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chart scale

A generic term that originated with paper charts, being the numerical ratio of the physical size of the chart to the size of the land or water it represents, usually express in the form 1:40,000. This means that 1 inch on the chart is equivalent to 40,000 inches on land (0.55 nmi).

The size of a chart scale is described by thinking of the ratio as a fraction, so that 1/40,000 is a larger number than 1/80,000 so 1:40,000 is considered a larger chart scale than 1:80,000.

The advent of electronic charts viewed on a computer screen that can be zoomed to larger or smaller scales calls for more specific terminology. A specific electronic chart is usually based in large part on a specific paper chart with a specific chart scale, whose details were presented at a level considered best for use on a chart of that scale. This scale of the electronic chart is then called the compilation scale for that specific electronic chart.

When viewed on the screen, the user can zoom in (to larger scales) or zoom out (to smaller scales). The actual scale being viewed at any moment is called the display scale.

If the display scale is zoomed in to more than twice the compilation scale (a 1:40,000 chart viewed at more than 1:20,000 display scale), then the display will be marked "over zoomed." Different navigation programs present this announcement in different ways.

Likewise, when viewing an electronic navigational chart (ENC) if you zoom out to display scales approaching one half of the compilation scale (a 1:40,000 chart viewed at less than 1:80,000 or so), some objects on the chart might no longer be shown, depending on the "scale minimum" (SCAMIN) attribute of the object.


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