Is it possible to convert 3D octomap (bt/ot) to a 2D costmap (png)?

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I'm running Melodic on Ubuntu 18.04 on x86-64 (kernel version is 4.15.0-43-generic).

I am able to save octomaps as a bt/ot file (planning to use a Velodyne Puck, but I am simply using Gazebo/Rviz for simulation atm). I am trying to get a costmap that indicates the locations of obstacles though. I was curious as to whether there was a simple way to get a 2D representation of a 0-255 range costmap. If there is a better way than using octomaps, then I would appreciate knowing as well.

I need the costmap to be in a greyscale, 2D format due to processing later with another program. If it's possible to associate a point in the map with a GPS coordinate, that would be amazing.

Thanks in advance.