Applying a newly developed tangent-circle method (TCM), we derive a volume density map of H?i gas in the inner Galaxy as a function of galactocentric distance R and height Z. The H?i hole around the Galactic Center (GC) is shown to have a crater-shaped wall, which coincides with the brightest ridge of the North Polar Spur and emanates from the 3 kpc expanding ring. The crater structure is explained by sweeping of the halo gas by a shock-wave from the GC. The unperturbed H?i halo outside 3 kpc is shown to be in hydrostatic equilibrium, obeying the sech2Z/h density law with a scale height h ～ 450?pc.
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