  In Paper I we present a new model for the Galactic distribution of free
electrons. In this paper we describe the input data and methodology for
determining the structure and parameters of the model. Tables of the input data
are provided and several figures are used to demonstrate why particular
Galactic structures are needed. We identify lines of sight on which discrete
regions either enhance or diminish the dispersion or the scattering. Most do
not coincide with known \ion{H}{2} regions or supershells, most likely because
the enhancements correspond to column densities smaller than detection
thresholds for the emission measure in recombination-line surveys.
