The relative astrometry between the r and the i, u,
z, and g CCDs is independent of the astrometric catalog
used, and typically has rms errors of 20 to 30 mas per coordinate, and
systematic errors of order 20 mas. The quality of the absolute
astrometry (based on the r astrometric solutions and centroids)
is dependent on the astrometric catalog used, and is dominated by
systematic errors which vary on timescales of minutes. Within a given
run, the distribution of systematic errors is well characterized by a
Gaussian. Reductions against UCAC have rms systematic errors of order
50 mas per coordinate. Reductions against Tycho-2 have rms systematic
errors of order 100 mas, and show additional systematic errors
constant over entire scans of up to 50 mas. Centroiding errors
contribute an additional random source of error, of order 20 mas, for
objects brighter than 
. Comparison with the astrometry of
the Two-Micron All Sky Survey for stars in common
shows systematic offsets under 50 mas, well within our quoted errors and
the expected systematic astrometric calibration effects quoted by the 2MASS team.
Atmospheric conditions contribute significantly to image wander.
These affect the Tycho-2 reductions more than the UCAC reductions due
to the shorter integration times on the astrometric CCDs. We attempt
to follow this wander by fitting the residuals with cubic splines.
Figure 6 shows the astrometric residuals as a
function of frame number for a typical run. The spline-fitted
solution is superposed on the points showing the residuals. The top
plot shows 
residuals (along the direction of the scan), and the
bottom plot shows 
residuals (the cross-scan direction).
Approximately 100 frames per hour are obtained, so the figure shows
roughly half an hour of scanning. Note that the residuals wander
several tenths of an arcsecond over time scales of minutes. The
frequency and amplitude of these wanderings vary from night to night
(and, occasionally, hour to hour).