Charts: Projections, Scale, Relief, Chart Properties and Use
Projections and properties
Aeronautical charts are selected for the property the navigation task needs: conformality (angles and shapes preserved), a straight rhumb line, a straight great circle, or manageable scale. Mercator is conformal; rhumb lines plot straight, but great circles curve poleward and scale expands with latitude. Lambert conformal conic is conformal with straight standard parallels; great circles are nearly straight over the chart, which suits mid-latitude navigation. Polar stereographic is used near the poles. The projection, standard parallels and scale must be read from the chart legend.
Scale
Scale is a representative fraction such as 1:500 000, or a graduated bar. Large-scale charts show more detail over a smaller area; small-scale charts cover more area with less detail. On a Mercator chart the stated scale is exact only at the standard parallel(s); measure distance against the latitude scale at the same latitude, not against a fixed bar if the chart warns of scale change.
Relief and chart use
Relief is shown by contours, spot heights, hill shading and layer tints. Contour interval, highest obstacles, airspace boundaries, magnetic variation, units and chart date all affect a safe plot. Use the chart’s own graticule and compass rose; do not transfer a bearing from another projection without correction.
Worked example
A chart has scale 1:500 000. A straight leg measures 6.4 cm. Ground distance = 6.4 × 500 000 cm = 3 200 000 cm = 32 km. Convert to NM: 32 ÷ 1.852 ≈ 17.3 NM. If the chart is 1:250 000, the same 6.4 cm represents 16 km ≈ 8.6 NM; the larger-scale chart gives more detail but covers less area.
Traps
Do not measure a great-circle distance with a ruler on a Mercator chart and call it exact. Do not use the scale bar at a latitude far from its standard parallel on a projection with scale variation. Do not confuse contour height with obstacle clearance. Check chart edition and variation date; an old chart can produce a legal-looking but wrong track.