Answer by tfoote for Hi,I have question, on odom and base_footprint, especially when the robot is falling down or 4-legged walking in REP 120. I assume that humanoid robot did not know about the current waist position, but can estimate orientation from IMU sensors.does odom correspond to the sensor based orientation and world reference based position of the waist?does base_footprint respect the sensor based orientation and model based calculation of the height of the waist?if it is ok, the question ishow do you estimate the position of the odom, from the walking controllers? does that include any sensor data?does base_footprint does not reflect any sensor data? what's happens if the robot falling down or 4-legged walking mode?

1. odom is an inertial frame defined in [REP 105](http://www.ros.org/reps/rep-0105.html) It can be initialized anywhere in any orientation, but for ease of use usually it's x forward y left z up under the robot's initial position. 2. Nominally the base_footprint is the projection of the robot's shape into a 2D form on the ground. Usually this is approximated as a set of primitives (sometimes one) in the ground plane directly under the base_link in the XY plane. 3. You can/should use any/all sensors to compute the best odometric updates for your robot. They will provide the transform from odom -> base_link. 4. When you consider a robot which is tall but might change to a recumbent position the footprint extend to make sure to remain a bounding box if using the footprint for navigation. Note the coordinate frame will not really update as it will still be the projection of the base_link (recommended attached to the waist) into the XY plane.