The profile of a structure having a region with a spatially varying property is modeled using an optical metrology model. A set of profile parameters is defined for the optical metrology model to characterize the profile of the structure. A set of layers is defined for a portion the optical metrology model that corresponds to the region of the structure with the spatially varying property, each layer having a defined height and width. For each layer, a mathematic function that varies across at least a portion of the width of the layer is defined to characterize the spatially varying property within a corresponding layer in the region of the structure.