Abstract: Terrain parameters,such as slope and aspect,are the most frequently surface geomorphic parameters in terrain analysis,hydrological modeling,environmental studies and other geosciences fields.Currently,these terrain parameters are often estimated by 3×3 local moving window at grid-based digital elevation model(DEM);and the estimated value is set to the centre cell in 3×3 window.In other words,the slope/aspect map derived from DEM is the same resolution as the DEM.There exists a question in this observation whether the resolution of slope/aspect map is the same as the DEM resolution or not? Based on the vector algebra and real DEM,it demonstrates that the slope/aspect estimated by 3×3 window is larger than a grid cell of DEM,which is 1.7 to 2.7 times of DEM resolution.