Sub-Gauss magnetic fields are crucial for many physical processes but hard to measure. In Zhang et al. 2019 (arXiv:1903.08675), we report the first observational results that measure 3D topology as well as the strength of a sub-milliGauss magnetic field simultaneously beyond our solar system. This is the first observational proof in line with the decade-long theoretically-proposed magnetic tracer ground state alignment (GSA). Our analysis has enhanced a 5-digit precision comparing to the previous observational limit set by the non-detection Zeeman splitting. We anticipate our observation to be the starting point of the revolution of magnetic field mapping in the universe, where much more information and higher precision can be achieved with current observational facility if accounting for GSA. The potentials that GSA has shown, including abundant observables shown in our work as well as the possibility of multi-waveband observation from UV to submillimeter fine-structure lines, will provide us observational access to the sub-Gauss field from our Galaxy to the early universe.