Evaluation of fractal dimension of soft terrain surface

•Structure function method and rescaled (R/S) analysis method are not suitable on soft terrain because of the precision.•The power spectral density method is not robust.•Root mean square method is the best method compared with other four methods.•The fractal dimension of a plowed surface is smaller than that of a sowed surface.•The plowed surface has a more complex internal structure than the sowed surface.