Plotting with variation and deviation (T-V-M-D-C)

Bowditch Ch. 4 §401

Nautical Charts A nautical chart is a graphic representation of the navigable waters and adjacent coast. It is the most fundamental tool of marine navigation; modern ECDIS systems use vector ENC files but paper or raster equivalents remain the back-up.

Bowditch Ch. 4 §402

Nautical Charts The Mercator projection presents lines of latitude and longitude as straight lines crossing at right angles, with rhumb lines (constant bearings) appearing as straight lines. Distances must be measured at the latitude of the line, since meridian length varies with latitude.

Bowditch Ch. 4 §403

Nautical Charts US charts show soundings in feet, fathoms, or metres referenced to Mean Lower Low Water (MLLW). The unit is stated in the chart title; always verify before plotting.

Bowditch Ch. 4 §404

Nautical Charts Chart No. 1 (US) — "Symbols, Abbreviations and Terms used on Paper and Electronic Navigational Charts" — is the authoritative key to every symbol and abbreviation on a nautical chart: aids to navigation, bottom characteristics, depth contours, hazards, and the meaning of chart colours. A mariner reads a chart by decoding its symbols and abbreviations against Chart No. 1 before plotting.

Bowditch Ch. 4 §405

Nautical Charts Latitude is measured 0 to 90 degrees north or south of the equator; longitude 0 to 180 degrees east or west of the Greenwich meridian. Position is stated latitude first. On a Mercator chart latitude is read from the side (left and right) scales and longitude from the top and bottom scales. One minute of latitude equals one nautical mile, so distance is always measured on the latitude scale alongside the vessel's own latitude, never on the longitude scale.

Bowditch Ch. 4 §406

Nautical Charts To plot the true course and distance between two charted points, draw a straight line joining them, then walk a set of parallel rulers from that line to the nearest compass rose and read the true course where the ruler crosses the outer (true) ring. Measure the line's length with dividers against the latitude scale at the working latitude — one minute of latitude is one nautical mile — to obtain the distance. Label the course line C (course) above and S (speed) below.