Cartograms are proving increasingly popular, but relatively little is known about the problems related to their use. A test program measured the performance of relatively inexperienced map users in identifying locations on a topological cartogram and on the geographic map from which it was derived. The spatial pattern of errors and the variation in task difficulty showed the dominant influence of two map transformation characteristics: change in angular location with respect to other units and change in unit shape. Also, it took significantly longer to perform the task on the map than on the cartogram.