Airways - Miscellaneous Charts — Page 188, Lesson 195

I want to walk you through a topic that becomes critical once you operate in the high-latitude regions — the polar areas above about 70° north. Standard navigation using true north or magnetic north starts to break down there, and we need a different reference. That reference is grid north, and the chart we use for it is the North Polar Stereographic chart. Let me start with the examples given in the book, because they show you the problem before we solve it. Example 1 gives us a series of waypoints. We have STN, which is Stornoway — coordinates N58°12.4', W006°11.0'. From there we join UN615, which is an airway, and proceed to MATIK at N61°00.0', W008°04.0'. Then we go to a point labelled 66PR at N66°00.0', W012°30.0'. The book asks you to work out magnetic tracks, true tracks, and grid tracks between these points. Now look at Example 2. It asks for magnetic variation at KARLL — N70°00.0', W151°00.0' — and at EUREKA NDB, identifier YEU, on 205 kHz, at N79°59.5', W085°53.9'. These are high-latitude positions where magnetic variation changes very rapidly. Here is the key observation the book makes. On the sector from MATIK to 66PR, the outbound true track is 340°(T). The return true track is 156°(T). Now, 340° plus 180° should give you 160° as the reciprocal — but we get 156°. That is a 4° difference. The return track is not the reciprocal of the outbound track when measured in true direction. Why? Because the track between two points on a chart that uses converging meridians is not a straight line in terms of direction — the true direction changes as you move along the route. However, when they measure the same leg using grid direction, the outbound grid track is 348°(G) and the return grid track is 169°(G). 348 minus 180 is 168 — and we get 169. That is only 1° off, and the book explains that 1° difference is because the track between the two points is not a perfectly straight line. So the grid reference gives you a nearly constant direction that is reciprocal. That is the whole point. Therefore, to measure and fly a track that has an unchanging direction, we must use grid north as the datum. That is the central principle of polar navigation. Now let me explain why true and magnetic references fail in high latitudes. The book gives three reasons. First, the rapid convergence of the meridians as latitude increases. Meridians are the lines of longitude, and they all meet at the North Pole. As you go north, they get closer together. That means if you try to follow a constant true track, the angle between your heading and the local meridian keeps changing as you cross different longitudes. Your true track direction changes rapidly over short distances. Second, the rapid change of magnetic variation over short distances. Magnetic variation is the difference between true north and magnetic north. Near the magnetic pole, that variation can change by many degrees over just a few nautical miles. So a magnetic track that works at one point is useless a hundred miles away. Third, the degraded directional capability of magnetic compasses. Near the magnetic pole, the horizontal component of the Earth's magnetic field becomes very weak. A magnetic compass becomes sluggish, unreliable, and prone to large errors. You simply cannot rely on it. So the solution is grid north. On a Polar Stereographic chart, we select one meridian as the grid north datum. In the case of this chart — and similar Polar Stereographic charts — the Greenwich Meridian, which is 0° longitude, is selected as the grid north datum. It is highlighted on the chart by a thick blue line, and it is paralleled east and west at 300-nautical-mile intervals. That means lines parallel to the Greenwich Meridian are drawn across the chart, and those lines define grid north. Grid north is the same direction everywhere on the chart — it does not converge. Example 3 drives this home. You have point A at N85°00.0', E040°00.0', and point B at N85°00.0', W040°00.0'. Both are at 85° north, but 80° of longitude apart. The true track direction at A is one value. The true track direction at the point where the route crosses the Greenwich Meridian — N85°, E/W 000° — is different. And the true track into B is different again. The book notes the change of direction over just 388 nautical miles. That is the convergence problem in action. But the grid track from A to B is a single, constant value. That is what we use. Now, the relationship between true north and grid north at any point on a grid chart is called convergence. Let me define it precisely. Convergence is the angular difference between true north and grid north at a given point. If true north is west of grid north, we call it westerly convergence. If true north is east of grid north, we call it easterly convergence. The book gives you a memory aid. It says: Convergence west, true best (grid least). Convergence east, true least (grid best). What does that mean? If convergence is westerly, then the true direction is a larger number — 'best' meaning bigger — and the grid direction is a smaller number — 'least' meaning smaller. If convergence is easterly, the true direction is the smaller number and the grid direction is the larger number. So you add or subtract the convergence value depending on which direction you are converting. Now here is the convenient part. On a Polar Stereographic chart, the convergence at any point is simply equal to the longitude with the sign changed. For example, if you are at 30° west longitude, the convergence is 30° east. If you are at 45° east longitude, the convergence is 45° west. That makes the conversion between true and grid very straightforward. So to summarise what we have covered: in high latitudes, true and magnetic references become impractical because of converging meridians, rapidly changing variation, and unreliable compasses. We switch to grid north, using the Greenwich Meridian as the datum on the Polar Stereographic chart. The difference between true and grid at any point is convergence, and on this chart convergence equals longitude with the sign changed. That allows us to measure and fly a constant direction track that actually works as a reciprocal when you reverse course.