Accessing 3D map data saved in bt file/formats to save octomap

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bt ("Bonsai Tree") files are a very compact binary serialized representation of OctoMap octree, and thus not directly human readable.

You can easily write a program that reads in the .bt, creates an OcTree from it and then traverse the full tree with an octree leaf_iterator to do whatever you want with the coordinates or voxels. For details see the doxygen documentation of OctoMap: http://octomap.sourceforge.net/doxygen/index.html

You can also have a look at the file "bt2vrml.cpp" in your octomap package. It reads in a .bt file and creates a VRML file of the occupied voxels (which will result in a quite large file).