ABSTRACT
All planar graphs can be embedded in a two-dimensional space and can be represented as spatial networks but the converse is not necessarily true: there are some spatial and non-planar graphs. In general, however, most spatial networks are, to a good approximation, planar graphs such as road or railway networks. But there are some important exceptions such as maritime networks, or the airline network. For many infrastructure networks, however, planarity is unavoidable. Power grids, roads, rail and other transportation networks are good approximation planar networks. For many applications, planar spatial networks are the most important and most studies have focused on these examples. Spatial networks were actually the subject, a long time ago, of many studies in quantitative geography. The chapter also discusses some tools which can be helpful to characterize some aspects of spatial networks and how people distinguishes them from generic, non-spatial complex networks.
