The raster approach involves sampling attributes for a set of cells having a fixed size. Many of the features shown on paper maps, including transportation routes, rivers, and political boundaries, can be represented effectively in digital form using the vector data model. The vector strategy is well suited to mapping entities with well-defined edges, such as highways or pipelines or property parcels. The vector data model is consistent with how surveyors measure locations at intervals as they traverse a property boundary. In this case, two polygons (filled with blue) represent the reservoir. A series of line segments that begin and end at the same node form polygon features. In this case, the line feature colored red represents the highway. Line segments connect nodes to form line features. The small squares are nodes: point locations specified by latitude and longitude coordinates. The graphic above right illustrates how the entities might be represented with vector data. The aerial photograph above left shows two entities, a reservoir and a highway. Requires the QuickTime plugin, which is available free at the Apple Quicktime download site. View the same animation in QuickTime format (vector.mov, 1.6 Mb) here. When the points are connected by lines or arcs, the sampled points form line features and polygon features that approximate the shapes of their real-world counterparts.Ĭlick the graphic above to download and view the animation file (vector.avi, 1.6 Mb) in a separate Microsoft Media Player window. The vector approach involves sampling either specific point locations, point intervals along the length of linear entities (like roads), or points surrounding the perimeter of areal entities (like water bodies such as lakes or oceans). Vector and raster data are, in essence, two distinct sampling strategies: vector and raster. And just as serious opinion polls are based on sound principles of statistical sampling, so, too, do geographic data represent reality by measuring carefully chosen samples of locations. In much the same way that public opinion is measured through polls, geographic data are constructed by measuring representative samples of locations. The Earth is too big, and the number of unique locations is mathematically infinite. No geographic data set represents every possible location, of course. Digital geographic data are encoded as alphanumeric symbols that represent locations and attributes of locations measured at or near Earth's surface. Recall that data consist of symbols that represent measurements. The two predominant data representation strategies are known as " vector" and " raster." Several different approaches to representing locations and extents in digital form were developed. One of the first challenges they faced was to convert the graphical data stored on paper maps into digital data that could be stored in, and processed by, digital computers. Innovators in many fields, including engineers, computer scientists, geographers, and others, started developing digital mapping systems in the 1950s and 60s.
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