Global Multihazard Frequency and Distribution is a combination of seven surfaces. These surfaces include each of the frequency and distribution deciles of the six hazards considered in the study (cyclones, droughts, earthquakes, floods, landslides, and volcanoes) of which only the top three deciles, the most hazardous, were retained (deciles 8-10). The seventh surface is a global decile of population density; explanations of the processes determining population and area per grid cell follow shortly. In the combined surface, each of the six hazard decile values and the population density decile value function together as a unique means of identifying and grouping grid cells. The item Value serves as a numeric identifier of these various unique, decile-based combinations. Theoretically, any results of aggregate analyses are applicable only to the hazards/population density decile combination and not to the individual grid cell. The population per grid cell (item Pop) is based on the Gridded Population of the World, Version 3 (GPWv3). The area per grid cell (item Areakm), in square kilometers, is calculated by subtracting from the area of the grid cell those areas identified as being permanently inundated using VMAP(0). The deciles of population density are determined by dividing the population per grid cell by the area per grid cell and classifying the resultant range of values into 10 classes of an approximately equal number of grid cells. VMAP(0) is also utilized to determine the length, in kilometers, of any major roads or railroads.
A multihazard index (item Indexmhz) is generated by summing all the decile values of the individual hazards. The multihazard index itself is not classified into deciles and has a theoretical range of 8-60.
Building upon a methodology presented by Sachs et al. (2003), a Gross Domestic Product (GDP) value (US$, 2000, purchase power parity adjusted (PPP)) is estimated for each grid cell. The process begins by determining the contribution of each subnational unit to national GDP using data of varied origin. The ratio of the subnational production to the national GDP is the contribution rate. To standardize estimates across countries, these contribution rates are applied to published World Bank GDP estimates to realize subnational GDP estimates.
The standardized, subnational GDP estimate is divided by the total population within the subnational unit to produce a spatially variable per capita GDP value. The GDP per grid cell is determined by multiplying the subnational, per capita GDP by the grid cell population density.
The GDP value presented in Global Multihazard Frequency and Distribution (item Gdpvalue) is specific to the unique hazards/population density decile combinations and not to individual grid cells (unless it is an artifact that there happens to be only one grid cell in a specific hazards/population density decile combination).
Agricultural GDP (item Agvalue) (US$, 2000, PPP) is also based on a process of spatial reallocation. At the level of the subnational unit, estimates are made of the agricultural GDP and of the total agricultural area within the subnational unit. Agricultural area is estimated from a re-interpretation of USGS Land Cover Characterization Data. A value of agricultural GDP per unit of agricultural area is determined at the subnational level; the agricultural GDP of a grid cell is determined by multiplying the agricultural area within the grid cell by the agricultural GDP per unit area. Agvalue is specific to the hazard/population density decile combination.
The values presented in the aggregtion analyses, items Gdpvalue, Agvalue, and Transkm, are not estimates of risk. These values are determined without any consideration of any type of hazard impact whatsoever. Their function is as a baseline from which to gauge any potential hazard impacts.