A position determination apparatus for determining the position of a working element arranged within an object having an inner structure with respect to a model of the object. The position and shape of a registration element within the inner structure of the object are provided and used for determining a transformation relating the inner structure of the model and the position and shape of the registration element with respect to each other, wherein the position of the working element with respect to the model is determined depending on a provided spatial relation between the working element and the registration element and the determined transformation.
