A Sectional Aeronautical Chart, often called sectional for short, is designed for navigation under visual flight rules. (In Canada, the equivalent charts used for visual flight are called VFR Navigation Charts (VNCs)).

A sectional chart shows topographical features that are important to pilots, such as terrain elevations, ground features that are recognizable from the airplane cockpit such as rivers, dams, bridges, towers, etc. The chart also shows information on airspace classes, ground-based navigation aids, radio frequencies, longitude and latitude, navigation waypoints and navigation routes.

Sectionals are in 1:500,000 scale and are named for a city on the chart. The charts are updated at six-month intervals. The Federal Aviation Administration (FAA) in the United States publishes over 50 charts covering the continental United States, Alaska, and Hawaii. A more detailed chart is the Terminal Area Charts (TACs) at 1:250,000 scale for the areas around major U.S. airports. The Sectional Chart Legend is the same for both. There is a link at the end of this lesson where you can download Sectional and Terminal Area charts.
Comparison of the detail from a Sectional Chart and the same region on a Terminal Area Chart.  Note that on the sectional (left) the VFR Waypoint at the Framingham Shopping Center does not appear, nor does the private airport  at Sudbury.
The National Airspace System
49 USC § 40103 - Sovereignty and use of airspace
(a) Sovereignty and Public Right of Transit.-
(1) The United States Government has exclusive sovereignty of airspace of the United States.

"Sovereignty and use of airspace" is a result of the crash between two airliners over the Grand Canyon on June 30, 1956, President Eisenhower signed the Airways Modernization Act in August 1957 and the FAA began operations on December 31, 1958 creating the Air Traffic Control (ATC) system we have today.
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