An algorithm for estimating mean square error of the pixel position of scanned and thinned image is put forward in this paper. Based on some experiments

the conclusion is obtained that mean square error of pixel position is generally composed of both constant error and ratio error

the effect of different thinning approach on the accuracy is too small and the scanner may exist regular error etc. Thinned and scanned image is the basis of tracing vectorization and its accuracy is important in such areas as translating it into datum used by GIS. So it is necessary to study pixel accuracy of scanned and thinned image.