Moderate-resolution Imaging Spectroradiometer（MODIS） data have been widely used in land cover classification with its advantages in multi-spectral and high-temporal features.The classification may suffer from uncertainty relevant to its moderate spatial resolution

yet the uncertainty remains unclear.To explore the issue

this paper used high spatial resolution data acquired from theAdvanced Space borne Thermal Emission and Reflection Radiometer（ASTER）.SinceASTER and MODIS are onboard the same satellite platform

which allows simultaneous multi-resolution observation at coincident nadirs.The Poyang Lake area was taken as the study area for its diverse land covers in low-water period.The high-resolution ASTER data were upscaled to the same coarse resolution as MODIS

with a scaling function developed from a triangular spread function.Different classification methods were applied respectively for land cover classification using either MODIS or upscaled ASTER data.The uncertainties in classified results were subsequently analyzed based on error matrix and a linear model.The evaluation revealed that the uncertainties contributed from scale difference and that from differences in spectral resolution and imaging mode were（6.6—11.2）:2.The scale-induced uncertainty also showed to be land-cover dependent.It was small for water body

but could be as large as 63% for forestland.The signif icance of scale effect is valuable for land cover classification

change detection analysis

scale effect evaluation

and uncertainty analysis in landscape ecology.