Left panel: rms noise map of a 3°  ×  3° field centred on ℓ = 19.5° and b = 0 showing how the noise varies as a function of Galactic longitude and latitude. Right panel: histogram of the pixel values found within  | b |  < 1° where the noise is approximately constant. The red line show the result of a Gaussian fit to the noise, from which we estimate the average noise for this field (σ_rms ≃ 60 mJy).