Chapter 3: Nautical Charts
A chart is a projection of the curved Earth onto a plane. Each projection distorts something; the navigator must know what is preserved and what is not.
Projections
The Mercator projection is conformal and cylindrical: meridians are parallel straight lines, rhumb lines plot as straight lines, and scale expands as the secant of latitude, which is why it is unusable near the poles. The gnomonic (great circle) projection plots great circles as straight lines and is used for great-circle planning charts. The Lambert conformal conic is used for aeronautical and some polar charts, with little distortion between its standard parallels. The transverse Mercator serves high latitudes.
- Mercator — rhumb line straight, areas exaggerated toward poles
- Gnomonic — great circle straight, used to lift waypoints to a Mercator chart
- Polar stereographic / transverse Mercator — high latitude work
Scale, accuracy and source
A large-scale chart (1:10 000) shows a small area in great detail; a small-scale chart (1:1 000 000) shows a large area with little detail. Always navigate on the largest scale available. The source diagram or zone-of-confidence (CATZOC) data tells you the survey vintage and quality; a lead-line survey from 1890 deserves a wide berth even on a modern-looking chart.
Symbols and reading
Chart No. 1 (US) and INT 1 explain every symbol and abbreviation: magenta for cautionary and radio information, blue and green tints for shoal water and drying heights, and light characteristics such as Fl(3)WR 15s 24m 12M meaning group-flashing three, white and red sectors, 15-second period, 24 metres elevation, 12 miles nominal range.
Formula cards
Mercator scale expansion
scale at latitude L = scale at equator × sec L
Explains why a mile measured near the top of a Mercator chart is longer on paper.
Key takeaways
- • Mercator preserves angles, not areas; rhumb lines are straight.
- • Great circles are straight only on a gnomonic projection.
- • Always work on the largest-scale chart of the area and check its survey source.
Knowledge check
Immediate feedback with rationales. Answer each question to see the explanation.
Chapter 3 quiz
0 of 3 answered
1.On which projection does a great circle appear as a straight line?
2.A chart at 1:20 000 compared to one at 1:200 000 is:
3.Fl(2)WR 10s 18m 9M describes a light that is: