  The astrometric calibration of the Sloan Digital Sky Survey is described. For
point sources brighter than r ~ 20 the astrometric accuracy is 45
milliarcseconds (mas) rms per coordinate when reduced against the USNO CCD
Astrograph Catalog, and 75 mas rms when reduced against Tycho-2, with an
additional 20 - 30 mas systematic error in both cases. The rms errors are
dominated by anomalous refraction and random errors in the primary reference
catalogs. The relative astrometric accuracy between the r filter and each of
the other filters (u g i z) is 25 - 35 mas rms. At the survey limit (r ~ 22),
the astrometric accuracy is limited by photon statistics to approximately 100
mas rms for typical seeing. Anomalous refraction is shown to contain components
correlated over two or more degrees on the sky.
