A charged beam apparatus, such as an electron microscopy apparatus, and a method for determining an aerial dimensional map of a charged beam within the charged beam apparatus, each use a test structure that includes a feature located upon a substrate. One of the feature and the substrate is conductive and the other of the feature and the structure is non conductive. The charged beam within the charged beam apparatus is scanned in a plurality of non-parallel linear directions with respect to the substrate and the feature to provide a corresponding plurality of current versus position response curves from which may be determined the aerial dimensional map of the charged beam within the charged beam apparatus.