def _crs_authority_id():
"""
Returns selected coordinate
reference system (CRS) authority ID
:return auth_id: CRS authority ID
:return auth_id: String
"""
proj_selector = QgsGenericProjectionSelector()
proj_selector.setOgcWmsCrsFilter(NAM_CRS)
proj_selector.exec_()
auth_id = proj_selector.selectedAuthId()
return auth_id
