Not being able to calculate the height in ardrone gazebo simulator

edit

Hey guys, I am relatively new to ros and c++, i am trying to implement pid controller on ardrone using ros indgio. for doing this I want to calculate(print) the height at which the quadcopter is flying. when I am doing rostopic echo "ardrone_autonomy/Navdata" , i can read the height but when I tried to subscribe to the topic to print the height, I am getting blank in the terminal. following is my code

#include "ros/ros.h"
#include "std_msgs/Empty.h"
#include "ardrone_autonomy/Navdata.h"
#include <iostream> 

using namespace std; 

class trajectory{
public:
  void height(const ardrone_autonomy::Navdata::ConstPtr& hmsg)
  { 
    double x = hmsg->altd; `    
   (ROS_INFO("HEIGHT IS : %f",x); 
  }
}; 

int main(int argc, char **argv)`
{ 
  ros::init(argc, argv, "pid"); 
  ros::NodeHandle n; 
  ros::Publisher takeoff_pub = n.advertise<std_msgs::Empty>("/ardrone/takeoff", 10);
  std_msgs::Empty msg;
  ros::Duration(0.5).sleep();
  takeoff_pub.publish(msg); 
  trajectory traj;
  ros::Duration(0.3).sleep();
  ros::Subscriber sub = n.subscribe("/ardrone_autonomy/Navdata",100,&trajectory::height,&traj);` 
  ros::spin();
  return 0; 
}

Thank you