Gmapping with V-REP

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Hello,

I'm doing a course project whose goal is to build a SLAM simulation with ROS and V-REP.

I'm new to ROS and V-REP and I'd like some help from you guys. Here is what I did:

1. I created a scene with a p3dx robot in V-REP.
2. I publish Twist message on ROS to control the motion of the robot in V-REP.
3. V-REP is publishing 3 frames to ROS: "odom": the fixed world coordinate; "base_link": the robot coordinate; "laser": the laser sensor coordinate. Also, V-REP is publishing laserScan data as well.
4. I run gmapping node, subscribing to the LaserScan data.
5. I run RVIZ and changed the fixed frame to /map, so that I can visualize the ongoing mapping while the robot moves in V-REP.

Here comes the question, I'm getting a error message for the global state in RVIZ: "Fixed Frame [map] does not exist" How should i fix this? I assume that I need to define a new frame corresponding to "map" but how am I supposed to do it? Just create a new coordinate and add a tf between "map" and "odom"?