This paper provides a methodology to examine the impact of transmission constraints on the efficient operation of large scale power markets. The Northeast USA is presented as a case study. A system model was first constructed using the publicly available US Federal Energy Regulatory Commission (FERC) Form 715 filings to provide a detailed representation of the transmission system. FERC Form 1 data and information from the US Energy Information Administration's (EIA) National Energy Modeling System model were used to represent generator costs. An optimal power flow (OPF) was then used to optimally dispatch a large system consisting of the New England Region (NEPOOL), New York (NYPP), and the NERC MAAC and ECAR regions, both under base case and modified conditions. Using the OPF results, the costs associated with transmission constraints are determined. Finally, given the large amount of data generated by these studies, methods for the efficient visualization of the results are also discussed.