When used to map the amplitude of the EEG or evoked potential at a specific point in time, interpolation between the voltages measured at the electrodes must be carried out to present a smooth contour on the map. The most practical interpolation method is the one based on spherical splines . Unlike magnetic resonance imaging and computed tomography, in which the intensity of every pixel is based on a measurement, topographic maps are derived from measurements at only 16 to 32 points, with the balance obtained via interpolation, creating the illusion of a higher resolution than actually exists.