uses
  Math;

procedure MidPoint(Lat1, Lon1, Lat2, Lon2: Double; out Lat3, Lon3: Double);
var
  Bx, By, DLon: Double;
begin
  DLon := DegToRad(Lon2 - Lon1);

  Lat1 := DegToRad(Lat1);
  Lat2 := DegToRad(Lat2);
  Lon1 := DegToRad(Lon1);

  Bx := Cos(Lat2) * Cos(DLon);
  By := Cos(Lat2) * Sin(DLon);
  Lat3 := RadToDeg(ArcTan2(Sin(Lat1) + Sin(Lat2), Sqrt((Cos(Lat1) + Bx) * (Cos(Lat1) + Bx) + By * By)));
  Lon3 := RadToDeg(Lon1 + ArcTan2(By, Cos(Lat1) + Bx));
end;