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As you may have seen, the documentation for global_planner states:

~<name>/allow_unknown (bool, default: true)
    Specifies whether or not to allow the planner to create plans that traverse unknown space. NOTE: if you are using a layered costmap_2d costmap with a voxel or obstacle layer, you must also set the track_unknown_space param for that layer to be true, or it will convert all your unknown space to free space (which planner will then happily go right through)

What you may have missed is that your cost map does contain layers, even when defined with pre-Hydro parameters. From the costmap_2d wiki:

Pre-Hydro Parameters

Hydro and later releases use plugins for all costmap_2d layers. If you don't provide a plugins parameter then the initialization code will assume that your configuration is pre-Hydro and will load a default set of plugins with default namespaces. Your parameters will be moved to the new namespaces automagically. The default namespaces are static_layer, obstacle_layer and inflation_layer. Some tutorials (and books) still refer to pre-Hydro parameters, so pay close attention. To be safe, be sure to provide a plugins parameter.

Therefore, it seems you'll still need to set track_unknown_space to true in the obstacle_layer namespace.