Plotting — Page 491, Lesson 491

Let me walk you through the key plotting technique here, because this is the heart of how we actually put a radio bearing onto a chart. We've been looking at how an aircraft measures a bearing of an NDB — a non-directional beacon — and we need to plot that as a position line. The critical idea is this: when you measure a bearing at the aircraft, you get a true great circle bearing of the NDB from the aircraft. But to draw that on a Lambert chart, you have to decide which meridian you're measuring from. Here's the subtlety. If you draw the angle of 090° from a line parallel to the aircraft's meridian drawn through the NDB, the position line will pass through the aircraft. That's exactly what Figure 28.4(b) shows. And here's the beautiful part — chart convergence has been automatically allowed for by drawing that line parallel to the aircraft's meridian through the NDB. This method is always used in plotting. That's the standard technique. Now, if instead you need to plot the bearing from the true meridian of the NDB itself, reference to Figure 28.4(b) shows that in this case it equals 090° minus chart convergence. So the two methods differ by the convergence angle. Let me give you a concrete example. An aircraft flying on a heading of 330°(T) measures a bearing of 090°(R) of an NDB. The question asks: what is the bearing to plot on a Lambert chart, first from a line parallel to the aircraft's meridian drawn through the NDB, and second from the meridian of the NDB, given chart convergence is 3°? For part (a): we add the heading to the relative bearing. 330° + 090°(R) = 060°(T). That's the true great circle bearing of the NDB from the aircraft. To plot it, we draw 240° from a line parallel to the aircraft's meridian drawn through the NDB. Notice we plot the reciprocal — 060° plus 180° gives 240° — because the position line runs through both the aircraft and the NDB, and we draw it from the NDB's position. That's Figure 28.5(a). For part (b): the bearing to plot from the meridian of the NDB is 240° + 3° = 243°. So we add the chart convergence of 3° to the 240° we plotted in method (a). Now, here's the practical takeaway. In practice, we would not normally use method (b), plotting from the true meridian. For practical plotting, we draw in a parallel false meridian, as in method (a). That's the standard approach — it automatically handles convergence, so you don't have to compute it. One more critical point about variation. In the case of bearings measured at the aircraft, it's the aircraft's compass which is used to add to relative bearing in order to obtain true bearing. Therefore, if the heading is magnetic and it's necessary to correct it to true, we always use variation at the aircraft. So the rule is simple: bearings measured at a ground station — VOR, VDF — use variation at the ground station. Bearings measured at the aircraft — NDB/ADF, AWR — use variation at the aircraft. That distinction matters because the compass error you correct for depends on where the measurement was actually made. So to summarize the whole technique: you take your heading, add the relative bearing to get the true great circle bearing, plot the reciprocal from the NDB using a line parallel to the aircraft's meridian, and that automatically handles chart convergence. And always correct variation at the point where the bearing was measured — the aircraft for NDB/ADF, the ground station for VOR/VDF.