The ArcView GIS and its Spatial Analyst extension provide tools for watershed delineation from digital elevation models. Dr. David Maidment's research group at UT Austin has developed a system of Avenue scripts for ArcView called the CRWR PrePro which allows a user to digitally delineate watersheds and stream networks. The result is the irregular termination of drainage paths in pits. Filling pits is typically the first processing step taken after a DEM is projected into the chosen map projection. Pits are cells or groups of cells that are completely surrounded by cells of higher elevations.
Some pits are artificial while others represent real inland drainage basins.
It is sometimes difficult to distinguish between real pits and artificial pits automatically by using the DEM alone. Since water flows down-gradient, this command inspects every cell in the grid and decides which of its eight neighboring cells offers the steepest elevation gradient. This function considers every cell and counts the number of cells up-gradient from a particular cell. In addition, from the topographic gradient map, watershed boundaries can be established by connecting points associated with the lowest gradients. In this step the drainage area, which corresponds to the minimum number of cells that contribute to a stream, is specified.