Mention has already been made of the lines that often constitute the objects of spatial data. They include streets, roads, railroads, and highways; rivers, canals, and streams; and the more abstract interpersonal networks studied by sociologists. Networks constitute one-dimensional structures embedded in two or three dimensions. Discrete point objects may be distributed on the network, representing such phenomena as landmarks or bridges on road networks, or observation points on rivers. Discrete line objects may also be found, in the form of tunnels, stretches of highway with a constant number of lanes, or river reaches. Continuous fields may also be distributed over the one-dimensional network, defining such variables as travel speed, railroad gradient, traffic density, or stream velocity. Mathematically, a network forms a graph, and many techniques developed for graphs have applications to networks. These include various ways of measuring a network’s connectivity, or of finding shortest paths between pairs of points on a network. These methods are explored in detail in Chapter 7, Network and Location Analysis.