  Sets of short exposure, guided CCD frames are used to measure the noise added
by the atmosphere to differential astrometric observations. Large nightly
variations that are correlated with the seeing have been found in the data
obtained over 2 years at the KPNO and CTIO 0.9-meter telescopes. The rms noise
added by the atmosphere, after a linear transformation of the raw $x,y$ data,
is found to be 3 to 7 mas, normalized to 100 seconds exposure time and a field
of view of 20 arcminutes near the zenith. An additional nearly constant noise
(base-level) of 8.5 mas = 0.012 pixel is found for the KPNO and 6.0 mas = 0.015
pixel for the CTIO telescope. This implies that a ground-based, all sky,
astrometric survey from guided CCD frames is more likely limited by the
base-level noise than by the atmosphere and could reach an accuracy better than
10 mas under good seeing conditions.
