Map Projections
A map is a flat depiction of a portion of the Earth's sphere. Because a sphere cannot be unrolled onto a plane without distortion, maps must sacrifice some property.
Three possible properties — cannot be achieved simultaneously
- Conformal (angle-preserving): angles between lines on the map = angles in reality. Important for navigation, because rhumb-line angles = chart angles.
- Equal-area: areas scale proportionally to reality. Important for statistical maps, not for navigation.
- Equidistant: distances on the map proportional to reality — possible only in a limited sense (along certain axes or standard parallels).
Carl Friedrich Gauss proved (1827): no projection can be simultaneously conformal, equal-area, and equidistant.
Main projection families
| Projection | Construction | Distortion |
|---|---|---|
| Cylindrical (Mercator) | Globe onto a cylinder tangent to the equator | small at the equator, grows strongly toward the poles |
| Conic (Lambert Conformal Conic) | Globe onto a cone cutting two standard parallels | small between the standard parallels, grows outside |
| Azimuthal (Polar Stereographic) | Globe onto a tangent plane at the pole | small near the pole, grows toward the equator |
Lambert Conformal Conic (LCC) — the aeronautical standard projection
Properties:
- Conformal (angle-preserving).
- Very low distortion in mid-latitudes between the standard parallels.
- Great circles appear as nearly straight lines (exactly straight only in special cases) — ideal for cross-country flight.
- Meridians: straight lines converging toward the poles.
- Parallels: concentric circular arcs.
Use: standard for ICAO 1
,000 and WAC charts in mid-latitudes. German, Swiss, Austrian, US VFR charts are LCC.Mercator (cylindrical)
Properties:
- Conformal.
- Rhumb lines are straight lines → important for maritime navigation with a constant compass course.
- Great circles appear as curves.
- Strong area distortion poleward (Greenland appears larger than Africa!).
Use: nautical charts, early long-range air charts, world overview maps.
Transverse Mercator and UTM
- Transverse Mercator: cylinder tangent to a meridian instead of the equator → optimal for narrow N-S strips.
- Universal Transverse Mercator (UTM): 60 zones of 6° longitude each — common for military and topographic maps.
Polar Stereographic
- Conformal, pole at the centre.
- Great circles through the pole are straight lines.
- Used for polar routes (e.g. ETOPS polar).
What is shown on every chart?
- Projection type (e.g. "Lambert Conformal Conic with standard parallels at 49°N and 53°N").
- Scale (1,000, 1,000, …).
- Standard parallels (for conic projections).
- Datum for elevations and coordinates (typ. WGS-84).
- Magnetic variation and annual change.
- Isogonals (lines of equal variation).
Choosing a projection in practice
For PPL: the projection is given by the ICAO chart (always LCC for VFR in Europe). The pilot does not actively choose a projection, but must know the properties of the chart in use:
- Distances on an LCC chart can be measured directly with a plotter — within the standard parallels, the scale is practically constant.
- Outside the standard parallels (well south/north of the typical design area), scale errors apply.