Measuring maps graphical density via digital image processing method on the example of city maps

During the centuries the main problem on mapping was to obtain the suffi cient and reliable source data; presently, an appropriate selection of the desired information from the deluge of available data is a problem. An availability of large amount of data induces to transfer the possibly rich information by means of map. It often results in overloading the cartographic documents, that is why they become less communica- tive and diffi cult to read. This situation is well illustrated by the example of city maps which are the most commonly used and thus the most frequently published cartographic products. Many user groups with different needs as well as preparation to read maps use these high volume publications. Therefore, the maps com- munication effectiveness problem is of particular importance. The city maps are the most complex cartographic presentations, because the presented areas are the places with the greatest concentration of different kinds of objects and forms of human activity arising from the civiliza- tion development. Conveying these specifi c features on the city maps leads to the problem of selecting the most relevant elements of content in terms of user's needs, since presenting all objects and their characteristics is impossible if the city map readability is to be kept. Although complexity has been the cartographers' object of interest for many years, because it exerts an im- pact on readability and effectiveness of cartographic documents, none of the measures used so far may be applied for automatic determination of complexity of such graphically complicated objects as city maps. Therefore a novel approach was needed for these applications. For that purpose digital image processing techniques have been proposed and successfully applied by the authors. The analysis of the spatial distribution of the objects' edges on the map surface, calculated using continuous wavelet transform, is the basis of the proposed measure. The method allows for comparison of complexity of city maps loaded by different type of graphical elements (point signatures, lines, text, etc.). Extended analyses of selected cartographic materials proved the usability of the method for quantitative estimation of city map complexity via formal index.