This paper introduces the methods of compiling the land-cover map of Northwest China（with an area of approximately 3

100

000 km 2）using SPOT4-VEGETATION data sets and the validation techniques using high spatial resolution TM images. Based on the spectral reflectance

NDVI（Normalized difference Vegetation Index） and NDWI（Normalized Difference Water Index）time series data sets from SPOT4-VEGETATION

the land-cover map of Northwest China is compiled by applying the ISODATA unsuperviesd classification method. In order to evaluate this land-cover map’s accuracy

47 sampling units are selected from the whole mapping region. Each sample has a 25km×25km unit and evenness distribution as well as high heterogeneity. With the assumption of the TM interpretation in the 47 sampling units as the true land cover condition

we calculated every land cover type and its area percentage existing in every sampling unit in the SPOT4-VEGETAION map and the TM interpreted map

respectively. According to the statistical results

the land-cover classification system of the SPOT4-VEGETATION is modified

the sampling statistical histogram of the validation results in every province is established and the regression coefficient value is also calculated respectively. The validation results show that the land cover mapping of Northwest China using SPOT4-VEGETATION data sets and the ISODATE method can get an improving accuracy attributing to the high quality of SPOT4-VEGETATION data products and the effective method of spectral index combination of NDVI and NDWI. The reasons that reduce or impact the landcover classification accuracy mainly derive from two aspects:one is attributed to different landcover classes with same spectral characteristics

and the other is due to the mixed pixel problem. TO the former

adding some auxiliary information data such as DEM can reduce the possibility of misjudgment; to the later

the methods of mixed pixel decomposition and sub-pixel mapping may be good ways to increase the land cover classification and mapping accuracy.