An apparatus for associating a position in a construction data set with a position in a building section of the apparatus, comprising: a building section for generating a three-dimensional work piece, configured to receive a base element comprising a boundary surface; an irradiation unit configured to generate a radiation beam and to scan the radiation beam over a predefined scanning field, wherein the predefined scanning field comprises at least part of the boundary surface of the base element; a detecting unit configured to detect time-dependent electromagnetic radiation intensity of electromagnetic radiation emitted at a spot position of the radiation beam while the radiation beam is scanned over the scanning field; and a controller configured to: associate the detected electromagnetic radiation intensity with position information indicative of a current position of the radiation beam; determine a position of the boundary surface of the base element with regard to the scanning field, based on intensity variations of the detected time-dependent electromagnetic radiation intensity and based on the position information, such that abrupt intensity variations over time are identified as corresponding to edges of the boundary surface; associate a position in a construction data set with a position in the building section, based on the determined position of the boundary surface; and associate a position of an additive boundary surface of an additive element defined by the construction data set with the determined position of the boundary surface of the base element. a building section for generating a three-dimensional work piece, configured to receive a base element comprising a boundary surface; an irradiation unit configured to generate a radiation beam and to scan the radiation beam over a predefined scanning field, wherein the predefined scanning field comprises at least part of the boundary surface of the base element; a detecting unit configured to detect time-dependent electromagnetic radiation intensity of electromagnetic radiation emitted at a spot position of the radiation beam while the radiation beam is scanned over the scanning field; and a controller configured to: associate the detected electromagnetic radiation intensity with position information indicative of a current position of the radiation beam; determine a position of the boundary surface of the base element with regard to the scanning field, based on intensity variations of the detected time-dependent electromagnetic radiation intensity and based on the position information, such that abrupt intensity variations over time are identified as corresponding to edges of the boundary surface; associate a position in a construction data set with a position in the building section, based on the determined position of the boundary surface; and associate a position of an additive boundary surface of an additive element defined by the construction data set with the determined position of the boundary surface of the base element. associate the detected electromagnetic radiation intensity with position information indicative of a current position of the radiation beam; determine a position of the boundary surface of the base element with regard to the scanning field, based on intensity variations of the detected time-dependent electromagnetic radiation intensity and based on the position information, such that abrupt intensity variations over time are identified as corresponding to edges of the boundary surface; associate a position in a construction data set with a position in the building section, based on the determined position of the boundary surface; and associate a position of an additive boundary surface of an additive element defined by the construction data set with the determined position of the boundary surface of the base element.