The previous chapters have dealt with 2D and 3D images. Many surfaces are not perfectly flat but also not fully three-dimensional, and it is often useful to treat them as “two-and-a-half dimensional.” For these surfaces, the z, height, or elevation dimension is fundamentally different from the x and y lateral dimensions. This is true whether the surface in question is a machined part with fine-scale roughness, a microscopic crystal with atom-scale steps, or the surface of the earth with its mountains and valleys (which is less rough than the surface of an orange; the greatest elevation above sea level is about 0.14% of the radius).