With a convolution algorithm

urban land use proportions were calculated based on remote sensing classification data

which were used as thresholds to distinguish between urban and non-urban area

and then urban extent was extracted.Indexes system was proposed to determine the size of the convolution template and the threshold by evaluating extraction results.We took Beijing as a study area and validated the feasibility of the method by using classification data of SPOT 5 multi-spectral images.Combining with the built-up area of statistical data

504 combinations of template and threshold were used to do the extraction test and the effects of templates and thresholds on the extraction results were discussed.Finally

combination of（205

51） was selected to extract the urban extent.The result showed that:（1） this method solved the problem that urban land acquired from remote sensing monitoring differed from urban extent that geographical studies cared about and provided a new way to prepare the basic data for urban study.（2） Test result indicated that both the lower or higher threshold will exaggerate or understate the extracted result

and that a small size of template cannot eliminate the random error meanwhile a large size would lead the result being too smooth.（3） For the classification data of SPOT 5 multi-spectral image

the extraction results were the best while the size of the template ranges from 193 to 205（around 4 km2） and the threshold is around 50.