Limitations and Scale

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In addition to the limitations of chart projections, cartographers and chart users are faced with the problems of scale, simplification, and classification. Finally, chart users, especially pilots, are faced with the crucial issue of current information. With visual charts only updated annually or semiannually and instrument charts updated every 56 days, the pilot must understand the system used to provide the latest information that is effective between routine chart revisions.

Charts provide a reduced representation of the Earth’s surface. Recall that scale defines the relationship between a distance on a chart and the corresponding distance on the Earth. Scale is generally expressed as a ratio. The numerator, customarily 1, represents chart distance, and the denominator, a large number, represents horizontal ground distance. For example, 1:500,000 (sometimes written 1/500,000) states that any single unit, whether inch, foot, yard, statute mile, nautical mile, or kilometer on the chart, represents 500,000 units on the ground. That is, 1 inch on the chart equals 500,000 inches on the Earth.

Chart makers provide scales for conversion of chart distance to statute or nautical miles or kilometers. Manufacturers of aeronautical plotters provide scales for standard aeronautical charts; however, the pilot must be familiar with the plotter used and chart scale for accurate calculations.

While flying in the annual Hayward-Bakersfield-Las Vegas air race, I use sectional charts (1:500,000), except in the Las Vegas area where a terminal area chart (1:250,000) is available. Sure enough, in my haste, I measured a leg using the wrong scale. This is disastrous in a race where the finishing order is based upon time in seconds and fuel in tenths of a gallon.

The smaller the scale of a chart, the less detail it can portray. For example, a chart with a scale of 1:1,000,000 cannot provide the detail of a chart with a scale of 1:250,000. Charts with a smaller scale increase the size of the area covered (assuming a constant size), but reduce the detail that can be shown.

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