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 requires the digitization of that entity into the TIGER data base and an independent summation of its component polygons.)

The Census Bureau tabulates and publishes demographic data for land blocks only. Because the TIGER data base identifies all water bodies with a separate series of unique numbers, the water area of a census block is always zero. The Census Bureau reports water area only for entities at the block group level and larger.

Land area measurements may disagree with the information displayed on the Census Bureau’s maps and in the TIGER data base because, for area measurement purposes, features identified as intermittent water and glacier are reported as land area. For this reason, it may not be possible to derive the land area for an entity (as opposed to its total area) by summing the land area of its component census blocks.

In addition, the water area measuementmeasurement [sic] figures reported for some geographic areas include water, while those for some lower-level geographic entities do not. This occurs, for example, where water is associated with a county but is not within the legal boundary of any MCD, or the water is associated with a State but is not within the legal boundary of any county. Therefore, because water area values are contained only in the totals for higher-level geographic entities, summing the water measurements for all component lower-level geographic entities will not necessarily yield the water area of that higher-level entity. Therefore, at any given higher level of geography, there may be water area that is not part of a particular lower-level geographic entity.

Some census tracts, BNAs, and census blocks are classified as crews-of-vessels entities, populated entirely by people aboard one or more civilian or military ships. Such entities do not encompass territory (the ships are assumed to be docked and part of the adjacent land block for this purpose) and therefore have no separate area measurement value. Area Measurement/Water Classification15-9