Coast charts, scale 180000, for approaching the coast at any point and for inside passages.
Harbor and channel charts, of various large scales from 15000 to 160000, for entering harbors and rivers and passing through channels.
The expression of scales by miles to the inch or inches to the mile is the more familiar. The expression of scale in the manner used by the Coast Survey and by most of the European countries, by standard fractions as 180000, meaning that any distance on the chart is 180000 of the actual distance on the earth, has some advantages. For instance, the relation of these fractions gives at a glance the relation of the scales of the charts. Thus a 180000 chart is on a scale five times as large as a 1400000 chart.
For the more important harbors charts have been published on several different scales to meet various needs. Thus New York Harbor is shown on charts of scales of 110000, 140000, 180000, 1200000, 1400000 and 11200000, each of course including a different area.
FIG. 26. NEW YORK HARBOR, PORTIONS OF CHARTS ON FOUR DIFFERENT SCALES.
The selection of suitable publication scales is of prime importance; a large scale permits of greater clearness and of showing more detail, but on the other hand restricts the area and the points that can be shown on a single sheet, or else makes a chart of excessive dimensions. In general in chart preparation the scale should be restricted to the minimum that can be used to fulfill the particular object and clearly represent what is desired. A chart of very large scale is not convenient for plotting, and a moving vessel may pass quickly beyond it or into range of objects beyond the limits of the chart.