Distance for air travel is calculated by using an aeronautical chart. An aeronautical chart is a map used for air navigation that contains information on the land and air between two locations.
The land and air information contained in an aeronautical chart generally includes topographical features, hazards and obstructions, navigation aids and routes, and designated air space and airports. Using the aeronautical chart, one must then locate the departure and destination airports. After these are located, the flight course must then be plotted using a series of straight lines between the two locations. When plotting the course, one must keep in mind the aircraft's flight restrictions, any physical obstructions such as mountains, and other restrictions such as restricted airspace. These restrictions could affect the flight's distance, for instance, if the pilot must go around certain obstructions or restricted airspace.
After the course is plotted, the distance must then be calculated by using the aeronautical chart's scale. Scales differ by chart but generally follow a one-to-500,000 formula, which means that one single unit on the chart is equal to 500,000 units on the ground. So if the chart uses feet, one unit on the chart is equal to 500,000 feet on the ground. The chart then provides a formula for converting to miles that can be used to calculate the total units to get the total air distance in miles.