Terrain Detection

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In Flightgear 2017.2.1 a new Nasal method was introduced by Richard Harrison.

It works like geo.elevation but in a more general way such that one would define an arbitrary line and the method will return any terrain detection along that line.


Returns where the given position in the specified direction will intersect with the ground. Returns whether or not a certain position and direction pair intersect with the ground, and if so the intersection point.

Useful for radars, terrain avoidance (GPWS), etc.

Input parameters: (geocentric coordinate system)

1. hash {"x","y","z"} position

2. hash {"x","y","z"} direction

Returns nil or geod hash {lat:deg,lon:deg,elevation:Meters} intersection

Time till impact with terrain

This example could be used in terrain detection systems that warn about impending collision with terrain.

if (getprop("velocities/speed-east-fps") != 0 or getprop("velocities/speed-north-fps") != 0) {
      var start = geo.aircraft_position();
      var speed_down_fps  = getprop("velocities/speed-down-fps");
      var speed_east_fps  = getprop("velocities/speed-east-fps");
      var speed_north_fps = getprop("velocities/speed-north-fps");
      var speed_horz_fps  = math.sqrt((speed_east_fps*speed_east_fps)+(speed_north_fps*speed_north_fps));
      var speed_fps       = math.sqrt((speed_horz_fps*speed_horz_fps)+(speed_down_fps*speed_down_fps));
      var heading = 0;
      if (speed_north_fps >= 0) {
        heading -= math.acos(speed_east_fps/speed_horz_fps)*R2D - 90;
      } else {
        heading -= -math.acos(speed_east_fps/speed_horz_fps)*R2D - 90;
      heading = geo.normdeg(heading);

      var end = geo.Coord.new(start);
      end.apply_course_distance(heading, speed_horz_fps*FT2M);

      var dir_x = end.x()-start.x();
      var dir_y = end.y()-start.y();
      var dir_z = end.z()-start.z();
      var xyz = {"x":start.x(),  "y":start.y(),  "z":start.z()};
      var dir = {"x":dir_x,      "y":dir_y,      "z":dir_z};

      var geod = get_cart_ground_intersection(xyz, dir);
      if (geod != nil) {
        end.set_latlon(geod.lat, geod.lon, geod.elevation);
        var dist = start.direct_distance_to(end)*M2FT;
        var time = dist / speed_fps;
        setprop("/sim/model/radar/time-until-impact", time);
      } else {
        setprop("/sim/model/radar/time-until-impact", -1);

Check for terrain between 2 aircraft

This example is typically for radar use.

var myPos = geo.aircraft_position();
var otherPos = geo.aircraft_position().apply_course_distance(100, 20000);# another plane 20 Km out, bearing (true) 100

# Check for terrain between own aircraft and other:
v = get_cart_ground_intersection({"x":myPos.x(), "y":myPos.y(), "z":myPos.z()}, {"x":otherPos.x()-myPos.x(), "y":otherPos.y()-myPos.y(), "z":otherPos.z()-myPos.z()});
if (v == nil) {
 print("No terrain, planes has clear view of each other");
} else {
 var terrain = geo.Coord.new();
 var maxDist = myPos.direct_distance_to(otherPos);
 var terrainDist = myPos.direct_distance_to(terrain);
 if (terrainDist < maxDist) {
   print("terrain found between the planes");
 } else {
  print("The planes has clear view of each other");