How to use GetMap-Service by gmapping

edit

I'm using gmapping to create a map. To this point everything works fine.

Now I want to call the GetMap-Service to receive the map for further investigation:

#include <nav_msgs/GetMap.h>

ros::ServiceClient client = n.serviceClient<nav_msgs::GetMap>("map");
nav_msgs::GetMap srv;

  if (client.call(srv))
  {
    ROS_INFO("Service GetMap succeeded.");
  }
  else
  {
    ROS_ERROR("Service GetMap failed.");
    return 1;
  }

But when running this node I always get "Service GetMap failed." I added gmapping to "find packages" in CMakefile and to build & runtime dependencies in package xml.

What's my fault?