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Usually, that kind of algorithms rely on TF to transform incoming sensor data into a global frame.

As you probably know, all your sensor data are supposed to carry a frame_id and a time stamp, which are used by TF to perform adequate transformations.

Checking the documentation of http://www.ros.org/wiki/planning_environment, you can find the following:

~global_frame (string, default: "") If the global frame is specified to be the empty string, the root frame in which all monitoring will take place is the root link of the robot ("base_link" for the robot). Otherwise, this parameter can be set to a fixed frame in the world (e.g. "odom_combined", "map")

so in the end, it is up to you to chose what frame is to be used as reference.

Raph