Digital Terrain Models (DTMs) represent the elevation of bare terrain whilst Digital Surface Models (DSMs) represent the upper height of all objects on the land surface.
The slope of the terrain is defined as the steepness or degree of inclination of the terrain to the plane surface. It ranges from 0 to 90 degree. Slope influences wind speed and directions and hydrological flows.
Aspect is defined as the orientation of slope and it is measured clockwise from north (0 to 360deg). Along with slope, aspect regulates land surface processes including plant productivity, climate and hydrology.