🏙️ The Ground Floor

The power ranking is a good instrument for one question and a useless one for any other. It cannot tell you whether the people living somewhere breathe clean air, can reach work, or can afford to live near it. We rank each metro on each condition independently and take the median of those ranks. There are no weights, because we are not in a position to tell you that clean air matters more than water. The position underneath this is stated rather than concealed: a metro that concentrates extraordinary capital while failing the people inside it is not succeeding, whatever its rank says.

Three dimensions, each measured the same way for every metro in the set, each ranked independently. A metro's Ground Floor rank is the median of its three dimension ranks. No weights and no normalisation, so there is nothing to argue with except the measurements themselves.

Air quality is annual mean PM2.5 from SatPM2.5 V6GL03 (Atmospheric Composition Analysis Group, Washington University in St. Louis), satellite-derived and calibrated against ground monitors, CC BY 4.0. Nitrogen dioxide is annual mean ground-level NO₂ from GlobalNO2_AiT (Mu and Tao, Earth System Science Data), CC BY 4.0. NO₂ is almost entirely combustion-derived, so unlike PM2.5 it carries no natural dust or sea-salt component. Water and sanitation is the mean of the shares of population with unimproved drinking water and unimproved sanitation, from WRI Aqueduct 4.0.

What the gap is not. It is bounded by position: a metro at the very top of the power ranking cannot have a negative gap, and one at the bottom cannot have a large positive one. It is a legible way of saying two ranks are far apart, not an effect size, and it should not be averaged. Across the whole set the correlation between accumulation rank and conditions rank is -0.19, which reads as though gathering more means living worse. Do not trust it. Accumulation is largely a measure of size, and larger metros have worse measured air; hold population constant and the relationship reverses sign, so that among metros of similar size the ones that accumulate more have better conditions, not worse. The gap is a sound description of an individual metro. It is not evidence of a general rule, and we would rather say so here than let the number imply one.

How every dimension is averaged. All three are population-weighted across the whole metro: each square kilometre inside the boundary contributes in proportion to the people living in it, so a figure describes what a typical resident experiences rather than what the geographic centre point happens to record. That distinction is not cosmetic. A metro's centre point sits on its traffic core more or less by construction, and before this change the value recorded for most large metros fell in the worst tenth of anything their own residents breathe. Population comes from GHS-POP, the European Commission's gridded population surface.

Limits worth knowing. The concentration grids resolve to about eleven kilometres, so weighting redistributes within that grid rather than seeing below it. PM2.5 is total particulate mass and includes desert dust, which counts against arid metros for their geology rather than their governance. The water and sanitation figures are survey-derived and resolve to province level, so they are coarser than the two satellite measures and produce more ties. A small number of remote island metros have no water data and are not ranked. Figures are shown rounded; the stored values carry the source grid's precision, which is not the same as measurement accuracy.