Adopting non-contact electric coupling for voltage monitoring avoids electrical connection to high-voltage transmission lines, making installation of the sensing platforms easier and enabling wide-area deployment. However, coupled transformation matrix is difficult to determine in practice as the exact spatial positions of the transmission lines are typically unknown and dynamic. This problem can be overcome by integrating magnetic field sensing with stochastic optimization algorithm. In this technique, magnetic signals emanated from the overhead transmission lines are measured by magnetic sensor arrays to determine the coupling coefficients so that the high voltage of the transmission lines can be deduced from the induced voltage in the copper bars on the ground. This proposed technique can be potentially implemented with low-cost copper induction bars and magnetoresistive (MR) sensors. Thus sectional and wide-area deployment may become possible and Ferranti effect and travelling lightning waves can then be monitored. Its enhanced sensing ability over potential transformers (PTs) can largely improve transient-fault identification and save cost for transmission-network inspection.