Position operators as integrals with respect to Euclidean systems of covariance

Position operators (p.o.) for relativistic elementary quantum systems are constructed as operator-valued integrals with respect to Euclidean systems of covariance (ESC), i.e., positive operator-valued (POV) measures being covariant under the Euclidean group, and are expressed in terms of the generators of the Poincaré transformations. These p.o. are partly well-known in the literature where they are found by other methods.