Assessing the impact of coordinated use of Electric Vehicles (EVs) on power distribution grids is of basic importance for design of integrated transportation-energy management. In this paper, we address the assessment problem on how the charging operation of a large population of EVs impacts on spatial voltage profile of a power distribution grid. Different from the conventional space-discretization approach (the so-called power-flow equations), we introduce an alternative approach for the assessment based on a continuum representation of distribution voltage profile. The continuum representation explicitly keeps spatial (or geographical) information on a distribution grid and hence enables us to quantify the spatial impact of EV charging, for example, how far the charging operation of EV at a particular location affects the voltage profile. Here we demonstrate the approach by numerical simulations of a simple distribution grid that are incorporated with operational data on EVs in a demonstration project of EV-sharing system.