TF error in topological roadmap [closed]

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I am running topological navigation (Revision: 54057) on ROS Diamondback. On launching move_base_topo_stage.launch in topological_roadmap package, a lot of TF errors similar to following appears:

[ERROR] [1332863239.641246114, 733.000000000]: Lookup would require extrapolation into the past.  Requested time 732.100000000 but the earliest data is at time 733.000000000, when looking up transform from frame [/grid6] to frame [/grid42]
[ERROR] [1332863239.955981566, 733.000000000]: TF Exception that should never happen for sensor frame: , cloud frame: base_laser_link, Lookup would require extrapolation into the past.  Requested time 732.200000000 but the earliest data is at time 733.000000000, when looking up transform from frame [/base_laser_link] to frame [/grid42]
[ERROR] [1332863239.956976386, 733.000000000]: TF Exception that should never happen for sensor frame: , cloud frame: base_laser_link, Lookup would require extrapolation into the past.  Requested time 732.300000000 but the earliest data is at time 733.000000000, when looking up transform from frame [/base_laser_link] to frame [/grid42]
[ERROR] [1332863239.957876360, 733.000000000]: TF Exception that should never happen for sensor frame: , cloud frame: base_laser_link, Lookup would require extrapolation into the past.  Requested time 732.400000000 but the earliest data is at time 733.000000000, when looking up transform from frame [/base_laser_link] to frame [/grid42]
[ERROR] [1332863239.958767605, 733.000000000]: TF Exception that should never happen for sensor frame: , cloud frame: base_laser_link, Lookup would require extrapolation into the past.  Requested time 732.500000000 but the earliest data is at time 733.000000000, when looking up transform from frame [/base_laser_link] to frame [/grid42]
[ERROR] [1332863239.969647689, 733.100000000]: TF Exception that should never happen for sensor frame: , cloud frame: base_laser_link, Lookup would require extrapolation into the past.  Requested time 732.600000000 but the earliest data is at time 733.000000000, when looking up transform from frame [/base_laser_link] to frame [/grid42]

what could be the cause of these? What could be solution? Or I can just ignore them (hoping that rejection of grid points will not change the map much!)

Thank you prince