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CHART FUNDAMENTALS

300. Definitions A nautical chart represents part of the spherical earth on a plane surface. It shows water depth, the shoreline of adjacent land, prominent topographic features, aids to navigation, and other navigational information. It is a work area on which the navigator plots courses, ascertains positions, and views the relationship of the shipto the surrounding area. It assists the navigator in avoiding dangers and arriving safely at his destination. Originally hand-drawn on sheepskin, traditional nautical charts have for generations been printed on paper. Electronic charts consisting of a digital data base and a display system are in use and are replacing paper charts aboard many vessels. An electronic chart is not simply a digitalversion of a paper chart; it introduces a new navigation methodology with capabilities and limitations very different from paper charts. The electronic chart is the legal equivalent of the paper chart if it meets certain International Maritime Organization specifications. See Chapter 14 for a complete discussion of electronic charts. Should a marine accident occur, the nautical chart in use at thetime takes on legal significance. In cases of grounding, collision, and other accidents, charts become critical records for reconstructing the event and assigning liability. Charts used in reconstructing the incident can also have tremendous training value. 301. Projections Because a cartographer cannot transfer a sphere to a flat surface without distortion, he must project the surface of a sphereonto a developable surface. A developable surface is one that can be flattened to form a plane. This process is known as chart projection. If points on the surface of the sphere are projected from a single point, the projection is said to be perspective or geometric. As the use of electronic charts becomes increasingly widespread, it is important to remember that the same cartographic principlesthat apply to paper charts apply to their depiction on video screens. 302. Selecting a Projection Each projection has certain preferable features. However, as the area covered by the chart becomes smaller, the differences between various projections become less noticeable. On the largest scale chart, such as of a harbor, all projections are practically identical. Some desirable properties of aprojection are: 1. 2. 3. 4. 5. 6. True shape of physical features Correct angular relationships Equal area (Represents areas in proper proportions) Constant scale values Great circles represented as straight lines Rhumb lines represented as straight lines

Some of these properties are mutually exclusive. For example, a single projection cannot be both conformal and equal area. Similarly, both greatcircles and rhumb lines cannot be represented on a single projection as straight lines. 303. Types of Projections The type of developable surface to which the spherical surface is transferred determines the projection’s classification. Further classification depends on whether the projection is centered on the equator (equatorial), a pole (polar), or some point or line between (oblique). The nameof a projection indicates its type and its principal features. Mariners most frequently use a Mercator projection, classified as a cylindrical projection upon a plane, the cylinder tangent along the equator. Similarly, a projection based upon a cylinder tangent along a meridian is called transverse (or inverse) Mercator or transverse (or inverse) orthomorphic. The Mercator is the most commonprojection used in maritime navigation, primarily because rhumb lines plot as straight lines. In a simple conic projection, points on the surface of the earth are transferred to a tangent cone. In the Lambert conformal projection, the cone intersects the earth (a secant cone) at two small circles. In a polyconic projection, a series of tangent cones is used. In an azimuthal or zenithal projection,...