The terrain model based on fractal character can sufficiently represent the statistical texture features of the terrain. Better model and higher resolution DEM (Digital Elevation Model) data can be obtained from the lower resolution data by using fractal interpolation. The determination of fractal scaleless range is very important for computing the fractal characteristic parameters and modeling the digital elevation. The traditional method for determining the fractal scaleless range usually adopts the mutual test between people and compute. However, this method tends to be too subjective. Hence, we offer an auto-determining method, which has been proved to be concise, effective and efficient by experimental results.