Image matching for historical maps comparison

Summary In cartographic heritage we suddenly find maps of the same mapmaker and of the same area, published in different years, or new editions due to integration of cartographic, such us in national cartographic series. These maps have the same projective system and the same cut, but they present very small differences. The manual comparison can be very difficult and with uncertain results, because it's easy to leave some particulars out. It is necessary to find an automatic procedure to compare these maps and a solution can be given by digital maps comparison. In the last years our experience in cartographic data processing was opted for find new tools for digital comparison and today solution is given by a new software, ACM (Automatic Correlation Map), which finds areas that are candidate to contain differences between two maps. ACM is based on image matching, a key component in almost any image analysis process.