Solar cycle 25 has begun in earnest. Multiple magnetic data sources will be used to characterize the global photospheric and coronal magnetic field during cycles 24 and 25, comparing them with the three previous cycles well covered by full-disk synoptic magnetograph observations, in particular the polar fields and their relationship with the transport of decayed active region magnetic flux. An end-to-end model for magnetograph observations will be described, and applications to GONG and SDO/HMI magnetograms introduced and compared. Such 'ground-truth' calibration will explain well known but poorly understood discrepancies between results from different magnetographs, and will become essential as we combine the advantages of multiple diverse data sources.