Latitude, longitude, and geographic coordinates

Bowditch Ch. 1 §101

Introduction to Marine Navigation Marine navigation is the art and science of determining a vessel's position and conducting it safely from one point to another. Its four primary methods are dead reckoning, piloting, celestial, and electronic — modern voyages combine all four with electronic navigation as the principal real-time reference and the others as cross-checks.

Bowditch Ch. 1 §102

Introduction to Marine Navigation Dead reckoning (DR) is the determination of position by applying course steered and distance run from a known starting point. DR accuracy degrades over time and must be confirmed by external fixes whenever possible.

Bowditch Ch. 1 §103

Introduction to Marine Navigation Piloting is navigation by reference to landmarks, aids to navigation, and soundings. It is the primary method used in confined waters where dead reckoning alone is insufficient and electronic positioning may be obstructed by terrain or interference.

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.