Geographic Information Systems

Geographic information systems are used to collect, analyze, and present information describing the physical and logical properties of the geographic world. Geographically refer-enced data is the spatial data that pertain to a location on the earth's surface. Using object-oriented database technology to model the real world. There are four major functional units in a typical geographic information systems (GIS): • Data Input Unit. Measurements in GIS are taken by sensors such as cameras and global positioning systems. A manual process is then used for inputting data that cannot easily be processed automatically. The measurements are discretized, for example, by imposing a regular, multidimen-sional discrete grid over the surface to be measured , allowing points of interest to lie only at the intersection of the grid lines. In addition to the error imposed by this discretization process, measurement errors also reduce the accuracy of attribute values. The data input therefore needs validation. Various integrity constraints including topological constraints also need to be checked. An example of a topological constraint is " Min-neapolis should be inside Minnesota. " • Data Model. A conceptual data model is a type of