Shape and Dimensions of the Earth, Coordinate Systems and Position
Latitude, longitude and the Earth model
General Navigation begins with the shape of the Earth and the coordinate grid used to fix position. The exam tests definitions, units and conversions rather than geodesy for its own sake.
The Earth is modelled as an oblate spheroid: equatorial radius 6 378.137 km, polar radius 6 356.752 km, flattening about 1/298.257. A geoid is the equipotential surface approximating mean sea level; a chart datum (e.g. WGS 84) ties coordinates to an ellipsoid. Latitude is the angle between the equatorial plane and the ellipsoid normal at the point (geodetic latitude); geocentric latitude uses the radius from the centre and differs slightly. Longitude is measured east or west of the Greenwich meridian from 0° to 180°. Parallels are small circles except the equator; meridians are great-circle semicircles.
Distance on the graticule
One minute of latitude is approximately one nautical mile (1 NM = 1 852 m). One degree of latitude is therefore about 60 NM. A minute of longitude, called departure, equals 1 NM × cos(latitude); it shrinks to zero at the poles. Use departure only for short east-west legs or as a rhumb-line approximation. Convert DMS to decimal degrees before arithmetic, and keep east positive/west negative when using formulae.
Worked example
Point A is 45°30′N 020°15′E; point B is 46°00′N 020°15′E. Latitude change = 30′ = 30 NM. If instead B is 022°45′E at the same latitude, longitude change = 2°30′ = 150′. At 45°30′N, departure = 150 × cos 45.5° ≈ 105 NM. The direct great-circle distance will be slightly less than this rhumb-line departure for a longer leg.
Traps
Do not treat a minute of longitude as a nautical mile away from the equator. Do not mix geodetic and geocentric latitude. Watch the anti-meridian: 179°E to 179°W is 2° of longitude, not 358°. Keep latitude and longitude in the correct order and state N/S, E/W explicitly.