CHAPT03 - CHAPTER 3 NAUTICAL CHARTS CHART FUNDAMENTALS 300....

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23 CHAPTER 3 NAUTICAL CHARTS 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, 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 ship to the surrounding area. It assists the navigator in avoiding dangers and arriving safely at his destination. The actual form of a chart may vary. Traditional nauti- cal charts have been printed on paper. Electronic charts consisting of a digital data base and a display system are in use and will eventually replace paper charts for operational use. An electronic chart is not simply a digital version of a paper chart; it introduces a new navigation methodology with capabilities and limitations very different from paper charts. The electronic chart will eventually become the le- gal equivalent of the paper chart when approved by the International Maritime Organization and the various gov- ernmental agencies which regulate navigation. Currently, however, mariners must maintain a paper chart on the bridge. See Chapter 14, The Integrated Bridge, for a discus- sion of electronic charts. Should a marine accident occur, the nautical chart in use at the time 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 sphere onto a developable surface . A developable sur- face is one that can be flattened to form a plane. This process is known as chart projection . If points on the sur- face 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 car- tographic principles that apply to paper charts apply to their depiction on video screens. 302. Selecting A Projection Each projection has certain preferable features. How- ever, as the area covered by the chart becomes smaller, the differences between various projections become less no- ticeable. On the largest scale chart, such as of a harbor, all projections are practically identical. Some desirable proper- ties of a projection are: 1. True shape of physical features. 2. Correct angular relationship. A projection with this characteristic is conformal or orthomorphic . 3. Equal area, or the representation of areas in their
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CHAPT03 - CHAPTER 3 NAUTICAL CHARTS CHART FUNDAMENTALS 300....

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