Numerical techniques are used to compare two-dimensional and three-dimensional analyses of the spreading of ionospheric electric fields. Specific cases are considered in which a source electric field is applied at 150-km altitude and within a limited horizontal distance L. It is found that horizontal spreading of fields at ionospheric heights can be accurately calculated by using a two-dimensional model with height-integrated conductivities, provided that L is 10 km or greater. For L<10 km a full three-dimensional model must be used. Three-dimensional calculations must also be used to relate electric fields measured at balloon altitudes (~30 km) to overhead ionospheric fields if L<100 km. Calculated ionospheric fields depend strongly on horizontal conductivity gradients, fields spreading more easily in the direction of decreasing conductivity.