Different fits to the spatially-resolved total Schmidt relation for NGC 0628. Only blue points with green error bars are used for fitting. The red data points with errorbars correspond to points with S/N <  3, and have not been included in the fits. The error bars shown here include the random error and systematic error on flux measurements as explained in Sect. 3.2, but not the additional calibration error. Dashed red line: standard unweighted regression, dashed orange line: inverse weighted regression. solid black line: maximum likelihood estimate where each point on both axes is given equal weight, solid blue line: maximum likelihood estimate where 20% calibration error has been included on both axes in addition to the uncertainties estimated in Sect. 3.2, solid red line: maximum likelihood estimate where points have been weighted solely with respect to the uncertainties estimated in Sect. 3.2.