Adverse Conditions and Advanced Navigation: Polar, Oceanic, Great-Circle and Rhumb-Line Planning
Polar and oceanic navigation
Polar navigation relies on gyros, GNSS and inertial systems because magnetic compasses are unreliable near the magnetic poles (steep dip, weak horizontal intensity). Grid navigation uses a grid overlay on polar charts so headings are referenced to grid north rather than true north. Oceanic navigation uses organised track systems (e.g. North Atlantic tracks), with position reporting, strategic lateral offsets and reduced vertical separation. Traps: forgetting grid convergence (the angle between grid north and true north), and assuming magnetic headings are usable at high latitudes.
Great-circle and rhumb-line planning
A great circle is the shortest distance between two points; it appears curved on a Mercator chart. A rhumb line is a constant-heading line, longer than the great circle except on the equator or a meridian. For long routes, the great-circle track changes continuously; the initial and final tracks differ. Compute great-circle distance with the spherical cosine formula or approximate using the mid-latitude method for short legs. The vertex is the highest latitude reached on a great circle.
Worked example
A flight from 60°N 030°W to 60°N 030°E along the parallel: departure = 60° of longitude × 60 × cos 60° = 3600 × 0.5 = 1800 NM. The great-circle distance is shorter, because the great circle curves poleward of the parallel. If the great circle reaches 70°N, the saving is significant.
Rule of thumb
"Great circle = shortest, rhumb line = constant heading." On a Mercator, the great circle bulges toward the pole. Near the poles, use grid headings and GNSS.