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A detailed analysis of the astrometric characteristics of the Automated Plate Scanner catalog of POSS I is presented. Internal estimates of the random catalog position errors are determined from our astrometric plate solutions and comparisons between the positions on O and E plates. An estimate of the systematic catalog position errors has been obtained by stacking residual maps from a large number of POSS I plates. External position error estimates have been determined using plate overlap regions and several different high surface-density astrometric catalogs. We find that our astrometric quality is sufficient for most applications for which APS POSS I catalog positions will be important (finder charts, CCD astrometric alignment, optical cross-identifications, the positioning of multi-fibers and multi-slits, etc.). The limitations at the present epoch resulting from stellar proper motions over the 40 yrs since POSS I are discussed in light of the APS program to determine proper motions from the Luyten Palomar survey. In addition, the advantages and disadvantages of our diffraction-spike centering technique are illustrated.
