Very Low Frequency (VLF, 3 – 30 kHz) waves propagate long distances in the waveguide formed by the Earth and the lower ionosphere. External sources such as solar flares and lightning discharges perturb the upper waveguide boundary and thereby modify the waves propagating within it. Therefore, studying the propagation of VLF waves within the waveguide enables us to probe the ionospheric response to external forcing. However, the wave propagation also depends on the lower waveguide boundary property, i.e. the ground conductivity. We tackle two main questions: how accurate should the path ground-conductivity description be to obtain a given accuracy on the ionospheric electron density? Are the currently-available ground conductivity maps accurate enough ? The impact of the ground conductivity values and their spatial extension on VLF-wave propagation is studied through modeling with the Longwave Mode Propagator (LMP) code. First, we show that knowledge of the ground conductivity value should be more accurate as the ground conductivity decreases, in particular in regions where σ ‹ 10-3 S/m. Second, we find that wave propagation is strongly sensitive to the spatial extension of ground-conductivity path segments: segments of down to 10 km should be included in the path description when 10% accuracy is required on the estimate of the electron density. These results highlight the need for an update of the ground conductivity maps, to get better spatial resolution, more accurate values, and an estimate of the time-variability of each region.