How to use geographic_msgs?

edit

Hi,

I'm trying to write a little client to access osm maps. What I did so far is:

ros::init(argc, argv, "Geo");
ros::NodeHandle n;
ros::ServiceClient client = n.serviceClient<geographic_msgs::GetGeographicMap>("geographic_msgs");
geographic_msgs::GetGeographicMap srv;

srv.request.url = "http://openstreetmap.org/node/123456";
if(client.call(srv))
{
     ROS_INFO("connected");
}
else
{
     ROS_ERROR("failed");
}

If I run the programm I always get "failed" as an output. Could anyone tell me how to connect to the maps correctly?

Thanks in advance, Chris