We present a method for comparing and combining maps with different resolut
ions and beam shapes, and apply it to the Saskatoon, QMAP, and COBE-DMR dat
a sets. Although the Saskatoon and QMAP maps detect signals at the 21 sigma
and 40 sigma levels, respectively, their difference is consistent with pur
e noise, placing strong limits on possible systematic errors. In particular
, we obtain quantitative upper limits on relative calibration and pointing
errors. Splitting the combined data by frequency shows similar consistency
between the ka and Q bands, placing limits on foreground contamination. The
visual agreement between the maps is equally striking. Our combined QMAP+S
askatoon map, nicknamed QMASK, is publicly available on the web together wi
th its 6495 x 6495 noise covariance matrix. This thoroughly tested data set
covers a large enough area (648 square degrees-currently the largest degre
e-scale map available) to allow a statistical comparison with COBE-DMR, sho
wing good agreement.