Raytrace not clearing the costmap

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Hi, I am using navigation stack for 2d navigation of UAV. I am using Hokuyo Lidar for obstacle detection. When the laser scan sees an obstacle it marks the points in costmap as obstacle. However if during the later scans if it doesnot find obstacle in the same spot it doesnt clear the obstacles from the costmap.

I havemarking: true, clearing: true, obstacle_range:2 and raytrace_range:20 in my costmap_common_params.yaml file. By these parameters I assume when no obstacles are seen by laserscan it shold clear area of 20m in front of it. But that doesnt happen. It clears around 1.5m area in front of it no matter what the raytrace_range is. Any insight as to what am I missing or what can i do will be very helpful.

I am using ROS Indigo.

Thanks. Please find below my costmap param files

costmap_common_params.yaml

obstacle_range: 2
raytrace_range: 20
footprint: [[-0.2,-0.2],[-0.2,0.2], [0.2, 0.2], [0.2,-0.2]]
#robot_radius: ir_of_robot
inflation_radius: 0.2

observation_sources: scan

scan: {sensor_frame: /laser, data_type: LaserScan, topic: /scan_filtered, marking: true, clearing: true}

global_costmap_params.yaml

global_costmap:
  global_frame: /map
  robot_base_frame: /base_link
  update_frequency: 1.0
  publish_frequency: 0.5
  static_map: false
  transform_tolerance: 5.0
  static_map: false
  rolling_window: true
  width: 50.0
  height: 50.0

local_costmap_params.yaml

local_costmap:
  global_frame: /map
  robot_base_frame: /base_link
  update_frequency: 50.0
  publish_frequency: 50.0
  static_map: false
  rolling_window: true
  width: 8.0
  height: 8.0
  resolution: 0.1
  transform_tolerance: 5.0
  planner_frequency: 5.0
  planner_patience: 5.0