GPS-survey technology is becoming an effective tool for determining topographic-related parameters. We hypothesized that the topographic-related parameters extracted from different scales of GPS survey are uncertain and the uncertainty will be especially large with respect to gully-related parameters. Four GPS-survey scales of 3

5

10

and 20-meter grid were conducted in Majiasongpo gully catchment located in Xichang area

SW-China. Topographic parameters including slope gradient

profile curvature

and gully system were derived from grid DEMs established using ArcGIS 8. 2 software. The results indicated that the average slope gradient and average profile curvature and gully density significantly decreased with the increase of GPS-survey grid

suggesting a lower value of gully-related parameters with big GPS-survey grid than with small GPS-survey grid. A GPS-survey scale of 5 m grid is suitable for establishing DEMs in quantifying gully-related parameters whereas a 3 or less than 3 m GPS-survey scale can meet with study needs of rills and shallow gullies in the upper Yangtze River basin.