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Gmapping doesn't mark free spaces

in the documentation for Gmapping it is said :

The maximum range of the sensor. If regions with no obstacles within the range of the sensor should appear as free space in the map, set maxUrange < maximum range of the real sensor <= maxRange.

now my laser sensor has a maximum range of 30 meters so i set the maxRange parameter to 30.0 and maxUrange parameter to 29.0 but gammping still doesn't mark free spaces when obstacles are out of range .

in fact when the laser doesn't detect any obstacles in the range of 30 meters this 30 meters area is not marked as free and remains unknown. I changed maxUrange and maxRange to many different values but that didn't help.

I also visited here and [here] ( but they didn't help . there's also an open issue for this problem on GitHub repo for slam_gmapping below are the parameters I use for gmapping :

<?xml version="1.0"?>
      <arg name="robotname" default="robot1"/>
      <group ns="$(arg robotname)">
                <node   pkg="gmapping"   type="slam_gmapping" name="mapping">
                   <param name="map_frame" value="/map" />
                   <param name="base_frame" value="$(arg robotname)/base_link"/>
                   <param name="odom_frame" value="$(arg robotname)/odom" />
                   <remap from="scan" to="/$(arg robotname)/hokuyo" />
                   <param name="map_update_interval" value="0.1" />
                   <param name="delta" value="0.2 "/>
                   <param name="particle" value="5" />
                   <param name="iterations" value="1" />
                   <param name="maxUrange" value="29" />
                   <param name="maxRange" value="30" />
                   <param name="linearUpdate" value="0.1" />
                   <param name="xmin" value="-75" />
                   <param name="xmax" value="75" />
                   <param name="ymax" value="75" />
                   <param name="ymin" value="-75" />


I was wondering If anyone has a solution. btw I am using ros kinetic on Ubuntu 16.04