Global MHD models, ring-current models, convection models, and a growing number of coupled models calculate the large-scale currents in the magnetosphere-ionosphere (MI) system. Traditionally, these models have been tested against single-point magnetic field measurements made by individual spacecraft. This often does not provide a clear picture of the pattern of model-data discrepancies. The global network of ground-based magnetograms constitutes a large source of data that is currently being underutilized for validation and analysis of these models. This is primarily due to the fact that a good code does not exist that accurately makes the connection between the models' output and ground magnetometer data. To address this, a numerical code has been developed to compute realistic ground magnetic field perturbations, based upon input from these models, and is described in this thesis. The code includes the effects of all large-scale current systems in the MI system. It also computer the effects from the inner magnetosphere currents by integrating the Biot-Savart integral over a spherical grid centered on the Earth. A scalar potential representation is used for the magnetic field due to the currents in the outer magnetosphere and solar wind. The effects of the ground induction currents are also included with a scalar potential by including the effects of an infinitely conducting sphere below the surface. Aside from the code's ability to compute realistic magnetic field disturbances at individual magnetometers, it is possible to compile synthetic geomagnetic indices and synthetic global disturbance maps. The software is intended as a general-use tool and was built to be flexible so that it can be integrated with a wide variety of large-scale MI coupling models. Initial synthetic magnetogram results are presented based on SWMF and RCM storm simulations. Synthetic Dst was computed for the SWMF simulation results, while synthetic magnetograms, several geomagnetic indices, and an LT-UT map were computed for the RCM simulation results. The magnetogram output is compared to observations to analyze both MI coupling codes.