Xplage For Mac

Xplage is a software utility that listens on an IP network for UDP datagrams representing the geographical coordinates of the simulated flight path of an instance of the X-Plane flight simulator, then transforms them into a series of continuously updated KML files that Google Earth can ingest to create "moving map" displays.

Xplage repeatedly generates two different KML files at a refresh interval specified by the user. The first is used by Google Earth to display an overhead map with North at the top and an icon in the center of the satellite image to indicate the position of the simulated aircraft.

In this view, the Google Earth "eye" is fixed directly above the airplane. Each refresh cycle shifts the map in the direction of the flight path so as to keep the icon centered. The second KML file is used by Google Earth to display a perspective view of the terrain ahead of the simulated aircraft.

In this view, the eye may be thought of as being right in front of the airplane and pointing slightly downward. Each refresh cycle repaints the perspective based on the simulated aircraft's roll and pitch orientation.

Note that because the refresh rates of Google Earth and xplage are asynchronous and independent, these moving map images lag slightly behind the flight simulation.

In effect, they represent still photo frames that capture the position of the aircraft a short time before they are displayed. This delay can be as great as the sum of the two refresh intervals.

In other words, if xplage is set to refresh at one-second intervals and Google Earth is set to refresh at two-second intervals, the moving map images may lag as much as three seconds behind the actual position of the simulated aircraft.

Delays in fetching satellite imagery through the public Internet from Google's servers may also contribute to the time lag.

Xplage is not a trivial program, but its logic is easy to understand once you grasp its structure. When it is executed, it parses its command line options and sets up a UDP socket to listen for incoming X-Plane datagrams.

Then the main routine launches three asynchronous lightweight child processes which respectively wait for an incoming datagram from the simulator, the end of the specified interval for updating the KML output files, or a manual interrupt from the user. When the user issues a request to the interrupt thread to exit the program, all four processes are simultaneously terminated.