_Rural-urban gradient in Latin America and the Caribbean_ _page_ 4 In practice we are constrained by the spatial resolution of available data. Census bureaus typically report this data for second-level administrative districts (e.g., municipios), although in some cases (such as Brazil) census tract level data is published. The reporting units differ widely in size.

Two consortia have independently tackled the task of assembling global population census data and using it to assign densities to finely-gridded geographic cells, so as to permit geographic analysis of the correlates of population. They have adopted complementary approaches, each with advantages and disadvantages.

Landscan, developed by the Oak Ridge National Laboratory <u>(http://www.ornl.gov/sci/gist/landscan/) uses census source data typically at the province</u> level. They then use an unpublished model to allocate population within the province level, based on road proximity, slope, landcover, and ‘nighttime lights’. The latter is a map derived from satellite images of artificial illumination, and is a useful proxy for the location of cities, towns, and major roads. (though the relation between population and illumination is of course modulated by income). Landscan’s goal is to represent population density averaged over a 24 hour day, thus blending both occupational and residential locations. The results are gridded to 0.5’ by 0.5’ grid cells<sup>2</sup> .

GPW3 (the Gridded Population of the World) is produced by CIESIN, Columbia University, and CIAT<sup>3</sup> . It employs 2.5’ x 2.5’ grid cells. It uses mostly lower-administrativelevel (higher resolution) census source data, typically at the level of the municipio or equivalent. The current version, GPW3, then assumes homogeneous distribution of population within the administrative unit. Table 6 reports, by country, the administrative level and the total number of units used. A forthcoming version, GRUMP v1<sup>4</sup> , documented in Balk _et al._ (2004) allocates reported within-unit urban population to grid cells with known light areas (to proxy for built up areas), based night-time illumination spots, coordinates of cities, and satellite imagery. It assumes homogeneous distribution of the remaining population to non-illuminated grid cells, within administrative areas.

A summary table comparing the data sources is displayed below. This draft is based on GPW3. GRUMP v1 is expected to be released shortly, and we intend to use it to reevaluate the results when it becomes available. The reader should keep in mind that inter-country 2 0.5 arc-minutes; there are 60 arc-minutes in a degree of latitude or longitude. At the equator, a one-degree grid cell is about 111 km on a side, so 0.5’ is approximately a kilometer at the equator. The length of a degree of longitude shrinks towards 0 at the poles, so grid cells vary in size.

3 Center for International Earth Science Information Network (CIESIN), Columbia University; and Centro Internacional de Agricultura Tropical (CIAT), 2004. Gridded Population of the World, version 3, Palisades, NY: CIESIN, Columbia University. available from http://beta.sedac.ciesin.columbia.edu/gpw 4 Center for International Earth Science Information Network (CIESIN), Columbia University; International Food Policy Research Institute (IPFRI),the World Bank; and Centro Internacional de Agricultura Tropical (CIAT),2004. Global Rural-Urban Mapping Project (GRUMP): Gridded Population of theWorld, version 3, with Urban Reallocation (GPW-UR). Palisades, NY: CIESIN,Columbia University