  We apply Velocity Channel Gradients (VChGs) and Reduced Velocity Centroids
Gradients (RVCGs) to H I data (the LAB survey), obtaining the plane-of-sky
component (POS) of the magnetic field as a function of the relative velocity.
Assuming a galactic rotation curve, we transformed the relative velocities to
distances, constructing the first map of the POS magnetic field at every point
of the celestial sphere as a function of the distance. To test the accuracy of
our 3D distribution, we used a set of stars with known distances from the
stellar polarization catalog. We compared the polarization directions that we
obtain with the VChGs and RVCGs against the starlight polarization directions.
We find a good correspondence between the derived magnetic field and measured
polarization directions, which testifies to the accuracy of this new way of
probing the 3D galactic POS magnetic field structure.
