Maps as Numbers • Question: How do we create a digital version of a map? • GIS requires that both data and maps be represented as numbers. • The GIS places data into the computer's memory ina physical data structure (i.e. files and directories). The Data Model • A logical data model is how data is organized for use by the GIS. • Geographic Information Systems have traditionally used either raster (lines) or vector (dots: binary code) for maps. Features and Maps • A GIS map is a scaled-down digital representation of point, line, area, and volume features. A Raster Data Model Uses a Grid • One grid cell is one unit or holds one attribute. • Every cell has a value, even if it is "missing". • A cell can hold a number or an index value standing for an attribute. • A cell has a resolution, given as the cell size in ground units. Rasters Are Faster • Points and lines in raster format have to move to a cell center. • Lines can become fat. Areas may need sparately coded edges. • Each cell can be owned by only one feature. • As data, all cells must be able to hole the maximum cell value. • Rasters are easy to understand, easy to read and write, and easy to draw on the screen. RASTER • A grid or raster maps directly onto a programming computer memory structure called an array. • Grids are poor at representing points, lines, and areas; but are good at surfaces. • Grids are good only at very localized topology, and weak otherwise. • Grida are a natural for scanned or remotely sensed data. • Grids suffer from the mixed pixel problem. • Grids must often include redundant or missing data. • Grid compression techniques used in GIS are run-length encoding and quad trees. The Vector Model • A vector data model uses points stored by their real (Earth) coordinates. • Lines and areas are built from sequences of points in order. • Lines have a direction to the order of the points. • Polygons can be built from points or lines. • Vectors can store information about topology. VECTOR • At first, GISs used vector data and cartographic spaghetti structures. • Vector data evolved from the arc/node model in the 1960s. • In the arc/node model, an area consisting of lines and a line consists of points. • Points, lines, and areas can each be stored in ther own files with links between them. • The topological vector model uses the line (arc) as a basic unit. Areas (polygons) are built up from arcs. • The endpoint of a line (arc) is called a node. Arc junctions are only at nodes. • Stored with the arc is the topology (i.e. the connecting arcs and left and right polygons). Vectors Just Seem More "Correcter" • TIN must be used to represent volumes. • Vector can represent point, line, and area (polygon) features very accurately. • Vectors are far more efficient than grids. • Vectors work well with pen and light-plotting devices and tablet digitizers. • Vectors are not good at continuous coverages or plotters that fill areas. Topology • Topological data structures dominate GIS software. • Topology allows automated error detection and elimination. • Rarely are maps topologically clean with digitized or imported. • A GIS has to be able to build topology from unconnected arcs. • Nodes that are close together are snapped. • Slivers due to double digitizing and overlay are eliminated.