What are rasters?
Rasters are a matrix of cells (pixels) that represent a place on the earth. Each cell corresponds to a single data value. The value can represent something categorical, like land cover, or something numeric, like average annual precipitation. Methods for cell value assignment vary across datasets, but it�s important to note that within a single raster, each cell will have only one value. These values can be integers or float (decimal). Raster data may be derived from satellite imagery or interpolated from data collected at points such as weather stations.
Each cell within a raster is the same size (in the raster�s unit of measurement). The cell size is a raster�s spatial resolution. For example, a 30-meter raster is a raster whose cells are 30m x 30m, and a 1-kilometer raster has 1km x 1km cells. The smaller the cell size, the higher the spatial resolution. In rasters in geographic coordinate systems (lat/long), resolution is measured in portions of degrees of latitude and longitude (e.g. 0.5 degrees, 30 arc seconds). The actual surface area covered by each cell varies with latitude, as the distance covered by a degree of longitude approaches zero at the poles.
TerraPop raster data come from academic, government, and other research organizations and are available as TIFFs.
TerraPop can transform area-level data into raster data by distributing data about geographic units over the grid cells within each unit. TerraPop can also derive area-level data from raster data by summarizing the values of the raster cells that fall within each geographic unit. Area-level data derived from rasters can be attached to microdata as contextual variables describing the geographic units in which individuals and households are located.