Plot showing how the noise varies as a function of Galactic longitude. The points show the standard deviation of the pixel to pixel variation averaged for each 3°  ×  3° field, however, we have only included pixels with  |b|  < 1° where the noise is relatively uniform and where the vast majority of the 870 μm emission is concentrated. A dashed line connects the point to emphasis the field-by-field variations in the noise.