export registered point clouds

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Hello, I'm very naive to ROS so please forgive me :)

I've a .bag file with velodyne data with the following topics:

topics: /mavros/global_position/global 2951 msgs : sensor_msgs/NavSatFix
/mavros/global_position/local 2951 msgs : nav_msgs/Odometry
/mavros/imu/data 4997 msgs : sensor_msgs/Imu
/tf 38012 msgs : tf2_msgs/TFMessage
/velodyne_points 4688 msgs : sensor_msgs/PointCloud2

I simply want to export all the point clouds in a common reference system, which from my understanding should be done by transforming each point cloud by the transformation in /tf. So far I managed to export the point clouds in the local reference system by running:

rosrun pcl_ros bag_to_pcd myfile.bag /velodyne_points ./pointclouds

I feel it must be really trivial but I'm pretty much lost. Thanks in advance, Pawel