Subscribe to quaternion

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Hi I am trying to subscribe to a quaternion being published by an nxt compass and using it for odometry orientation. So far the code doesn't work and I am wondering what I've done wrong. Thanks.

#include <ros/ros.h>
#include <tf/transform_broadcaster.h>
#include <nav_msgs/Odometry.h>
#include <nxt_msgs/Compass.h>
#include <boost/assign/list_of.hpp>
#include <math.h>
#include "std_msgs/String.h"

void chatterCallback(const std_msgs::String::ConstPtr& msg)
 {
  ROS_INFO("I heard: [%s]", msg->data.c_str());
 }



int main(int argc, char** argv) {
  ros::init(argc, argv, "odometry_publisher");

  ros::NodeHandle n;
  ros::Publisher odom_pub = n.advertise<nav_msgs::Odometry>("odom", 50);
  tf::TransformBroadcaster odom_broadcaster;

  double x = 0.0;
  double y = 0.0;
  double th = 0.0;

  double vx = 0.1;
  double vy = -0.1;
  double vth = 0.1;


  ros::Time current_time, last_time;
  current_time = ros::Time::now();
  last_time = ros::Time::now();



  ros::Subscriber sub = n.subscribe<nxt_msgs::Compass>("compass", 1000, chatterCallback);



  ros::Rate r(30.0);
  while(n.ok()){

    ros::spinOnce();               // check for incoming messages
    current_time = ros::Time::now();

    //compute odometry in a typical way given the velocities of the robot
    double dt = (current_time - last_time).toSec();
    double delta_x = (vx * cos(th) - vy * sin(th)) * dt;
    double delta_y = (vx * sin(th) + vy * cos(th)) * dt;
    double compass_yaw = atan (compass_angle);

    double compass_angle = 90;
    x += delta_x;
    y += delta_y;

    //std::cout << "x=" << x << " y=" << y << std::endl;
    //since all odometry is 6DOF we'll need a quaternion created from yaw
    geometry_msgs::Quaternion odom_quat = sub;

    //first, we'll publish the transform over tf
    geometry_msgs::TransformStamped odom_trans;
    odom_trans.header.stamp = last_time;//current_time;
    odom_trans.header.frame_id = "odom";
    odom_trans.child_frame_id = "base_link";

    odom_trans.transform.translation.x = x;
    odom_trans.transform.translation.y = y;
    odom_trans.transform.translation.z = 0.0;
    odom_trans.transform.rotation = odom_quat;

    //send the transform
    odom_broadcaster.sendTransform(odom_trans);

    //next, we'll publish the odometry message over ROS
    nav_msgs::Odometry odom;
    odom.header.stamp = current_time;
    odom.header.frame_id = "odom";

    //set the position
    odom.pose.pose.position.x = x;
    odom.pose.pose.position.y = y;
    odom.pose.pose.position.z = 0.0;
    odom.pose.pose.orientation = odom_quat;

    odom.pose.covariance =  boost::assign::list_of(1e-3) (0)   (0)  (0)  (0)  (0)
      (0) (1e-3)  (0)  (0)  (0)  (0)
      (0)   (0)  (1e6) (0)  (0)  (0)
      (0)   (0)   (0) (1e6) (0)  (0)
      (0)   (0)   (0)  (0) (1e6) (0)
      (0)   (0)   (0)  (0)  (0)  (1e3) ; 

    //set the velocity
    odom.child_frame_id = "base_link";
    odom.twist.twist.linear.x = vx;
    odom.twist.twist.linear.y = vy;
    odom.twist.twist.angular.z = vth;

    odom.twist.covariance =  boost::assign::list_of(1e-3) (0)   (0)  (0)  (0)  (0)
      (0) (1e-3)  (0)  (0)  (0)  (0)
      (0)   (0)  (1e6) (0)  (0)  (0)
      (0)   (0)   (0) (1e6) (0)  (0)
      (0)   (0)   (0)  (0) (1e6) (0)
      (0)   (0)   (0)  (0)  (0)  (1e3) ; 

    //publish the message
    odom_pub.publish(odom);
    //std::cout << "published: [" << last_time << " " << current_time << " " << dt << "]" << std::endl;
    last_time = current_time;
    r.sleep();
  }
}

Compile error:

In function ‘int main(int, char**)’:
/opt/ros/groovy/share/fakeOdometry/src/main.cpp:60:43: error: conversion from ‘ros::Subscriber’ to non-scalar type ‘geometry_msgs::Quaternion’ requested
make[2]: *** [CMakeFiles/fake_odometry.dir/src/main.cpp ...