  Astronomical seeing is quantified by a single parameter, turbulence integral,
in the framework of the Kolmogorov turbulence model. This parameter can be
routinely measured by a Differential Image Motion Monitor, DIMM. A new
instrument, Multi-Aperture Scintillation Sensor (MASS), permits to measure the
seeing in the free atmosphere above ~0.5km and, together with a DIMM, to
estimate the ground-layer seeing. The absolute accuracy of both methods is
studied here using analytical theory, numerical simulation, and experiments. A
modification of the MASS data processing to compensate for partially saturated
scintillation is developed. We find that the DIMM can be severely biased by
optical aberrations (e.g. defocus) and propagation. Seeing measurements with
DIMM and MASS can reach absolute accuracy of ~10% when their biases are
carefully controlled. Pushing this limit to 1% appears unrealistic because the
seeing itself is just a model-dependent parameter of a non-stationary random
process.
