The U.S. Census Grids are created by taking population and housing counts at the block level and proportionally allocating the counts in the census blocks to a latitude-longitude quadrilateral grid. If a grid cell contains 40% of the area of one census block and 30% of the area of a second census block, the population count for that grid cell will be 40% of the population of the first census block and 30% of the population of the second census block.
U.S. Census Grids uses TIGER/Line files for the census block boundaries and SF1 and SF3 tables for the demographic and socioeconomic characteristics of each census block. SF1 data are based on the census short form and therefore include counts for the total population. The SF3 data are based on the census long form, which is sent to approximately one out of every six households.
30 arc-second grids using SF1 data
The relevant fields are extracted from the SF1 tables. Some of the grids contain data from a single field, such as the number of non-Hispanic whites, which is taken from SF1 table P10, field P10001 for 1990 and P8, field P008003 for 2000. Other grids use data from several fields. This is true of all the age grids, which are derived from SF1 tables P12 and P14 for 2000. These tables report the number of people in each age category by gender. The 2000 grid for the population under age one uses SF1 table P14, field P14003 (the number of males under age one) plus SF1 table P14, field P14024 (the number of females under age one). The Variable Catalog contains a list of the fields used for each census grid year.
The TIGER/Line files are converted to ArcInfo coverages and joined to the SF1 field variables. The density of the variable being gridded is calculated for each census block, for example the number of foreign born residents per square kilometer. A 30 arc-second quadrilateral grid is intersected with the census block coverage. This divides each census block into pieces that fit into the grid cells. The total count for the grid cell is calculated by taking the area of each census block piece within the grid cell, multiplying it by the density of the variable being gridded, and summing these values for all census block pieces in the grid cell.
30 arc-second grids using SF3 data
The lowest level of geography for which SF3 data are released is the census block group. For the SF3 grids, these data are proportionately allocated to census blocks using the distribution of the underlying SF1 population. For instance, if 35% of the block group’s population aged 25 and older lives in a given census block, as reported in the SF1 tables, 35% of the block group’s population aged 25 and older with a high school diploma, as reported in the SF3 tables, is assigned to that census block. Once the SF3 data have been allocated to the census block level, the gridding process is the same as described above.
Metropolitan Statistical Area grids
The metropolitan statistical area (MSA) grids are created by selecting all census blocks within the MSA and gridding those blocks using a 7.5 arc-second quadrilateral grid.
1. The grids for Alaska are divided by hemisphere. Most of the state is in the western hemisphere. There are several islands, with a total population of 47, in the eastern hemisphere. These are gridded separately.
2. The initial beta version had an error with the datum used to create the grids for Hawaii. The TIGER/Line files use a local datum for Hawaii, but the beta version dated April 27, 2006 erroneously assumed that the datum was NAD83. The corrected Hawaiian data have a date of June 1, 2006.
3. Puerto Rico data for U.S. Census Grids 1990 will be available soon.