Map reading and checkpoints
The craft has its own vocabulary. Map reading is the determination of the aircraft's position by matching the ground with its representation on the chart, in both directions: from chart to ground when you know roughly where you are, from ground to chart when you do not. A visual checkpoint is a feature, chosen in advance, at which the aircraft's position is to be visually confirmed. A good one has the qualities of a good feature, sharpened: unmistakable identity (a bridge over a river at a town's edge beats an anonymous wood), a location near the planned track, and ideally a second confirming detail nearby. Preparation makes them usable: the chart is folded to the route, marked with the tracks and timing points, and aligned with the track ahead, track-up in the hand even when north-up on the table, so that left on the chart is left out of the window.
In flight the technique is a cycle the syllabus spells out and this course compresses to a rhythm: DR first, then look. The techniques the syllabus names:
use the DR position to know where and when to look for a landmark, rather than searching the whole window;
identify charted features positively, by several details, before trusting them;
weigh the factors affecting the choice of landmarks, and select suitable ones: distinctiveness, distance from the track, and visibility from the planned altitude;
allow for seasonal and meteorological effects on their appearance: a lake half its charted size in drought, snow burying field boundaries, haze softening everything;
estimate distance from a landmark by successive bearings: the swing of its bearing over a timed run gives the range by the one-in-sixty family, a landmark whose bearing changes 30° while the aircraft flies 4 NM lying about 60 × 4/30, which is 8 NM, abeam;
or estimate it from the sighting angle and the flight altitude: a feature seen about 45° below the horizon lies roughly one altitude away over the ground.
Two disciplined habits complete the cycle. If no visual checkpoint is available at a scheduled turning point, the turn is made on time, on the DR position, and the new leg is confirmed at the next opportunity: holding the old heading while hunting for a missed feature converts a small uncertainty into a large one. And some terrain simply reads badly: deserts, forests, tundra and open water offer few distinctive features; other regions are charted sparsely or inaccurately. There, checkpoints are chosen further apart, timing carries more weight, and the DR position is trusted over a doubtful identification.
Position lines and fixes
A sighting can be sharpened into geometry. A position line from a visual observation, a bearing to a church, a railway crossed at a known angle, runs through every place the aircraft could be. Two intersecting position lines, plotted on the chart, establish a fix, and the nearer their cut is to a right angle, the better the fix. A single observed position line, on its own, still checks flight progress: a line cutting across the track answers how far along it the aircraft is; one parallel to the track answers whether it is left or right. Evaluating the differences between DR positions and the actual positions found refines both the navigation and the flight's performance figures: the along-track difference corrects the ground speed, the cross-track difference corrects the drift, exactly the DR-versus-fix comparison of the last module, now fed by the eyes.