Orientation of orthogonal coordinate systems used for registration of cephalometric landmarks.

Accurate orientation of orthogonal coordinate systems facilitates repeated registration of cephalometric landmarks, addition of new data at a later date and the evaluation of superimposional methods in longitudinal analysis. The precision of a template designed to mark radiographs with four fiducial points was examined. It was found that the envelopes of error surrounding each point were circular, of similar magnitude and closely approached the previously established standard error of the measuring instrument employed. It was concluded that for most practical purposes the precision offered by the template was well matched to that of the coordinatograph used for registering cephalometric data and to the ability of the operator to align the instrument.