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.