Two different population products
Calamity publishes an urban exposure index and, where a supported footprint and grid are available, a separate gridded modelled-population estimate. They answer different questions and must not be added together or described interchangeably.
Urban exposure index (PEI)
Urban PEI = sum(city population × distance weight × source weight).
The Gaussian profile uses exp(−3(d/R)²) inside a heuristic radius R. Other recorded profiles use linear or step weights. Dataset, reference year, radius, profile and model version identify an estimate. These profiles are heuristics, not simulations of shaking, wind, floodplains or atmospheric transport. They have not been calibrated here against casualty outcomes.
A weighted contribution from a city is an index value, not an observed person. Rural coverage can be absent. Summing event PEIs can repeat the same city contribution; the result is not unique people. Zero means no covered urban contribution, not proof that nobody is present. Missing estimation remains unassessed.
Synthetic example: a city population of 1,000 multiplied by a distance weight of 0.5 and source weight of 0.8 contributes 400 index units. It does not establish that 400 people experienced a hazard.
Gridded population in an observed shaking footprint
The separate supported adapter currently uses WorldPop Italy 2025 cells and available USGS ShakeMap MMI≥5 footprints. It selects cell centres inside the footprint. The output records dataset/year, footprint source, method, computation time and missing-cell coverage. This coverage is Italy only; a missing footprint or territory outside the supported grid is not a zero estimate.
Shared grid cells are counted once when combining supported footprints. Synthetic example: cells with modelled populations 100 and 80 sum to a union of 180 even if two footprints both cover them. A newly covered-cell delta can be positive when a footprint moves, even if the total population remains unchanged.
These are modelled resident populations, not observed occupancy, displaced people, casualties or injured people. Cell-centre inclusion approximates boundary overlap. A historical footprint combined with a modern population grid is a retrospective spatial example, not information that was available when the event occurred.
Sources and limits
[USGS ShakeMap](https://earthquake.usgs.gov/data/shakemap/) documents ground-motion products. [WorldPop methods](https://www.worldpop.org/methods/) describe population mapping. Those source methodologies do not validate Calamity priority scores or cascade weights.
The [population model card](/methodology/models#population) gives the current contract and reproducible examples. Gridded exposure remains separate from the urban PEI used in the priority-index formula.