Many GIS programs support analysis of surface water hydrology and hydrological processes. In almost all cases the data input is a grid file of surface elevations (a DEM file) rather than a TIN, plus optional seed or pour points, and output is a number of vector and grid files. Programs with a focus on grid processing such as SAGA, PCRaster, Idrisi, GRASS, TNTMips and ANUDEM provide a wide range of facilities for this type of analysis. Vector-oriented programs with strong grid support, such as ArcGIS and Manifold (V6.5 onwards) provide similar facilities. The central (pragmatic) notion is that of a uniform pattern of raindrops falling throughout the study region, creating water flows from location to location down the locally steepest path, combining with other paths and accumulating the total of upstream flows in the process. Where flows in adjacent cells are away from each other it suggests that a local watershed exists. As streams combine larger and larger regions which drain into these streams are determined, and these regions are described as stream basins with boundaries known as watersheds.