How to subscribe a new costmap2dROS object to the global_costmap/costmap

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Hi all,

I am planning to write my own node in ROS and I need to access the global_costmap which is being published. For that, I have created a new Costmap2DROS object:

int main(int argc, char** argv){

    ros::init(argc,argv,"publish_multiple_goals");
    ros::NodeHandle n;

    tf::TransformListener tf_(ros::Duration(10));

    costmap_ros_ = new costmap_2d::Costmap2DROS("my_costmap", tf_);
    costmap_ros_->start();

    costmap_ = costmap_ros_->getCostmap();

    return 0;
}

Now, I have a my_costmap_params.yaml file where I have specified the map_topic : /move_base_node/global_costmap/costmap (this is being published by move_base) so that the new costmap2DROS object subscribes to it:

my_costmap:
  #Set the global and robot frames for the costmap
  global_frame: /map
  robot_base_frame: /base_link

  #Set the update and publish frequency of the costmap
  update_frequency: 2.0
  publish_frequency: 2.0

  #We'll use a map served by the map_server to initialize this costmap
  static_map: true
  rolling_window: false

  use_dijkstra: false

  plugins:
  - {name: static_layer, type: "costmap_2d::StaticLayer"}
  - {name: obstacle_layer, type: "costmap_2d::ObstacleLayer"}
  - {name: inflation_layer, type: "costmap_2d::InflationLayer"}

  static_layer:
    map_topic: /move_base_node/global_costmap/costmap

  #Configuration for the sensors that the costmap will use to update a map
  obstacle_layer:
    observation_sources: base_scan
    base_scan: {data_type: LaserScan, sensor_frame: /base_laser_link_1, topic: /base_scan, expected_update_rate: 0.4,
      observation_persistence: 0.0, marking: true, clearing: true, max_obstacle_height: 0.4, min_obstacle_height: 0.0}

I am loading the above parameters to parameter_server here:

publish_multiple.xml

<launch>
 <node pkg="publish_multiple_goals" type="publish_multiple_goals" respawn="false" name="publish_multiple_goals" output="screen">
    <rosparam file="$(find drsim_control)/move_base_config/my_costmap_params.yaml" command="load" />
  </node>
</launch>

Now the problem is it still subscribes to the "map" topic published by the map_server and does not subscribe to the map_topic I mentioned above that is /move_base_node/global_costmap/costmap using the costmap2DROS object I created above. I want to make sure that the costmap2DROS object created above subscribes to the map_topic to get the global_costmap. Does anyone have any idea what is going wrong here or what can I do to achieve the above mentioned task? Let me know if you need more information from me.

Update:
I think I am able to subscribe to the global_costmap but still when I use functions like: costmap_ros_->getRobotFootprint() , it gives me wrong readings (when I use this function in the global planner plugin, it gives me the correct values). Also, the Cost values of the costmap is either 0 or 254. Shouldn't the inflation be there according to http://wiki.ros.org/costmap_2d#Inflation ? Any help will be appreciated.

Thanks in advance.
Naman Kumar