The average roughness (or “Ra”) value of a surface is the most common number describing the “amount” of roughness on that surface. While the Ra value (or “Sa” for areal / 3D measurements) may give a general sense of the surface texture, it cannot distinguish between two surfaces of different shapes. For example, a jagged surface with sharp spikes could have the same Ra value as a smoothly plateaued surface with lots of deep porosity. As we show in “Average Roughness (Ra)“, describing a surface using only Ra is like describing a concert only by loudness! Yet, Ra (or Sa) may still have its uses in some production settings.