[ros-users] Converting PointCloud to LaserScan

Hello,
i want to convert PointCloud to LaserScan.
I have a Topic wich publishs point cloud.
if i try to make a map like this:

rosrun gmapping slam_gmapping scan:=point_cloud

i become a warning:
[WARN] 1297772305.065463: Could not process inbound connection: topic types do not match: [sensor_msgs/LaserScan] vs. [sensor_msgs/PointCloud]{'topic': '/point_cloud', 'tcp_nodelay': '0', 'md5sum': '90c7ef2dc6895d81024acba2ac42f369', 'type': 'sensor_msgs/LaserScan', 'callerid': '/slam_gmapping'}

So now i want convert pointcloud to laserscan.
Or if there is another way to make a map, with pointcloud?

Greetings,
Bünyamin