A simulation is used to compare two different methods of generating maps of vertical total electron content (TEC) in the ionosphere for 24 days in 2001. The first method is the established thin shell, in which the ionosphere is represented by a thin shell at a fixed altitude. The second method uses a new four-dimensional inversion algorithm called Multi-Instrument Data Analysis System (MIDAS). The differences between the vertical TEC in the simulated ionosphere and that determined by each method provide a measure of the error in the vertical TEC maps. It was found that in general, the MIDAS method produced a threefold improvement in the accuracy of the determination of daytime TEC compared with the thin shell. At nighttime, the errors were much smaller and were similar for both methods.