how to get the resolution of a map from the map_server

edit

You can read resolution with map data.

nav_msgs::OccupancyGrid message has resolution variable inside MapMetaData

I found this method much easier to get the parameter value from the server

 double map_resolution;
  // **rosparam list** < use this command to see all the parameters
  if (ros::param::get("/move_base/global_costmap/resolution", map_resolution))
  {
    ROS_INFO("map resolution: %f",map_resolution);
  }
  else
      ROS_INFO("couldn't get map resolution");