TYPES OF CHARTS

distance on a Mercator chart, one must be careful to use the latitude scale at the proper place. Distance is measured by placing one end of the dividers at each end of the line to be measured and, without changing the setting of the dividers, transferring them to the latitude scale with the middle of the dividers at about the middle latitude of the two points between which the distance is desired. TYPES OF CHARTS Of the types of charts used today, Mercator projection charts are the most commonly used navigational charts. Other charts used include the transverse Mercator, polyconic, and gnomonic projections. For more information on the use of these charts, consult Dutton‘s Navigation and Piloting or the American Practical Navigator, Pub. No. 9. CHART SCALES The scale of a chart refers to a measurement of distance-not area. A chart covering a relatively large area is called a small-scale chart, and one covering a relatively small area is called a large-scale chart. Scales may vary from 1:1,200 for plans to 1:14,000,000 for world charts. Normally, the major types of charts fall within the following scales: Harbor and Approach Charts—Scales larger than 1:50,000. These charts are used in harbors, anchorage areas, and the smaller waterways. Those charts used for approaching more confined waters are called approach charts. Coastal Charts—1:50,000 to 1:150,000. These charts are used for inshore navigation, for entering bays and harbors of considerable width, and for navigating large inland waterways. General and Sailing Charts—l:150,000 or smaller. These charts are used for coastal navigation outside outlying reefs and shoals when the vessel is generally within sight of land or aids to navigation and its course can be directed by piloting techniques. They are also used in fixing the position as the ship approaches the coast from the open ocean and for sailing between distant coastal ports. The size of the area portrayed by a chart varies extensively according to the scale of the chart. The larger the scale, the smaller the area it represents. It follows then that large-scale charts show areas in greater detail (fig. 9-6.) Many features that appear on Figure 9-6.—Comparison of small- and large-scale charts. a large-scale chart do not show up on a small-scale chart of the same area. The scale to which a chart is drawn usually appears under its title in one of two ways: 1:25,000 or l/25,000. These figures mean that an actual feature is 25,000 times larger than its representation on the chart. Expressed another way, an inch, foot, yard, or any other unit on the charts means 25,000 inches, feet, or yards on Earth's surface. Thus, the larger the figure indicating the proportion of the scale, the smaller the scale of the chart. A chart with a scale of 125,000 is on a much larger scale, for instance, than one whose scale is 1:4,500,000. You must exercise more caution when working with small-scale charts than with large-scale charts. A small error, which may be only a matter of yards on a large-scale chart, could amount to miles on a chart depicting a much more extensive area. When navigating the approaches to land, use the largest scale charts available. 9-5