The International Terrestrial Reference Frame (ITRF) is a set of points with their 3-dimensional cartesian coordinates which realize an ideal reference system, the International Terrestrial Reference System ( ITRS), as defined by the IUGG resolution No. 2 adopted in Vienna, 1991 ( IUGG1991 Resolutions (not accessible) [PDF, 105 KB] ).
The ITRS Combination Centres provide also alternative realizations of the ITRF.

