FlightGear addon for geodesy things, like measuring distances, angles, areas, etc.
Install it like any other FlightGear addon - download this repository, either as ZIP archive (and unzip that) or with git clone, then add the resulting folder to the Addons list in the FGLauncher's Addons tab, or, if you are not using the launcher, add:
--addon=/path/to/addon/folder
to the command line, where /path/to/addon/folder is the path to the addon folder which contains this README.md file.
To use the addon in the simulator, hold the Alt key and click anywhere in the scenery. At each click, a tooltip in the main simulator window will display the coordinates of the clicked position, and the total distance of a line formed by the points you clicked, in the order you clicked. To clear the points and start measuring anew, press Alt+d.