Problem with gmapping and tf topic

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Hi all, i have this problem using the gmapping node. I'm trying to use gmapping to read data from a laser scanner and build a map . I know that gmapping needs two transformations, the first from map to odom, and the last from odom to base_link (which represents the movements of the robot, it's odometry, right?). I also know, reading its wiki page, that the gmapping node subscribes the /laser_scan topic (data from laser scanner) and the /tf topic (transformations). My problem is that the gmapping node itself publishes the /tf topic with the transformation from map to odom. So, how can I give the odom to base_link transformation to gmapping, if the /tf topic is already published??

Thanks a lot for your help, Flavio