A nuclear bomb detonated above the earth's surface can cause a high-altitude electromagnetic pulse (HEMP). HEMPs create an electric field at the earth's surface, which induces unwanted slowly varying dc currents, called geomagnetically induced currents (GIC) on transmission lines. The magnitude of the electric field highly depends on the conductivity of the earth, hundreds of thousands of meters below the surface. The earth's conductivity is very complex and there exist different models to represent it. HEMP electric fields are commonly evaluated using a simple model called the uniform model, which models the earth using a single value of conductivity. This thesis describes a method to convert HEMP electric fields under a more detailed conductivity model, called the 1D model, which is based on geological surveys and includes regional variations. Using the 1D model enables locationally dependent simulations of HEMP electric fields, yielding more realistic results. This methodology has been automated by a tool, created with MATLAB, and was applied to several publicly available HEMP electric field waveforms at different locations across the continental United States. These electric fields are analyzed by comparing their magnitudes and their impact on a 10,000-bus synthetic grid. The results show the extent that HEMP electric field magnitudes can vary from region to region. Also, evaluations of different HEMP electric field waveforms show that each waveform may have characteristics that impact the grid differently. Based on the analysis performed in this thesis, it is recommended that comprehensive HEMP vulnerability studies utilize multiple worst-case HEMP waveforms while considering regional differences in earth conductivity.