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Robots using URDF rarely follow DH conventions -- and a number or ROS packages (calibration, for sure) don't necessarily work with all the extra fixed rotations that are needed to make URDF follow DH.

For the PR2, and a number of other robots, the convention has been that "when the joints are at their 0.0 position, all of the TF frames will line up parallel to the base_link orientation". Thus, if you put the PR2 arm out forward and level with the ground, with all the roll joints zeroed, all the links will be red (X) forward, blue (Z) up.