UTM means Universal Transverse Mercator.... Its a projection system... A projection is a way of representing the earth's surface on a map. A map projection is the means by which you display the coordinate system and your data on a flat surface, such as a piece of paper or a digital screen. When we first downloads the digital elevation model, then its projection is World Projections, i.e, World Geographic Coordinate System (WGS 1984). For hydrological analysis we need some physical parameters like Length of River, Area of Watershed/Basin etc. By projecting Digital Elevation Model (DEM) to UTM Zone which is a type of linear projection, hydrologist calculates the physical parameters of the watershed. our final dem projections will be WGS 1984 UTM Zone 42...