Charts: Projections and Practical Use of Aeronautical Charts
Why projections matter
A chart is a projection of a curved surface onto a plane; every projection distorts something. You must know which properties are preserved and which are not. A conformal projection preserves angles and shapes locally (so a straight line is a rhumb line on a Mercator), but scale varies. An equal-area projection preserves area but distorts shape. A great circle plots as a straight line on a gnomonic chart, which is why it is used for planning long routes. The Mercator is conformal, with scale increasing away from the equator; a rhumb line is a straight line, and a great circle is curved (concave toward the pole in the northern hemisphere).
Common chart types
Lambert conformal conic is standard for many aeronautical charts: conformal, with two standard parallels, a straight-line great circle approximation over small areas, and scale error small within the standard parallels. Polar stereographic is used for polar regions, where a straight line is approximately a great circle. Transverse Mercator is used for some grid systems.
Practical use
On a Lambert chart, measure true track at the meridian nearest the midpoint of the route, and measure distance against the scale for that latitude. On a Mercator chart, remember that the scale bar is valid only at the latitude indicated; use the latitude scale on the side to measure distance. Traps include using the wrong scale bar, measuring track at the departure or destination meridian instead of the mid-meridian, and forgetting magnetic variation when converting a plotted true track to a magnetic heading.
Worked example
A route from 50°N to 55°N on a Lambert chart: the true track is measured at the mid-latitude (about 52.5°N). If variation there is 5°W, the magnetic track is 005° greater than the true track (east of true track? No: magnetic = true + west variation). Apply the sign carefully.
Rule of thumb
"Gnomonic for great circles, Mercator for rhumb lines and plotting, Lambert for regional aeronautical charts, polar stereographic for the poles."