Publishing a xform from /map to /base_link?

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Hey all -

Trying to get up to speed on the NavStack. Presently, when I run the launch file, I get the following error:

Waiting on transform from base_link to /map to become available before running costmap, tf error: Frame id /map does not exist!

Presently, my tf tree looks like this odom->base_link->base_laser. It makes sense that the tree should look like this: /map->odom->base_link->base_laser, however, I'm not sure A) who's responsibility it is to broadcast that transform and B) how to compute said transform. Any and all assistance would be greatly appreciated.