How to Create an Airspace File

This is an abbreviated defintion of the OpenAir (.FAS) airspace format (which works for most flight instruments) and description of how to create an FAS file to define a custom airspace. An alternate format called SUA (Special Use Airspace) exists which I will not go into here.

Definition

Header Information: I would recommend creating entries with information in the order it is presented in this list though I do not know if ordering matters.

• *: Any line starting with an asterisk is a comment and ignored when rendering
• AC <X>: Airspace Class, usually the first line of the defintion where <X> can be:
• R: Restricted
• Q: Danger
• P: Prohibited
• A: Class A
• B: Class B
• C: Class C
• D: Class D
• GP: Glider Prohibited
• CTR: Controlled Traffic Region
• W: Wave Window
• AN <name>: Airspace name where is whatever you want to call the airspace, no clue if it needs to be unique
• AH <alt>: Airspace ceiling, expressed in feet like '8000' or 'UNLIMITED' for no ceiling or 'FL600' for top of class A (60,000'). 'MSL' can also be appended but I think it is used by default. Adding 'FT' afterwards appears to be optional as well since heights are always in feet.
• AL <alt>: Airspace floor, expressed in feet like '3000' or 'SFC' for surface, '0' for no floor or '1500 AGL' to attach floor relative to surface height. 'MSL' can also be appended but I think it is used by default. Adding 'FT' afterwards appears to be optional as well since heights are always in feet.
• AT <coord>: Optional. Coordinate of where to place a name label on the map. Multiple AT records for single airspace is ok. Example '38:56:00 N 120:02:00 W'

Drawing Definitions: This is information used to draw the airspace and I assume it draws the lines in the order it is written. Co-ordinates need to be written as <lat> N <long> W (or similar) however co-ordinate formats appear to support at least these types DD.dddd, DD:MM.mm, DD:MM:SS. Longtitude degrees must be 3 digits so for 2 digit longtitudes it would be '085'. Minutes and seconds must be 2 digits long so instead of writing '5' you would write '05'. For DD.dddd, it appears to support at least 13 digits of precision (13 digits after the period) however ALL co-ordinates must be the same length in the list, basically all the 'N's and 'W's should line up nicely in the list. The default format that is used for co-ordinates is DD:MM:SS which looks like '31:55:16 N 087:03:15 W'

• V <X>=<n>: Variable assignment used before the following commands DA, DB, DC. The following formats are supported for <X>=<n>
• D={+|-}: Sets direction for: DA and DB records. '-' means counter-clockwise direction; '+' is the default. Automatically reset to '+' at the beginning of new airspace segment
• X=<coord>: Sets the center for the following records: DA, DB, and DC
• W=<number>: Sets the width of an airway in nautical miles (NYI)
• Z=<number>: sets zoom level at which the element becomes visible (WP version 1.130 and newer)
• DP <coord>: Add polygon point at specified coordinate, example "DP 39:36:00 N 118:53:30 W"
• DA <radius>, <angle start>, <angle end>: Add an arc, angles in degrees, radius in nautical miles (set center using V X=...) example "DA 10,270,290"
• DB <coord1>, <coord2>: Add an arc, from coordinate1 to coordinate2 (set center using V X=...)
• DC <radius>: draw a circle (center taken from the previous V X=... record, radius in nautical miles. Example "DC 5"
• DY <coord>: Add a segment of an airway (NYI)

Sample

Creating Your Own Airspace File

• Figure out the dimensions that you want for your airspace. Is it a circle or a polygon or a mix of the two? What is the surface height and what is the ceiling height?
• Provided your object is not a circle map it out on Google Earth. You can then export that shape as a KML file. Open up the KML in a text editor of your choice, I prefer Notepad++
• Now create a new file and name it whatever you want. You can leave it as a TXT file or rename the extension to FAS. Enter the header information as described above.
• Now figure out how to draw your area using lines, circle and arcs from the drawing definitions. If the area is just a straight polygon it is easy to transpose the data from a KML to an FAS format by just massaging the data using "Search/Replace" functionality in a good text editor. The co-ordinates in the MKL are under the 'coordinates' tag and is made of a list of ,, with a space separating each set. Pull out that co-ordinate list and replace all spaces with newlines, remove the elevation which is usually just ',0'. Now reverse the order of , so it becomes , and add in the DP, N, W tags at the appropriate places and make sure everything lines up in nice columns now. If it is just a circle then figure out where the center is and what the radius is in nautical miles. If it's a combination then set your brain to engaged and use a combination of these techniques to achieve your desired shape. Look for examples of similar airspaces if you get stuck.
• Once done, I recommend testing your FAS file here
• If everything looks good then just append the contents of your FAS file into whatever airspace file you are currently using and reload that file on your device. See below for links to FAS files for all USA airspace.

Resources

• OpenAir Defintion
• FAS to Google Maps Tool
• Bulk Co-ordinate Conversion Tool
• Custom FAS Generator - Creates an FAS file in a selected region for known airspaces
• Pre-made USA Airspace files
• XCSoar Airspace Downloads