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All geographic information systems (GIS) are built using formal models that describe how things are located in space. A formal model is an abstract and well-defined system of concepts. It defines the vocabulary that we can use to describe and reason aboutthings. A geographic data model defines the vocabulary for describing and reasoning about the things that are located on the earth. Geographic data models serve as the foundation on which all geographic information systems are built. We are all familiar with one model for geographic information—the map. A map is a scale model of reality that we build, using a set of conventions and rules (for example,map projections, line symbols, text). Once we construct a map, we can use it to answer questions about the reality it represents. For example, how far is it from Los Angeles to San Diego? Or, what cities lie along the Mississippi River? The map model also serves as a tool for communicating facts about geography visually: Is the terrain rough? Which way is north? In fact, when we see a map, weoften understand things that might not even occur to us as specific questions. Maps work because we know the “rules” of conventional map reading: blue lines are rivers, North is toward the top of the page, and so on. In a similar way, geographic data models define their own set of concepts and relationships, which must be understood before you can expect to create or interpret your own data model....