A sectional chart (or just 'sectional') is a type of navigation chart (map) used by the aviation community (pilots, etc). In the United States, there are officially approved (by the FAA) sectionals available for use when flying anywhere in the world. There are a few things about sectionals that make them a distinct form of map.

First of all, they have expiration dates! Because the information on a sectional is more critical to safety than that on, say, a regular highway map, there are dates past which it is illegal to use them for flight navigation or planning. Even if the sectional you have is still valid, however, you (as the pilot) are still responsible for carefully checking for any errata or revisions that may have been posted for them before using them to plan a flight.

They have a fixed scale, namely 1:500,000 (1 inch = 6.86 nautical miles). There are Wide Area Charts which have a 1:1,000,000 scale and VFR Terminal Area Charts which are 1:250,000. The Sectional is the best all-around navigation chart for use in the airplane.

They contain a great deal of information; as such, reading them is a skill that must be learned with care. Unlike regular highway maps, which may contain three or four types of information (road placement, road type, distance, and place names) the sectional contains far more. Here are some of the things you will find on a sectional chart:

This is an incredible amount of information. Couple this with the fact that a sectional must use a limited range of colors (both due to potential color-blindness on the part of the user and, more likely, because the user is in a low-light or red-light environment) and the problem of information density begins to become apparent.

Sectionals utilize a great deal of text, but try to compartmentalize it into bounded boxes to denote application to a particular location; hence, airports and regions will usually have 'information boxes' adjoining them. These boxes and text are printed in relatively low contrast colors so as not to obscure terrain and obstruction shapes and symbols. It is nearly impossible to use a sectional without some form of straight edge or rule. The most important skill one can learn when using a sectional is the ability to quickly translate symbols into 'reality' so that if you are lost, you can locate yourself on the sectional by placing the abstract symbols for objects into context outside your window. It's much harder than it sounds; plus, you swiftly learn that from the air, all railroads, rivers, roads, radio towers and power lines look alike. There are no handy signs to tell you what you're looking at. You learn to look for small-town water towers, which tend to have place names stenciled on their sides in letters large enough to read from afar; you look for oddly-shaped bodies of water, or intersections of multiple landmarks like roads, railroads and power lines. Nuclear power plant cooling towers, airport layouts, large groups of radio towers - all of these are fairly easy to locate both on the sectional and from the sky.

Finally, sectionals make fascinating wall-hangings; expired ones are available for next-to-nothing (or nothing, if you know a pilot or two) and make great informational decoration or toys.

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