MapAnalyst is a new software application for the visualization and study of the planimetric accuracy of old maps. It illustrates local map distortion by generating distortion grids, displacement vectors, and new isolines of scale and rotation. MapAnalyst additionally computes the old map's scale and rotation, as well as statistical indicators summarizing the map's geometric accuracy. It offers a user- friendly interface and is freely available for all major computer platforms at http:// mapanalyst.cartography.ch/. Map historians are invited to use MapAnalyst, and are encouraged to consult and improve the free Java source-code. This article describes the steps leading to visualizations of a map's planimetric accuracy. It provides basic algorithmic information that is necessary for the understanding and correct interpretation of displacement vectors and distortion grids. It also introduces isolines of equal scale and rotation, a new type of accuracy visualization. The last section interprets sample visualizations for an eighteenth century map.