In an example, a method includes receiving, at least one processor, object model data representing at least a portion of an object that is to be generated by an additive manufacturing apparatus by fusing build material within a fabrication chamber. An intended object placement location within the fabrication chamber may be determined, and a dimensional compensation to apply to the object model data may be determined using a mapping resource relating dimensional compensations to object placement locations. The determined dimensional compensation may be applied to the object model data to generate modified object model data using at least one processor.