Study of Visual Clutter in Geographic Node-Link Diagrams

An increasing amount of geographic data is now freely available on the Internet, and this number is expected to increase as monitoring systems and sensors are becoming a ubiquitous part of our environment. A substantial subset is structured as networks (or graphs). Notable examples are import/export of goods, networks of climate stations, flight connections, migration flows and Internet traffic. To fully comprehend such networks, it is needed to know both their geographic and relational information. For this purpose, the most common visual representation is the node-link diagram, where vertices are depicted as points and the edges connecting them are drawn as lines. This Thesis focuses on an instance of node-link diagrams, where nodes are over-imposed on a map and fixed according to their geographical information. The major issue of this visualization is the visual clutter of nodes or edges, defined in literature as ``the state in which excess items, or their representation or organization, lead to a degradation of performance at some task''. In particular, nodes and links may cause occlusion and ambiguity in the graph representation. Such problems characterize a cluttered diagram because they reduce the potential usefulness of the visualization. The goal of this thesis is to advance the state of the art in Graph Visualization with respect to visual clutter. On the theoretical perspective, our goal is to acquire a deep understanding of existing approaches for visualizing geographic networks and for reducing visual clutter. Initially, we provide a classification of geographic node-link diagrams and a survey on techniques to reduce the visual clutter on such visualizations. Afterwards, we present a schematization of techniques that helps the reader to decide, given a task and a geographical node-link diagram, which are the candidate solutions that help to reduce the visual clutter. On the practical perspective, our goal is the development of visualization and interaction techniques to overcome various issues of the state-of-the-art approaches. On the one hand, we present an interactive lens that faces the issues of links organization over a map. On the other hand, we describe a deformation-based technique that reveals nodes and links otherwise hidden behind the globe surface. Finally, we introduce a method that automatically generates flow map layouts starting from multivariate geographical datasets.