Characterisation of freeform surfaces is an increasingly importance area of metrology. Currently the characterisation of surfaces is performed using data that can be represented by a height map. Modern measurement devices allow the acquisition of surfaces with complex shapes that cannot properly be analysed using the current texture parameters definition. In this paper a method to characterise both freeform surfaces and surfaces with re-entrant features is proposed. Two surface approximation methods are investigated: triangular mesh and Bézier surface. The proposed method will be compared with a method able to extract the ISO 25178-2 areal texture parameters from a triangular mesh.