that point. If the value is positive, True North lies to the east of Grid North. If it’s negative, True North lies to the west. And the sign convention matters — you’ll use this to convert a Grid track into a True track, or vice versa, by adding or subtracting the convergence. Let me give you a concrete example. Look at the row for N60, and find the longitude 16 04. The value there is 5. That means at 60° North, 16° 04' West, the convergence is 5 degrees. So if you’re flying a Grid track of, say, 090°, your True track would be 090° plus 5°, giving 095°. That’s the essence of the conversion. Now, notice how the values change. At N60, the convergence values are small — around 5 to 6 degrees. But as you move south toward the Equator, the values increase. Look at the row for 0 — the Equator — and you’ll see values like 18 11, 18 12, 18 13, climbing up to around 18 30. That’s because at the Equator, the meridians are diverging most rapidly from the grid lines, so the convergence is largest. At the poles, the meridians converge to a point, so the convergence approaches zero. Also, watch the sign flip. At N60, the values are positive — True North is east of Grid North. But as you cross the Equator into the Southern Hemisphere, the values become negative. Look at S10 — you’ll see values like 26, 27, 29, 30, and then negative numbers like -18 40, -18 41. So south of the Equator, True North lies to the west of Grid North, and the convergence is negative. One more thing to notice: the values are not symmetric. At N60, the convergence is about 5 degrees, but at S60, it’s much larger — look at the bottom row, S60, and you’ll see values like 21 32, 21 43, 21 55, climbing to 23 10. That asymmetry comes from the specific projection and the central meridian chosen for this grid. So you can’t just mirror the Northern Hemisphere values — you have to read the chart directly. So, to use this chart in practice: you locate your position by latitude and longitude, read the convergence value from the table, apply the sign convention (positive east, negative west), and then convert between Grid and True tracks. That’s the whole tool — it’s a lookup table that turns a geometric relationship into a simple arithmetic step. Let me also point out the figure that goes with this — shows a point A at longitude 70W, with the line defining True North being the line joining A to the North Pole. That’s the geometric basis for everything we just did — True North is always toward the pole, and the grid is a separate, artificial reference. And shows the creation of the grid itself, so you can see how the grid lines are laid down relative to the meridians. That’s the convergence chart. You’ll use it constantly in navigation problems, so get comfortable reading it — latitude down the side, longitude across the top, and the convergence value in the cell. Positive east, negative west, and the magnitude grows as you move away from the pole.