We compare simultaneous magnetograms of a solar active region taken by the Advanced Stokes Polarimeter (ASP) and the Solar Optical Universal Polimeter (SOUP) in 1998. In addition we compare magnetograms taken by the La Palma Stokes Polarimeter (LPSP), the Michelson Doppler Imager (MDI) on SOHO, and the SOUP instrument in 2000. The SOUP instrument on the Swedish Vacuum Solar Telescope (SVST) attains the highest spatial resolution but has the least understood calibration; the ASP on the Dunn Solar Telescope (DST) at Sacramento Peak attains the highest magnetic field precision. The goal of the program is to better quantify the SOUP magnetograms and thereby study magnetic element dynamics in the photosphere with higher precision.