US Census Blocks Open Data

Census Block is the smallest geographic unit used by the United States Census Bureau for tabulation of 100-percent data (data collected from all houses, rather than a sample of houses). The number of blocks in the United States, including Puerto Rico, for the 2010 Census was 11,155,486.Census blocks are grouped into block groups, which are grouped into census tracts. There are on average about 39 blocks per block group. Blocks typically have a four-digit number; the first number indicates which block group the block is in. For example, census block 3019 would be in block group 3.Blocks are typically bounded by streets, roads or creeks. In cities, a census block may correspond to a city block, but in rural areas where there are fewer roads, blocks may be limited by other features. The population of a census block varies greatly. As of the 2010 census, there were 4,871,270 blocks with a reported population of zero, while a block that is entirely occupied by an apartment complex might have several hundred inhabitants. Census blocks covering the entire country were introduced with the 1990 census. Before that, back to the 1940 census, only selected areas were divided into blocks. Census blocks are maintained within the Administration Feature and is dissolved out weekly. Administration is a polygon feature consisting of the smallest statistical areas bounded by visible features such as roads, streams, railroad tracks, and mountain ridges, as well as by nonvisible boundaries such as jurisdictional limits, school district, public safety boundaries, voting precincts, and census blocks. This methodology allows for single stream editing to move coincidental boundaries across many aggregate datasets simultaneously. Administration is maintained though an ArcGIS topology class in conjunction with County Parcels and Zoning. The topology prevents self-intersection and gaps, while ensuring complete coverage amongst the participating features.人口普查街区（Census Block）是美国人口普查局用于汇总全量普查数据（即从所有住户而非抽样样本采集得到的数据）的最小地理统计单元。2010年美国人口普查（含波多黎各）的普查街区总数为11,155,486个。 普查街区会被组成为街区组（block groups），而街区组又隶属于普查分区（census tracts）。平均每个街区组约包含39个普查街区。普查街区通常采用四位数字编号，首位数字代表该街区所属的街区组编号。例如，编号为3019的普查街区即隶属于街区组3。 普查街区通常以街道、道路或溪流作为边界。在城市区域中，普查街区可对应城市街区；而在道路较少的乡村区域，普查街区的边界则可由其他地理特征划定。普查街区的人口规模差异极大：2010年人口普查时，共有4,871,270个普查街区的登记人口为零，而完全被公寓建筑群覆盖的普查街区则可能拥有数百名居民。 覆盖全美范围的普查街区体系是在1990年人口普查中首次引入的。在此之前，可追溯至1940年人口普查，仅部分区域被划分为普查街区。 普查街区依托行政要素图层（Administration Feature）进行维护，并按周执行要素溶解更新。行政要素图层是由最小统计区域构成的面状要素，其边界既包括道路、溪流、铁路线、山脊等可见地理特征，也涵盖司法辖区、学区、公共安全区划、投票选区以及普查街区等非可见边界。该方法支持通过单次流编辑，同步调整多个聚合数据集的重合边界。 行政要素图层通过ArcGIS拓扑类与县宗地（County Parcels）、区划（Zoning）协同维护。该拓扑规则可防止要素自相交与间隙，并确保参与要素间的覆盖完整性。