The Disp Approx modifier lets you apply Displacement mapping to an object.
Disp Approx (short for Displacement Approximation) converts its input object to an editable mesh, so you can use this modifier to add displacement mapping to geometry primitives and any other kind of object that can convert to an editable mesh.
Using an image to displace the surface of a cylinder
Displacement mapping uses a map to change surface geometry. You apply the map using the Material Editor.
You don't need to use this modifier with NURBS surfaces, patch objects, editable mesh objects, or editable poly objects, because you can apply displacement mapping directly to them.
Apply displacement mapping:
Now you can apply displacement mapping to the object. The Displacement Approx. rollout has parameters you can adjust, but displacement mapping will work using the default settings.