  We compare photospheric line-of-sight magnetograms from the Synoptic
Long-term Investigations of the Sun (SOLIS) vector spectromagnetograph (VSM)
instrument with observations from the 150-foot Solar Tower at Mt. Wilson (MWO),
Helioseismic and Magnetic Imager (HMI) on Solar Dynamics Observatory (SDO), and
Michelson Doppler Imager (MDI) on Solar and Heliospheric Observatory (SOHO). We
find very good agreement between VSM and the other data sources for both
disk-averaged flux densities and pixel-by-pixel measurements. We show that the
VSM mean flux density time series is of consistently high signal-to-noise with
no significant zero-offsets. We discuss in detail some of the factors -spatial
resolution, flux dependence and position on the solar disk- affecting the
determination of scaling between VSM and SOHO/MDI or SDO/HMI magnetograms. The
VSM flux densities agree well with spatially smoothed data from MDI and HMI,
although the scaling factors show clear dependence on flux density. The factor
to convert VSM to HMI increases with increasing flux density (from $\approx$1
to $\approx$1.5). The nonlinearity is smaller for the VSM vs. ~SOHO/MDI scaling
factor (from $\approx$1 to $\approx$1.2).
