Inferred gravitational potential at a height of |z| = 400 pc, as a function of Galactocentric radius. The results of the good data samples are marked with circles, which are colour coded according to the data samples’ respective Y coordinate. The dashed line shows a best fit exponential curve with respect to the good data samples. We also show the results coming from the dubious data samples, and the result of Paper II for the immediate solar neighbourhood.